Gustavo F. Gutiérrez-López, Ph.D.
Escuela Nacional de Ciencias Biológicas Instituto Politécnico Nacional México, DF.
Gustavo V. Barbosa-Cánovas, Ph.D.
Washington State University Pullman, Washington
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Library of Congress Cataloging-in-Publication Data
Food science and food biotechnology / edited by Gustavo F. Gutiérrez-López and Gustavo V. Barbosa-Cánovas. p. cm. -- (Food preservation technology series) Includes bibliographical references and index. ISBN 1-56676-892-6 1. Food--Biotechnology. 2. Food industry and trade. I. Gutiérrez-López, Gustavo F. II. Barbosa-Cánovas, Gustavo V. III. Series TP248.65.F66 F733 2003 664--dc21
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Introduction to Molecular Food Biotechnology Juan Alberto Osuna-Castro and Octavio Paredes-López Recent Developments in Food Biotechnology Humberto Hernández-Sánchez Bioprocess Design Ali Asaff-Torres and Mayra De la Torre-Martínez Gas Hold-Up Structure in Impeller Agitated Aerobic Bioreactors Ashok Khare and Keshavan Niranjan Production and Partial Puriﬁcation of Glycosidases Obtained by Solid-State Fermentation of Grape Pomace Using Aspergillus niger 10 Sergio Huerta-Ochoa, María Soledad de Nicolás-Santiago, Wendy Dayanara Acosta-Hernández, Lilia Arely Prado-Barragán, Gustavo Fidel Gutiérrez-López, Blanca E. García-Almendárez, and Carlos Regalado-González Protein Crystallography Impact on Biotechnology Manuel Soriano-García Stability of Dry Enzymes Mauricio R. Terebiznik, Viviana Taragano, Vanessa Zylberman, and Ana María Pilosof Trends in Carotenoids Biotechnology María Eugenia Jaramillo-Flores and Rosalva Mora-Escobedo Studies on the Reverse Micellar Extraction of Peroxidase from Cruciferae Vegetables of the Bajio Region of México Carlos Regalado-González, Miguel A. Duarte-Vázquez, Sergio HuertaOchoa, and Blanca E. García-Almendárez
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10 Improving Biogeneration of Aroma Compounds by In Situ
Product Removal Leobardo Serrano-Carreón
Lupines: An Alternative for Debittering and Utilization in Foods Cristian Jiménez-Martínez, Humberto Hernández-Sánchez, and Gloria Dávila-Ortíz
12 Recent Development in the Application of Emulsiﬁers: An Overview Victor T. Huang
13 Drying of Biotechnological Products: Current Status and New Developments Arun S. Mujumdar
14 An Update on Some Key Alternative Food Processing
Technologies: Microwave, Pulsed Electric Field, High Hydrostatic Pressure, Irradiation, and Ultrasound José J. Rodríguez, Gustavo V. Barbosa-Cánovas, Gustavo Fidel Gutiérrez-López, Lidia Dorantes-Alvárez, Hye Won Yeom, and Q. Howard Zhang
15 Emerging Processing and Preservation Technologies for Milk and Dairy Products Valente B. Alvarez and Taehyun Ji
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I am glad to welcome this book on food science and food biotechnology to the growing CRC Food Preservation Technology Series. This new book signiﬁcantly expands the scope of the series, opening new directions I would like to pursue. The blend of engineering with food science and food biotechnology is quite unique, as well as complex and intriguing. Wonderful things will come from this association, provided we make the right considerations and identify those products that match expectations. Biotechnology is vast, without boundaries, and offers great possibilities as well as concerns. Let’s use it correctly, let’s learn more about it, and let’s see how it will contribute to the science of making better foods on a daily basis. I am very enthusiastic about this book and hope that all readers will feel the same. It has signiﬁcant new material that will open new horizons; and it offers the opportunity to expand two critical disciplines at the same time: food biotechnology and food science. Gustavo V. Barbosa-Cánovas
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The relevance of biotechnology in food science has signiﬁcantly grown in the last few years and this trend will continue in the years to come. For this reason, it is becoming quite relevant to identify in an organized manner efforts toward the use of a very important area of scientiﬁc research and application (biotechnology) in a domain of tremendous relevance for the whole population (food science). This book presents a meaningful and up-to-date review of how food science and biotechnology are interacting and covers important aspects of both subjects as well as their interface. Distinguished scientists from key institutions have contributed chapters that provide deep analysis of their particular subjects and at the same time place each topic within the context of this interface. The premise of this book is that an effective discussion on these subjects (food science, biotechnology) requires an effective coupling to convey in a comprehensive manner the state-of-the-art for the fundamentals and applications of both areas. The book is mainly directed to academics, undergraduate and postgraduate students (including research students) in food science and technology, biotechnology, and bioengineering, who will ﬁnd a selection of topics ranging from the molecular basis of food preservation and biotechnology to industrial applications. Professionals working in food and biotechnology research centers also may ﬁnd this book useful. The ﬁrst chapter reviews molecular aspects of food biotechnology, presenting the reader a state-of-the-art introduction to the next 14 chapters. The second chapter covers some of the recent developments in food biotechnology. The reader may note a certain degree of overlap between these two chapters. However, Chapter 2 covers a wide range of food applications. Chapters 3, 4, and 5 describe speciﬁc examples of upstream processes and bioseparations. The following eight chapters cover basic and applied situations of food biotechnology such as the impact of crystallography, stability of enzymes, biotechnology of carotenoids, biogeneration of aroma compounds, potential usage of regional alternative food supplies, and developments in the application of emulsiﬁers. The last three chapters describe food preservation techniques, strongly stressing alternative food processing technologies as well as drying of biotechnological products. It is very likely that many of the procedures and techniques described in the book will be used in the food biotechnology industry. It is hoped that this text will constitute a worthy addition to the emerging literature on food biotechnology and that the readers will ﬁnd in it balanced and organized © 2003 CRC Press LLC
information, with chapters written with the common goal of stressing the interactions of food science and biotechnology and demonstrating, through speciﬁc examples, that the bases for many of the situations discussed in the various sections are very similar. Gustavo F. Gutiérrez-López Gustavo V. Barbosa-Cánovas
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Gustavo F. Gutiérrez-López received his bachelor of science in biochemical engineering and master of science in food science and technology from the National Polytechnic Institute of Mexico, and his master of science in food process engineering and Ph.D. in food engineering from the University of Reading, U.K. He is currently professor of food engineering at the National School of Biological Sciences of the National Polytechnic Institute of Mexico and president of the Mexican Society of Biotechnology and Bioengineering. Gustavo V. Barbosa-Cánovas received his bachelor of science in mechanical engineering from the University of Uruguay and his master of science and Ph.D. in food engineering from the University of Massachusetts at Amherst. He is currently professor of food engineering at Washington State University and director of the Center for Nonthermal Processing of Food.
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The Editors wish to express their gratitude to the following institutions and individuals who contributed to making this book possible: National School of Biological Sciences of the National Polytechnic Institute of Mexico (ENCB-IPN), CYTED (Ibero-American Program to promote science and technology) Subprogram XI, Mexican Society of Biotechnology and Bioengineering (SMBB), and Washington State University (WSU) for supporting the preparation of this book. Our fellow colleagues Lidia Dorantes, María Eugenia Jaramillo, Rosalva Mora, Gloria Dávila, and Humberto Hernández (IPN-ENCB) for their valuable comments and suggestions throughout the preparation of this book. J. Anderson (WSU) for her professionalism and dedication throughout the entire editorial process. V. Aguilar-Clark, L. Alamilla, M. Cornejo, A. Ortíz, and J. Chanona, all from ENCB-IPN, and Gipsy Tabilo (WSU) for their decisive participation in helping the editors prepare the ﬁnal version of this book by revising references, formatting all the manuscripts, and incorporating in the text all the editorial comments.
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Wendy Dayanara Acosta-Hernández Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Valente B. Alvarez, Department of Food Science and Technology, The Ohio State University, Columbus, Ohio Ali Asaff-Torres Departamento de Biotecnología y Bioingeniería CINVESTAV–IPN, Instituto Politécnico Nacional, México Gustavo V. Barbosa-Cánovas D e p a r t m e n t o f B i o l o g i c a l S y s t e m s Engineering, Washington State University, Pullman, Washington Gloria Dávila-Ortiz Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Lidia Dorantes-Alvárez Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Miguel A. Duarte-Vázquez Departamento de Investigación y Posgrado en Alimentos, PROPAC, Facultad de Química, Universidad Autónoma de Querétaro, Querétaro, México Blanca E. García-Almendárez Departamento de Biotecnología, CBS, Universidad Autónoma Metropolitana–Iztapalapa, Delegación Iztapalapa, México Gustavo Fidel Gutiérrez-López Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Humberto Hernández-Sánchez Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Victor T. Huang General Mills, Minneapolis, Minnesota
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Sergio Huerta-Ochoa Departamento de Biotecnología, CBS, Universidad Autónoma Metropolitana–Iztapalapa, Delegación Iztapalapa, México María Eugenia Jaramillo-Flores Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Taehyun Ji Department of Food Science and Technology, The Ohio State University, Columbus, Ohio Cristian Jiménez-Martínez Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Ashok Khare School of Food Biosciences, The University of Reading, Whiteknights, Reading, United Kingdom Rosalva Mora-Escobedo Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Naconal, Carpio y Plan de Ayala, México Arun S. Mujumdar Department of Mechanical Engineering, National University of Singapore, Singapore María Soledad de Nicolás-Santiago D e p a r t a m e n t o d e G r a d u a d o s e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala, México Keshavan Niranjan School of Food Biosciences, The University of Reading, Whiteknights, Reading, United Kingdom Juan Alberto Osuna-Castro CINVESTAV, Unidad Irapuato, Libramiento Norte, Carretera México-León, Irapuato, México Octavio Paredes-López CINVESTAV, Unidad Irapuato, Libramiento Norte, Carretera México-León, Irapuato, México Ana María Pilosof Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina Lilia Arely Prado-Barragán D e p a r t a m e n t o d e B i o t e c n o l o g í a , C B S , Universidad Autónoma Metropolitana–Iztapalapa, Delegación Iztapalapa, México
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Carlos Regalado-González Departamento de Investigación y Posgrado en Alimentos, PROPAC, Facultad de Química, Universidad Autónoma de Querétaro, Querétaro, México José J. Rodríguez Department of Biological Systems Engineering, Washington State University, Pullman, Washington Leobardo Serrano-Carreón Instituto de Biotecnología, Departamento de Bioingeniería, Universidad Nacional Autónoma de México, Morelos, México Manuel Soriano-García Departamento de Bioestructura, Instituto de Química, Universidad Nacional Autónoma de México (UNAM), Circuito Exterior, Ciudad Universitaria, Delegación Coyoacán, México Viviana Taragano Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina Mauricio R. Terebiznik Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad de Buenos Aires, Buenos Aires, Argentina Mayra De la Torre-Martínez D e p a r t a m e n t o d e B i o t e c n o l o g í a y Bioingeniería, CINVESTAV–IPN, Instituto Politécnico Nacional, México Hye Won Yeom Department of Food Science and Technology, The Ohio State University, Columbus, Ohio Q. Howard Zhang Department of Food Science and Technology, The Ohio State University, Columbus, Ohio Vanessa Zylberman Departmento de Industrias, Faculta de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina
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Introduction to Molecular Food Biotechnology
Juan Alberto Osuna-Castro and Octavio Paredes-López
CONTENTS Introduction Components of Molecular Biotechnology Recombinant DNA Technology Restriction Endonucleases Plasmid Cloning Vectors Polymerase Chain Reaction Biomolecular Engineering Metabolic Engineering Protein Engineering Protein Rational Design Protein Irrational Design Molecular Bioinformatics Functional Genomics and DNA Microarrays Proteomics Applications of Molecular Biotechnology Plant Biotechnology for Food Production Improvement of Plant Nutritional and Functional Quality Plant Proteins Nutritional Quality Functional Quality Lipids Saturated Fatty Acids Unsaturated Fatty Acids Carbohydrates Starch Quality Plants as Bioreactors Vaccines Plant Vaccines Antibodies
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Plantibodies Milk Proteins Reconstitution of Human Milk Proteins in Food Plants Micronutrients Carotenoids Vitamins Minerals Manipulation of Fruit Ripening Microbial Biotechnology in Industry Microbial Metabolites Microbial Production of Small High-Value Molecules Vitamins Lactic Acid Amino Acids Carotenoids Acknowledgment References
Molecular biotechnology is an exciting revolutionary scientiﬁc discipline based on the ability of researchers to transfer speciﬁc units of genetic information from one organism to another. This conveyance of a gene relies on the techniques of genetic engineering (recombinant DNA technology). The application of science and technology, with molecular biology being one of the more recent developments, has resulted in greatly increased yields per unit of cultivated area, due in great part to production of plants that are resistant to insect predation, fungal and viral diseases, and environmental stresses such as short-term drought, excessive heat, and acidic and alkaline soils. Some biotechnologists now see plants as biofactories or bioreactors that need only water, minerals, sunlight, and the proper combination of genes to produce high-value biomolecules such as enzymes, starches, oils, vitamins, pigments, nutraceuticals, and vaccines for the food and pharmaceutical industries. Biotechnology can also be applied to the production or transformation of food and food ingredients and animal feed by developing microorganisms able to produce chemicals, antibiotics, polymers, amino acids, and various food additives.
Components of Molecular Biotechnology
The ﬁrst experiments in which DNA fragments were joined in vitro and the recombinant molecules reintroduced into living cells were performed © 2003 CRC Press LLC
nearly 30 years ago (Cohen et al., 1973). The basic information obtained in these early experiments, together with numerous new ﬁndings in all ﬁelds of bioscience, as well as in chemical, physical, and computer sciences, have led to the development of modern molecular biotechnology. This new ﬁeld has at least three components: (1) recombinant DNA technology; (2) biomolecular engineering, including metabolic and protein engineering; and (3) molecular bioinformatics, including functional genomics and proteomics (Glick and Pasternak, 1998; Kao, 1999; Ryu and Nam, 2000).
Recombinant DNA Technology
Fundamental steps in recombinant DNA technology, also called gene cloning, molecular cloning, or engineering genetics, are the isolation, enzymatic cleavage, and joining (ligation) of a speciﬁc DNA fragment of interest into a cloning vector to make a recombinant DNA molecule (Olmedo-Alvarez, 1999). This construct is then transferred into a host (such as bacteria, yeast, or animal or plant cell), ampliﬁed, and maintained within the host. The introduction of DNA into bacterial host cells is termed transformation (Glick and Pasternak, 1998; Olmedo-Alvarez, 1999). Those host cells that take up the DNA construct (transformed cells) are identiﬁed and selected from those that do not carry the recombinant molecule desired.
Restriction Endonucleases Recombinant DNA molecules could not be constructed without the use of restriction endonucleases (or restriction enzymes). For molecular cloning, both the source DNA that contains the target sequence and the cloning vector must be consistently cut into discrete and reproducible fragments (Russell, 1998). It was only after restriction endonucleases were discovered that the development of recombinant DNA technology became feasible (Glick and Pasternak, 1998). The important feature of these enzymes is that they are able to cleave double-stranded DNA molecules internally at speciﬁc nucleotide pair sequences called restriction sites. Restriction enzymes are used to produce a pool of discrete and required DNA fragments to be cloned; they are also utilized to analyze the positioning of restriction sites in a piece of cloned DNA or in a segment of DNA in the genome, in this way allowing us to obtain their restriction maps (Russell, 1998). Over 400 different restriction enzymes have been characterized and puriﬁed; thus, endonucleases that cleave DNA molecules at many different DNA sequences are available (OlmedoAlvarez, 1999).
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Plasmid Cloning Vectors Plasmids are self-replicating, double-stranded circular DNA molecules that are maintained in bacteria as independent extra-chromosomal entities necessary to join or ligate in vitro DNA molecules or fragments and perpetuate them in a host cell. The particular plasmids used for cloning experiments are derivatives of naturally occurring bacterial plasmids engineered to have features that facilitate gene cloning. Because they are most commonly used, we focus here on particular features necessary for Escherichia coli plasmid vector cloning (Glick and Pasternak, 1998; Russell, 1998; Olmedo-Alvarez, 1999): 1. Small size, which is necessary because the efﬁciency of transfer of exogenous (foreign) DNA into E. coli decreases signiﬁcantly with plasmids that are more than 15 kb long 2. An origin of replication, which allows the plasmid to replicate in E. coli because it provides a sequence recognized by the replication enzymes in the cell 3. Unique restriction endonuclease cleavage sites for several different restriction enzymes (called a polylinker or multiple cloning site), into which the insert DNA can be cloned 4. One or more dominant selectable markers for identifying recipient cells that carry the construct from cells lacking it
The most commonly used markers are genes mediating resistance to antibiotics, such as ampicillin, chloramphenicol, kanamycin/neomycin, and tetracycline (Glick and Pasternak, 1998; Russell, 1998; Olmedo-Alvarez, 1999).
Polymerase Chain Reaction The polymerase chain reaction, usually referred to as PCR, is an extremely powerful procedure that allows the ampliﬁcation of a selected DNA sequence in vitro. A three-step cycling process achieves this ampliﬁcation, which can be more than a million-fold. A typical PCR process entails a number of cycles for amplifying a speciﬁc DNA sequence; each cycle has three successive steps: 1. Denaturation — The ﬁrst step in the PCR ampliﬁcation system is the thermal denaturation of the DNA sample by raising the temperature within a reaction tube to 95∞C. 2. Annealing — In the next step, the denatured DNA is annealed to primers by incubating at 35 to 60∞C. The ideal annealing temperature depends on the base composition of the primers. © 2003 CRC Press LLC
3. Elongation polymerization or replication — Taq DNA polymerase is used to replicate the DNA segment between the sites complementary to the oligonucleotide primers. The primers provide the 3¢hydroxyl ends required for covalent extension, and the denatured DNA provides the required template function. Polymerization reaction is usually carried out at 70 to 72∞C (Glick and Pasternak, 1998; Snustad and Simmons, 2000). The products of the ﬁrst cycle of replication are then denatured, annealed to oligonucleotide primers, and replicated again with Taq DNA polymerase. The procedure is repeated many times until the desired level of ampliﬁcation is achieved. The ampliﬁcation occurs in an exponential manner (Glick and Pasternak, 1998; Snustad and Simmons, 2000). All steps and temperature changes required during PCR cycles are usually carried out in an automated programmable block heater known as a PCR machine or thermal cycle (Snustad and Simmons, 2000). Polymerase chain reaction technologies provide shortcuts for many cloning and sequencing applications. These procedures permit scientists to obtain deﬁnitive structural data on genes and DNA sequences when very small amounts of DNA are available.
Metabolic Engineering The development of recombinant DNA technology has led to the emergence of the ﬁeld of metabolic engineering, the purposeful and directed modiﬁcation of intracellular metabolism and cellular properties. Metabolic engineering is generally deﬁned as the redirection of one or more enzymatic reactions to produce new compounds in an organism, to improve the production of existing compounds, or to mediate the degradation of compounds (Jacobsen and Khosla, 1998; Nielsen, 1998; Bailey, 1999; DellaPenna, 2001). The complete sequencing of genomes from several organisms has resulted in the increased availability of genes for metabolic engineering. The number of databases and computational tools to deal with this information has also increased. This development has stimulated, and will continue to stimulate, advances in metabolic engineering. Speciﬁc recent advances include the development of nuclear magnetic resonance (NMR)based methods for monitoring of intracellular metabolites and metabolic ﬂux and the application of metabolic control analysis and metabolic ﬂux analysis to a variety of systems. It has become possible to perform detailed analyses of cellular functions through both in vivo and in vitro measurements (Nielsen, 1998; Bailey, 1999; DellaPenna, 2001). NMR has been used for in vivo measurement of in vivo intracellular metabolites, and, based on
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experiments with 13C-enriched carbon sources followed by measurements of the fractional enrichment of 13C in cellular amino acids, it is possible to quantify intracellular distribution with high precision (Nielsen, 1998). Although progress in both pathway gene discovery and the ability to manipulate gene expression in transgenic plants has been most impressive during the past two decades, attempts to use these tools to engineer plant metabolism have met with limited success (DellaPenna, 2001; Lassner and Bedbrook, 2001). Though there are notable exceptions, most attempts at metabolic engineering have focused on modifying (positively or negatively) the expression of single genes affecting routes. In general, the ability to predict experimental outcomes has been much improved when targeting conversion or modiﬁcation of an existing compound to another, rather than attempting to increase ﬂux through a pathway. Modiﬁcation of metabolic storage products or secondary metabolic pathways, which often have relatively ﬂexible roles in plant biology, has also been generally more successful than manipulations of primary and intermediary metabolism (DellaPenna, 2001). Thus, exploiting the full biosynthetic capacity of food crops requires a thorough knowledge of the metabolic routes in plants and the regulatory processes involved in plant biochemistry (Galili et al., 2001). When novel branch-points in plant metabolic pathways are introduced by genetic engineering, the introduced enzyme or enzymes must possess a sufﬁciently high afﬁnity for their substrate(s) to compete with endogenous enzymes (Jacobsen and Khosla, 1998). In addition, the effects of novel carbohydrates, proteins, or lipids on plant physiology and development may limit the range and quantity of products that can be synthesized. The tissue and/or cellular compartment in which the compound is produced may also limit accumulation of the product; for example, the accumulation of a fructosyltransferase construct that had a vacuolar-targeting signal did not lead to an aberrant phenotype, whereas plants containing a construct with apoplastic-targeting signal sequence exhibited severe necrosis (Goddijn and Pen, 1995; Jacobsen and Khosla, 1998). Therefore, the potentially negative effects of engineered compounds on plant growth or development may be prevented by targeting gene product to appropriate cellular compartments; the same effect could also be obtained by using tissue-speciﬁc promoters (Goddijn and Pen, 1995; Lassner and Bedbrook, 2001). In metabolic engineering, a ﬁeld in continuous progress, hybrid approaches that involve both directed and evolutionary steps are likely to become increasingly important. For example, genetic engineering can ﬁrst be used to add a gene or set of genes to an organism; an evolutionary approach, such as selection in a continuous reactor, can then be used to achieve further improvements in factors such as growth rate, regulatory properties, or resistance to toxic metabolites (Jacobsen and Khosla, 1998). Also, the principle of breeding and in vitro evolution can be used to access natural product diversity rapidly and in simple laboratory microorganisms such as E. coli and in model plants such tobacco or Arabidopsis (Schmidt© 2003 CRC Press LLC
Dannert et al., 2000; Lassner and Bedbrook, 2001). Breeding new biosynthetic pathways may involve mixing and matching genes from different sources, even from unrelated metabolic routes, and at the same time creating new biosynthetic functions by random mutagenesis, recombination, and selection, all in the absence of detailed information on enzyme structure or catalytic mechanism. Because the gene functions introduced into a recombinant organism are not coupled to its survival, this approach can be freely used to explore a wide variety of possible product compounds (Schmidt-Dannert et al., 2000).
Protein Engineering It is possible with recombinant DNA and PCR technologies to isolate the gene for any protein that exists in nature, to express it in a speciﬁc host organism, and to produce a puriﬁed product that can be used commercially. However, the physicochemical and functional properties of these naturally occurring proteins are often not well suited for either industrial or food processing applications (Goodenough, 1995; Shewry, 1998). By using a set of techniques that speciﬁcally change amino acids encoded by a cloned gene, proteins can be created with properties that are better suited than their natural counterparts for therapeutic and industrial uses (Chen, 2001). Protein engineering provides an excellent opportunity to explore and manipulate the structures, composition, and functional properties of food proteins, with special emphasis on storage proteins from food crops (Goodenough, 1995; Shewry, 1998; Chen, 2001). The ﬁnal goal is to improve their quality for traditional end uses and to introduce new properties for novel food applications. In the enzyme case, some target properties that we may wish to improve or modify may include thermal tolerance or pH stability, or both, enabling the altered protein version to be used under conditions that would inactivate the native macromolecule (Glick and Pasternak, 1998; Chen, 2001). It may also be desirable to modify the reactivity of an enzyme in nonaqueous solvents so chemical reactions can be catalyzed under nonphysiological conditions. Another desired trait would be to change an enzyme so that a cofactor is no longer required for continuous industrial production processes in which the cofactor must be supplied on a regular basis (Goodenough, 1995; Glick and Pasternak, 1998; Chen, 2001). Finally, one can cite alteration of the allosteric regulation of an enzyme to diminish the impact of metabolite feedback inhibition and to increase product yield (Glick and Pasternak, 1998; Chen, 2001). Protein engineering is undergoing the most profound and exciting transformation in its history, promising unprecedented expansion in the scope and applications for modiﬁed or improved proteins and/or enzymes with desired properties. Two complementary strategies are currently available for achieving these goals: rational design and directed evolution or irrational design (Nixon and Firestine, 2000; Tobin et al., 2000). © 2003 CRC Press LLC
Protein Rational Design It is not a simple matter to produce a new protein with speciﬁed predetermined properties; however, it is quite feasible to modify the existing properties of known proteins. Theoretically, these changes can be carried out at either the protein or gene level (Glick and Pasternak, 1998); however, chemical modiﬁcations of proteins are generally harsh and nonspeciﬁc and, in the case of food proteins, have negative effects on nutritional quality (Glick and Pasternak, 1998; Shewry, 1998). They are also required repeatedly for each batch of protein, so it is preferable to manipulate the DNA sequence of a cloned gene to create an altered protein with novel properties. Unfortunately, it is not always possible to know in advance which individual amino acids or short sequence of those amino acids contribute to a particular characteristic. The process for generating amino acid coding changes at the DNA level is called directed mutagenesis (Chen, 2001). Determining which amino acids of a protein should be changed to attain a speciﬁc property is easier if the three-dimensional structure of the protein has been well characterized by x-ray crystallographic analysis or other analytical procedures (Goodenough, 1995; Chen, 2001). For most proteins, such detailed information is lacking. In these cases, individual amino acid substitutions or secondary structure engineering have generated enzymes or food proteins with desired properties. Despite these spectacular examples, however, numerous attempts at redesigning proteins have failed (Goodenough, 1995; Shewry et al., 2000; Chen, 2001). These failures might have resulted, to some extent, from an incomplete understanding of the underlying mechanisms or structure–function relationship bases required to enhance the desired enzyme or food functional properties and also because a signiﬁcant number were based on primary amino acid sequence homologies being the only criterion for amino acid replacements (Goodenough, 1995; Nixon and Firestine, 2000; Chen, 2001). Protein Irrational Design Protein irrational design is also called protein directed evolution and does not require information about protein structure related to function (Nixon and Firestine, 2000; Tobin et al., 2000; Chen, 2001). This technology accesses an important facet of natural evolution that was lacking in previous formats: the ability to recombine mutations from individual genes similar to natural sexual recombination. This approach employs a random process in which error-prone PCR is used to create a library of mutagenized genes. When coupled to selective pressure or to high-throughput screening, progeny sequences or mutants encoding desirable functions are identiﬁed. Desirable clones may be iteratively creating offspring that contain multiple beneﬁcial mutations, until the evolved sequence encoding the desired function is obtained (Nixon and Firestine, 2000; Tobin et al., 2000; Chen, 2001). One limitation of directed evolution is the prerequisite for having a sensitive and efﬁcient method for screening a large number of potential mutants (Chen, © 2003 CRC Press LLC
2001). Although either rational design or directed evolution can be very effective, a combination of both strategies will probably represent the most successful route to improve the properties and function of a protein (Nixon and Firestine, 2000).
Functional Genomics and DNA Microarrays The availability of complete sequence information for many different organisms is driving a revolution in the biological sciences. Unfortunately, the billions of bases of DNA sequence do not tell us what all the genes do, how cells work, how cells form an organism, what goes wrong in disease, or how cells respond to stimuli (Lockhart and Winzeler, 2000). This is where functional genomics comes into play (Kao, 1999; Lockhart and Winzeler, 2000). The goal of functional genomics is not simply to provide a catalog of all the genes and information about their functions, but also to understand how the components work together to comprise functioning cells and organisms (Lockhart and Winzeler, 2000). For the ﬁrst time, technologies that analyze thousands of genes in parallel are generating comprehensive, high-resolution, and quantitative information on the cellular states of living organisms (Lockhart and Winzeler, 2000; Ryu and Nam, 2000). Thus, the widespread and routine use of functional genomic tools that are based on DNA microarray technology promises to shed light on virtually all scientiﬁc arenas, from the fundamental issue of how cells grow to the medical challenge of understanding cancer and other human diseases; breeding plant crops, farm animals, and food-grade microorganisms in order to improve food quality and production; and the industrial goal of developing fermentation processes (Kao, 1999; Ryu and Nam, 2000; Lockhart and Winzeler, 2000). The development of microarray technology is intimately connected with the transition of molecular biology from its classical phase into its postgenomic era. DNA microarray technology promises not only to dramatically speed up the experimental work of molecular biologists but also to make possible an entirely new experimental approach in molecular biology (Kao, 1999; Blohm and Guiseppi-Eli, 2001). Instead of investigating the complexity of biological effects by analyzing single genes of putative importance one after the other, many or even all genes of an organism can now be tested at once: ﬁrst, to ﬁnd out which genes are involved in a biological event and, second, to analyze in detail their (inter)actions afterwards. Nucleic acid array works by hybridization of labeled RNA or DNA in solution with DNA molecules attached at speciﬁc locations on a surface. The hybridization of a sample to an array is, in effect, a highly parallel search by each molecule for a matching partner on an afﬁnity matrix, with the eventual pairings of molecules on the surface determined by the rules of molecular recognition (Lockhart and Winzeler, 2000). © 2003 CRC Press LLC
Traditionally, the arrays have consisted of DNA fragments, often with unknown sequence, spotted on a porous membrane such as nylon material. The arrayed DNA fragments often come from cDNA, genomic DNA, or plasmid libraries, and the hybridized material is often labeled with radioactivity (Lockhart and Winzeler, 2000). Recently, the use of glass as a substrate and ﬂuorescence for detection, together with the development of new technologies for synthesizing or depositing nucleic acids in glass slides at very high densities, have allowed the miniaturization of nucleic acid arrays with concomitant increases in experimental efﬁciency and information content (Blohm and Guiseppi-Eli, 2001). While making arrays with more than several hundred elements was, until recently, a signiﬁcant technical achievement, arrays with more than 250,000 different oligonucleotide probes or 10,000 different cDNAs per square centimeter can now be produced in signiﬁcant numbers (Lockhart and Winzeler, 2000; Blohm and Guiseppi-Eli, 2001). Although it is possible to synthesize or deposit DNA fragments of unknown sequence, the most common implementation is to design arrays based on speciﬁc sequence information, a process sometimes referred to as downloading the genome onto a chip (Kao, 1999; Lockhart and Winzeler, 2000; Ryu and Nam, 2000; Blohm and Guiseppi-Eli, 2001). The amounts of RNA or cDNA hybridized to each probe on a microarray can be measured by scanning the blot with an imaging system that measures the amount of radioactivity or ﬂuorescence and analyzing the results with a computer that compares the signals with those produced by known control probes and RNAs or cDNAs (Kao, 1999; Lockhart and Winzeler, 2000; Blohm and Guiseppi-Eli, 2001).
Proteomics It is becoming increasingly clear that the behavior of gene products is difﬁcult or impossible to predict from gene sequence. Even if a gene is transcribed, its expression may be regulated at the level of translation, and protein products are subject to further control by posttranslational modiﬁcations, varying half-lives, and compartmentation in protein complexes (Dove, 1999). Proteomics, more appropriately called functional proteomics, is a ﬁeld that promises to bridge the gap between genome sequence and cellular behavior (Blackstock and Weir, 1999). Proteomics aims to study the dynamic protein products of the genome and their interactions (Blackstock and Weir, 1999). Its rapid emergence in biotechnology is being driven by the development, integration, and automation of large-scale analytical tools, such as two-dimensional gel electrophoresis (2D-PAGE) and tandem mass spectrometry, which have simpliﬁed protein analysis and characterization, and the emergence of sophisticated bioinformatics approaches for simplifying complex and interrelated data (Blackstock and Weir, 1999; Patterson, 2000). © 2003 CRC Press LLC
Proteomics has its roots in traditional biochemical techniques of protein characterization. Over 25 years ago, high-resolution 2D-PAGE was developed, and its basic principles have remained unchanged (O’Farrell, 1975; Blackstock and Weir, 1999). Essentially, the proteins in a cell or tissue extract are separated, ﬁrst in one dimension on the basis of charge, and then in a second dimension on the basis of molecular size, resulting in deﬁned spots (O’Farrell, 1975). Early 2D-PAGE experiments focused on comparing the resulting pattern of spots for different tissues, or for bacteria cultured under different growth conditions. Because it was difﬁcult or impossible to determine which proteins the individual spots represented, the usefulness of the system was strictly limited (O’Farrell, 1975). At present, the dominant approach to proteomics combines 2D-PAGE, which separates, maps, and quantiﬁes proteins, with mass spectrometry (MS)-based sequencing techniques, identifying both the amino acid sequences of proteins and their posttranslational appendages (Patterson, 2000). This approach is allied with database search algorithms to sequence and characterize individual proteins. Proteomics relies greatly on genomic databases to facilitate protein identiﬁcation and in so doing indicates which genes within the database are important in deﬁned circumstances. Consequently, the two ﬁelds do not merely focus on complementary levels within the cell but have a synergistic relationship (Dove, 1999; Patterson, 2000). The use of standardized procedures, robotics, and sophisticated bioinformatics led to systematic evolution of industrialized approaches with an unprecedented level of sensitivity and selectivity (Patterson, 2000). Using the 2D-PAGE/MS approach, the leading proteomics laboratories can separate hundreds of proteins within individual gels at low femtomolar sensitivities and characterize amino acid sequences and posttranslational modiﬁcations of proteins at a rate of up to 1000 a week (Patterson, 2000). This approach to proteomics has limitations. There remain difﬁculties, for instance, in subjecting hydrophobic and very low molecular weight protein to gel-based analysis. Nevertheless, it represents by far the most powerful and comprehensive means yet devised to screen protein components of biological samples and to compare, for example, healthy and diseased specimens (Blackstock and Weir, 1999; Dove, 1999). On the other hand, two different approaches are currently being pursued to separate proteins in chips (Dove, 1999; Lueking et al., 1999). In one technique, chips are prepared with different surface chemistries, much like traditional protein chromatography columns, but in a ﬂat array. Proteins can then be separated on the basis of their chemical afﬁnities and identiﬁed with MS (Dove, 1999; Patterson, 2000). In the second strategy, which is more similar to the afﬁnity array used in DNA chips, scientists attach antibodies or nucleic acid sequences to the chip surface and then allow antigens or transcription factors to bind to the surface, where they can be detected and identiﬁed (Dove, 1999; Lueking et al., 1999). © 2003 CRC Press LLC
Applications of Molecular Biotechnology
Plant Biotechnology for Food Production Plant genetic engineering may be deﬁned as the manipulation of plant development, structure, or composition by insertion of speciﬁc DNA sequences (Halford and Shewry, 2000). These sequences may be derived from the same species or even variety of plant. This may be done with the aim of altering the levels or patterns of expression of speciﬁc endogenous genes — in other words, to make them more or less active or to alter when and where in the plant they are switched on or off (Halford and Shewry, 2000). Alternatively, the aim may be to change the biological (e.g., regulatory or catalytic) properties of the proteins they encode. However, in many cases, genes are derived from other species, which may be plants, animals, or microbes, and the objective is to introduce novel biological properties or activities (Halford and Shewry, 2000). To date, numerous transgenic plants have been generated, including many crop and forest species. In the near future, plant biotechnology will have an enormous impact on conventional breeding programs, because it can significantly decrease the 10 to 15 years that it currently takes to develop a new variety using traditional techniques; further, it will also be used to create plants with novel traits (Halford and Shewry, 2000; Lassner and Bedbrook, 2001; Ryals, 2001). From a biotechnology point of view, there exist two areas in which plant genetic engineering is being applied as a means of enhancing the rational exploitation of plants. In the ﬁrst, the addition of genes often improves the agronomic performance or quality of traditional crops (Ryals, 2001). Thus, tremendous progress has occurred in the genetic engineering of crop plants for disease-, pest-, stress-, and herbicide-resistance traits, as well as for traits that enhance shelf life and processing characteristics of harvested plant materials (Grierson, 1998; Dunwell, 1999). Some genetically determined agricultural traits, such as recombinant resistance approaches or delay of fruit senescence and ripening, are attractive because they involve only minor changes to the plant (i.e., the introduction of a single heterologous gene). Consequently, the characteristics of commercially successful cultivars are likely to remain unmodiﬁed due to genetic improvement (Grierson, 1998; Ryals, 2001). In the second major area, genetic manipulation is being exploited with the objective of improving the quality of plant products consumed by human beings, and that will affect their nutrition and health. This has already yielded more nutritious grains with modiﬁed oil, protein, carbohydrate content, and composition; process-improved ﬂours; designer oilseeds with tailored end-uses; and plants producing high-value biomolecules, such as milk and pharmaceutical proteins, industrial enzymes, vitamins, pigments, nutraceuticals, and edible vaccines for the food and pharmaceutical industries © 2003 CRC Press LLC
(DellaPenna, 1999; Dunwell, 1999; Hirschberg, 1999; Kishore and Shewmaker, 1999; Mazur et al., 1999; Fischer and Emans, 2000; Giddings et al., 2000; Ye et al., 2000; Rascón-Cruz et al., 2001). Among the approaches used to produce such transgenic plants with new quality attributes, which represent the second generation of genetically modiﬁed plants, are (1) manipulation of plant endogenous metabolic pathways in order to favor the accumulation of important and desired products, and (2) generation of transgenic plants that can act as living bioreactors or biofactories (Goddijn and Pen, 1995; DellaPenna, 1999; Dunwell, 1999; Hirschberg, 1999; Kishore and Shewmaker, 1999; Mazur et al., 1999; Fischer and Emans, 2000; Giddings et al., 2000).
Improvement of Plant Nutritional and Functional Quality Human beings require a diverse, well-balanced diet containing a complex mixture of both macronutrients and micronutrients in order to maintain optimal health. Macronutrients, carbohydrates, lipids, and proteins make up the bulk of foodstuff and are used primarily as an energy source (GuzmánMaldonado and Paredes-López, 1999). Modifying the nutritional composition of plant foods is an urgent worldwide health issue, as basic nutritional needs for much of the world population are still unmet (DellaPenna, 1999; Guzmán-Maldonado and Paredes-López, 1999; Kishore and Shewmaker, 1999; Mazur et al., 1999). Large numbers of people in developing countries exist on diets composed mainly of a few staple foods which usually present poor food quality for some macronutrients and many essential micronutrients (DellaPenna, 1999; Guzmán-Maldonado and Paredes-López, 1999; Kishore and Shewmaker, 1999; Mazur et al., 1999). Seeds and tubers are the most important plant organs harvested by humankind, in terms of their total yield and their use for food, feed, and industrial raw material. This exploitation is possible because they contain rich reserves of storage compounds such as starch, proteins, and lipids. Genetic engineering provides an opportunity to explore and manipulate the structure, nutritional composition, and functional properties of those macromolecules to improve their food quality for traditional end uses and to introduce new properties for novel applications (Goodenough, 1995; Shewry, 1998; Rooke et al., 1999; Chakraborty et al., 2000; Osuna-Castro et al., 2000; Shewry et al., 2000).
Plant Proteins Because animal proteins are more expensive, people in developing countries virtually depend on seed proteins alone for their entire protein requirement. But, unlike animal proteins such as casein and egg albumin, which are nutritionally more balanced in terms of essential amino acids, plant proteins are generally deﬁcient in some essential amino acids (Guzmán-Maldonado © 2003 CRC Press LLC
and Paredes-López, 1999). Animals, including humans, are incapable of synthesizing essential amino acids; these must be supplied from outside by food. Seed storage proteins have been classiﬁed into four groups on the basis of their solubility in water (albumins), dilute saline (globulins), alcohol/water mixtures (prolamins), and diluted acid or alkali (glutelins) (Shewry and Casey, 1999). Albumin and globulin storage proteins are also classiﬁed according to their sedimentation coefﬁcients into 2S albumin and 7–8S and 11–12S globulin, also known as vicilins and legumins, respectively (Shewry and Casey, 1999). Seeds form the major source of dietary proteins for humans and their livestock. The seed storage globulins of legumes are low in the sulfur-containing amino acids cysteine and methionine, such as in soybeans and common beans, whereas cereal prolamins are normally deﬁcient in lysine and tryptophan, such as in maize and rice (Shewry, 1998; Tabe and Higgins, 1998; Yamauchi and Minamikawa, 1998; Rascón-Cruz et al., 2001). Consequently, diets based on a single cereal or legume species result in amino acid deﬁciencies (Guzmán-Maldonado and Paredes-López, 1999). Storage proteins also confer functional properties which allow seeds to be processed in many food systems (Shewry, 1998). These proteins are particularly important in soybeans for texturing and gel (tofu) formation, and in the production of bread, pasta, and other products from wheat (Shewry, 1998). Thus, functional characteristics are largely determined by the amount and properties of the storage proteins. Processing quality poses some problems, as it may be determined by the biophysical properties of the seed proteins (rather than their composition) and their interactions with other seed components, such as starch, lipids, and nonstarch polysaccharides (cell wall components) (Miﬂin et al., 1999). In this case, it may be necessary to deﬁne quality criteria in molecular and structural terms before attempting to make improvements (Shewry, 1998; Shewry et al., 2000). In addition, functional attributes can only be assessed after grain has been broken down into its component parts. Small-scale grain processing facilities suitable for gram quantities of grain are necessary to isolate grain fractions similar to those produced in commercial scale milling and processing plants. These small-scale facilities eliminate the time-consuming seed multiplication step necessary for large-scale assays (Mazur et al., 1999; Rooke et al., 1999). Tiered functional assays, in which high-throughput functional, nutritional, and sensory evaluations are carried out, also increase the efﬁciency of functionality assessments (Mazur et al., 1999; Rooke et al., 1999). Due to the great importance of seed storage proteins, they are one of the major targets for creating food crops tailored to provide better nutrition for humans and improved food functional properties (Tabe and Higgins, 1998; Yamauchi and Minamikawa; 1998). Understanding the structural features of seed reserve proteins is important to provide a basis for proposed engineering mutations at suitable sites of these proteins, with the ultimate goal of enhancing nutritional and/or food processing quality, without affecting their folding/assembly, transport, deposition, and packing into protein bodies of plant seeds (Shewry, 1998; Shewry et al., 2000). © 2003 CRC Press LLC
Nutritional Quality The beneﬁts of food crops that produce seed proteins with improved amino acid composition are numerous. One way to reach this goal is to increase the amount of essential amino acids present in seed material used either for human consumption or for livestock feed (Tabe and Higgins, 1998). Currently, when corn is used as animal feed, it must be supplemented with soybean meal or puriﬁed lysine, or both (Glick and Pasternak, 1998). The development of high-lysine crops such as corn or soybeans is a good approach to replace the use of expensive lysine supplements. Lysine is a nutritionally important essential amino acid; its level in plants is largely regulated by the rate of synthesis and catabolism (Galili et al., 2001). Lysine is a member of the aspartate family of amino acids and is produced in bacteria by a branched pathway that also produces threonine, methionine, and isoleucine (Galili et al., 2001). The ﬁrst step in the conversion of aspartic acid to lysine is phosphorylation of aspartic acid by aspartokinase (AK) to produce b-aspartyl phosphate. The condensation of aspartic b-semialdehyde with pyruvic acid to form 2,3-dihydrodipicolinic acid, which is catalyzed by dihydrodipicolinic acid synthase (DHDPS), is the ﬁrst reaction in the pathway that is committed to lysine biosynthesis. AK and DHDPS are the two key enzymes in the lysine biosynthesis pathway, which are both feed-inhibited by lysine (Eggeling et al., 1998; Tabe and Higgins, 1998; Yamauchi and Minamikawa; 1998; Mazur et al., 1999; Galili et al., 2001). Falco et al. (1995) isolated bacterial genes encoding highly insensitive forms of AK and DHSPS from E. coli and Corynebacterium, respectively. A deregulated form of the plant DHSPS was created by site-speciﬁc mutagenesis (Karchi et al., 1993; Mazur et al., 1999). Expression of these genes in tobacco leaves produces high concentrations of free lysine, but no accumulation was observed in tobacco seed with the use of either constitutive or seed-speciﬁc promoters (Karchi et al., 1993). It was discovered that the failure to augment lysine concentrations in the seeds was due to the presence of an active catabolic pathway. However, in the seed-speciﬁc expression of a feedback-insensitive AK alone resulted in a 17-fold increase in free threonine and a 3-fold increase in free methionine in the seed of transgenic tobacco (Karchi et al., 1993; Tabe and Higgins, 1998). In soybean and canola seeds, lysine accumulated sufﬁciently to more than double the total seed lysine content (Table 1.1) (Falco et al., 1995; Mazur et al., 1999). In corn, expression with an endosperm-speciﬁc promoter did not lead to lysine accumulation, whereas expression with an embryo-speciﬁc promoter gave high levels of lysine sufﬁcient to raise overall lysine concentrations in seed 50 to 100%, with minor accumulation of catabolic products (Table 1.1) (Mazur et al., 1999). On the other hand, a number of studies have focused on increasing the sulfur (S)-amino acid contents in legume and rapeseed crops and lysine and tryptophan contents in cereal crops (Shewry, 1998). For example, a gene for b-phaseolin, a 7S globulin from Phaseolus vulgaris, was modiﬁed by addition © 2003 CRC Press LLC
TABLE 1.1 Advances in Improvement of Nutritional and Functional Quality of Seed Storage Proteins Improvement Nutritional Protein Engineered highmethionine bean phaseolin Bacterialinsensitive AK and DHDPS Methioninerich 2S albumin of Brazil nut Target Seed Crop Tobacco Results Degraded due to misfolding Ref. Hoffman et al., 1988
Soybean, canola, and maize Soybean and canola
Methioninerich sunﬂower 2S albumin Essential amino-acidrich amaranth 2S albumin Amarantin with elevated levels of essential amino acids Antisense cruciferin
Increase in total seed lysine level Spectacular increase in total methionine content; allergenic in humans Increase of methionine level Improved nutritional quality Improved nutritional quality
Falco et al., 1995; Mazur et al., 1999 Altenbach et al., 1992; Nordlee et al., 1996
Molvig et al., 1997 Chakraborty et al., 2000
Rascón-Cruz et al., 2001
Modiﬁed 7S/ 11S globulin ratios by cosuppression technology HMW subunit of wheat glutenin
Increase in sulfurcontaining amino acids and lysine levels Improved emulsifying, water- and fat-binding, and gel properties Massive increase in dough elasticity
Kohno-Murase et al., 1995
Mazur et al., 1999; Kinney et al., 2001
Rooke et al., 1999
Abbreviations: AK, aspartokinase; DHDPS, dihydrodipicolinic acid synthase; HMW, high molecular weight.
© 2003 CRC Press LLC
of a 45-bp nucleotide sequence encoding a methionine-rich region from a maize 15-kDa zein, a seed storage protein (Hoffman et al., 1988). The added peptide was predicted to form an a-helix structure and was inserted into an a-helical region of phaseolin. The modiﬁcation would increase the number of methionine residues on the protein from three to nine. The modiﬁed globulin gene was expressed, under the control of b-phaseolin speciﬁc-seed promoter at the same level as the wild-type gene in the seeds of tobacco, as measured by mRNA abundance. However, the engineered, high-methionine phaseolin accumulated to a much lower concentration than the unmodiﬁed protein. It was concluded that the high-methionine globulin was unstable in the developing seed (Table 1.1). The three-dimensional structure of phaseolin indicated that the introduction of methionine residues caused misfolding of the modiﬁed phaseolin. Spectacular success was achieved with the methionine-rich 2S storage albumin of Brazil nut, which contains 26% sulfur-containing amino acids. To elevate the methionine content of the rapeseed crop, the cDNA of the 2S albumin from Brazil nut seed was ligated to the promoter region of the phaseolin gene, and this chimeric gene was introduced into canola plants; an increase of up 30% in total seed methionine was obtained (Table 1.1) (Altenbach et al., 1992). When the promoter region of the legumin gene from ﬁeld bean directed synthesis of the Brazil nut albumin in Vicia narbonensis (narbon beans), the content of total protein was raised 6% (Saalbach et al., 1995). This was sufﬁcient to give a three-fold increase in total seed methionine. In addition, it was reported that high expression levels have also been achieved in seeds of transgenic soybean (Nordlee et al., 1996). Although Brazil nut albumin is unusually rich in cysteine and methionine, it has a major disadvantage for use in food or feed; it is highly allergenic in its puriﬁed form and in extracts of transgenic seeds and could therefore result in the development of allergenic reactions in humans or livestock. This has prevented the commercial development of transgenic crops with improved levels of sulfur-containing amino acid (Nordlee et al., 1996; Yamauchi and Minamikawa, 1998). A related 2S albumin from sunﬂower seeds contains 16% methionine residues and is not an allergen, making it an interesting target for improving nutritional traits in legume crops (Molvig et al., 1997). Its cDNA was ligated to the promoter of the vicilin gene from pea, and then transferred to lupin. Methionine content in seed proteins of transgenic lupin was approximately twofold higher than the non-transgenic lupin (Molvig et al., 1997). Results of a rat feeding trial indicated that the nutritional value of the seeds from the transgenic lupin was signiﬁcantly higher than that of the untransformed lupin. A different approach to improving the nutritional quality of rapeseed (Brassica napus) has been taken by Kohno-Murase et al. (1995). They transformed plants with an antisense gene for the 11S storage protein cruciferin. This did not signiﬁcantly affect the total protein content of the seeds, but the nutritionally poor cruciferins were decreased and the 2S albumins increased. The net result was increases of about 32, 10, and 8% in the levels of cysteine, lysine (which is © 2003 CRC Press LLC
lower in rapeseed [Brassica napus] than in soybean), and methionine, respectively (Table 1.1). Potato is the most important noncereal food crop and ranks fourth in terms of total global food production, besides being used as animal feed and as raw material for manufacture of starch, alcohol, and other food products. The essential amino acids that limit the nutritive value of potato protein are lysine, tyrosine, methionine, and cysteine. Chakraborty et al. (2000) used an interesting strategy for improving potato nutritional quality. The 2S albumin from amaranth seed (Amaranthus hypochondriacus), termed AmA1, is nonallergenic in nature and in its puriﬁed form and is rich in all essential amino acids, including those in which potato is deﬁcient; the amino acid composition corresponds well with World Health Organization standards for optimal human nutrition (FAO/WHO, 1991). These authors reported the tuber-speciﬁc expression of AmA1 in potato by using granule-bound starch synthase (GBSS). The expression of AmA1 in transgenic tubers resulted in a signiﬁcant increase in all essential amino acids. Unexpectedly, the transgenic plants also contained more total protein in tubers (35 to 45% increase) compared to control plants (with 1.1 g per 100 g tuber), which was in broad correlation with the increase of most essential amino acids. One highly expressing tuber population, labeled pSB8, showed a signiﬁcant 2.5- to 4-fold increase in lysine, methionine, cysteine, and tyrosine contents; interestingly, in other highly expressing tubers, their amounts appeared to be 4- to 8-fold (Table 1.1). These ﬁndings are consistent with immunoblot data, wherein the expression of AmA1 was found to be 5- to 10-fold higher in pSB8G tubers than that in pSB8 tubers. A striking increase in the growth and production of tubers was observed in transgenic populations. Hypersensitivity tests in mice did not evoke an IgE response, which negated the possibility that the protein is allergenic (Chakraborty et al., 2000). In addition, the literature has not revealed any allergenicity associated with amaranth grain or amaranth forage. In fact, amaranth grain and its food products, even eaten as fresh vegetables, have been exploited and consumed in Mexico and Latin American countries for many centuries without the development of allergies (Guzmán-Maldonado and Paredes-López, 1999; Segura-Nieto et al., 1999). The pseudocereal amaranth has been identiﬁed as a food crop comparable with most potential food and feed resources because of the exceptional nutritional–functional quality of its storage proteins (Guzmán-Maldonado and Paredes-López, 1999; Segura-Nieto et al., 1999). Speciﬁcally, its 11S globulin, called amarantin, one of the most important amaranth proteins, contains a good balance of essential amino acids that nearly meets the needs of human protein nutrition in reference to protein requirements established by the World Health Organization (FAO/WHO, 1991; Segura-Nieto et al., 1994, 1999; Barba de la Rosa et al., 1996). A previous report showed that amarantin cDNA may be synthesized in E. coli, exhibiting electrophoretic, immunochemical, and surface hydrophobicity properties similar to those of native amarantin from amaranth seed © 2003 CRC Press LLC
(Osuna-Castro et al., 2000). Thus, it would be advantageous to express novel seed proteins such as amarantin in maize grains with the objective of improving amino acid composition. Very recently, Rascón-Cruz et al. (2001) found that the amarantin cDNA with signal peptide and under control of the tissue-speciﬁc promoter from rice glutelin 1 (osGT1) was synthesized, processed correctly, and accumulated speciﬁcally in the seed endosperm of tropical maize. In addition, they achieved a remarkable change in respect to total protein content (44 to 75%) in transgenic maize seeds, ﬁnding a signiﬁcant increase in the limiting essential amino acids ranging from 23 to 83%, when compared with untransformed maize. Thus, the highest expression was achieved in the maize line, which showed a signiﬁcant 1.3- and 1.6-fold increase in lysine and tryptophan, respectively, which are deﬁcient in ordinary maize, and the third limiting amino acid, isoleucine, also increased around 2-fold (Table 1.1). In addition, the content of other essential amino acids in transgenic maize was improved. Amarantin has an elevated content of essential amino acids and, because maize constitutes an important component of the diet of people in many developing countries and in particular of Mexico, the ﬁnal target of this research work is to overexpress amarantin in order to more fully exploit its nutritional potential.
Functional Quality In soybeans, two classes of storage proteins, 7S and 11S globulins, predominate, each with useful functional characteristics. Transgenic lines in which one or the other of these classes was eliminated by cosuppression technology, and lines with altered 7S/11S ratios were then tested at increasing scale for emulsiﬁcation, water- and fat-binding properties, gel-forming ability, and other parameters relevant to processing, cooking, and food manufacture. From these assessments, speciﬁc transgenic soybeans have been selected for testing in milk and meat replacement products (Table 1.1) (Mazur et al., 1999; Kinney and Jung, 2001). The ability to make leavened bread from wheat ﬂour depends largely on the unusual properties of the gliadin and glutenin proteins, which together constitute gluten; these properties are not shared by the prolamins of barley and wheat, although they are related structurally to the gluten proteins (Shewry, 1995, 1998; Miﬂin et al., 1999). Gluten storage proteins form a continuous network in dough, conferring the viscoelastic properties necessary to entrap carbon dioxide released during the prooﬁng of leavened bread. The protein network also provides the cohesiveness required for other foods; for example, high elasticity is required for making noodles and pasta, while more extensible doughs are needed for making cakes and biscuits (cookies). The quality of wheat is determined by genetic and environmental factors, with poor quality generally resulting from low gluten elasticity (Shewry, 1998; Shewry et al., 2000). © 2003 CRC Press LLC
Wheat gluten is a complex mixture of proteins with over 50 individual components, but one group of proteins, the high-molecular-weight (HMW) subunits of glutenin, which form HMW (above 1¥106-Da) polymers stabilized by inter-chain disulﬁde bonds, are particularly important. A range of studies provide strong evidence that the glutenin polymers are responsible for gluten elasticity, and the variation in quality is associated with differences in the number (three, four, or ﬁve), amounts (about 6 to 12% of the total protein), and properties of the glutenin polymers and their constituent subunits (Shewry, 1998; Shewry et al., 2000). Shewry’s research group has shown that increasing the number of expressed subunits in a wheat line with a poor-quality background from two to three and four results in stepwise increases in dough elasticity, mirroring the effects of manipulating gene dosage by conventional breeding; however, it has been possible to go beyond the gene dosage obtainable by classical breeding. This results in gluten that contains over 20% of HMW subunits and a massive increase in dough elasticity (Table 1.1) (Rooke et al., 1999). The resulting ﬂour is actually too strong to be used for bread making, but may be valuable for blending to fortify poor-quality wheat. This is, however, a coarse approach, and future improvements may require a ﬁner adjustment of gluten structure. This could be achieved by making speciﬁc mutations — for example, changing the pattern of disulﬁde bonds. Similarly, the amounts and properties of other types of gluten protein could also be manipulated. Protein engineering is a powerful tool for studying the structures and biophysical properties of individual gluten proteins, and it is even possible to determine the functional properties of heterologously expressed proteins by incorporation into dough using a small-scale mixograph (Rooke et al., 1999; Shewry et al., 2000). However, such incorporation experiments are difﬁcult to perform and it is not possible to ensure that incorporated protein will form the same molecular interactions as it would if expressed in the developing wheat grain. The developing of wheat transformation systems, therefore, provides an opportunity to determine the roles of individuals proteins, engineered or not, in wheat gluten structure and functionality. This will eventually allow for the production of a variety of wheats in which the structure and properties of gluten are ﬁne-tuned for various end uses, including bread, pasta and noodles, and other baked products (Shewry, 1998; Shewry et al., 2000).
Lipids In most commercial seed oils, more than 95% of the mass can be attributed to only a few fatty acids — speciﬁcally, lauric, mirystic, palmitic, stearic, oleic, linoleic, and linolenic acids (Murphy, 1996, 1999; MacKenzie, 1999). Plants produce a wide diversity of fatty acids, the majority of which accumulate in the seed as triacylglycerols. Vegetable oils generally are preferred to oils and fats from other sources because of their higher content
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of mono- and polyunsaturated fatty acids (Figure 1.1). Unsaturated fatty acids are healthier than saturated fatty acids, and the monounsaturated form, oleic acid, is also more stable in frying and cooking applications than are the polyunsaturated forms, linoleic and linolenic (MacKenzie, 1999; Napier et al., 1999). It is now possible to engineer “designer” oilseeds tailored to speciﬁc end uses. Plant lipids have different end uses, from industrial applications such as lubricants and detergents to food and nutrition (Murphy, 1996, 1999). They also have important roles as nutraceuticals and pharmaceuticals. The global market for plant-derived oils is immense; consequently, the major biotechnology companies have driven much of the research.
Saturated Fatty Acids Laurate, which is used in confectionery, is normally obtained from either coconut or palm oil. Although both plants yield relatively high levels of the fatty acid, they are limited in their agricultural utility. Research work carried out at Calgene Company has demonstrated the feasibility of engineering canola to produce lauric acid by introducing the gene encoding a lauroylspeciﬁc acyl-carrier protein (ACP), thioesterase, from the California bay plant. This enzyme causes premature chain termination of the growing fatty acid, resulting in the accumulation of lauric acid, a 12-carbon saturated fatty acid, rather than normal C18 oils (Figure 1.1). Canola normally contains less than 0.1% lauric acid; however, with the latter approach, Calgene has produced a number of canola lines that accumulate around 40% laurate (Voelker et al., 1992). The novel fatty acids are recovered from transgenic canola by standard processing methods. These oils, trivially named canola laurate, are now marketed as a partially hydrogenated vegetable oil for use in confectionery under the trade name Laurical. This work has also demonstrated the feasibility of producing large amounts of transgene-modiﬁed plant oils to supplement or replace ﬂuctuating natural sources (Murphy, 1996, 1999; MacKenzie, 1999). Saturated fatty acids are relatively uncommon in most plant storage lipids because of the presence of highly active desaturases in developing seeds. The composition of canola oil has been modiﬁed by expressing an antisense stearoyl-ACP desaturase cDNA, under the control of a napin or ACP promoter, in seeds of Brassica rapa and B. napus. The activity and amount of stearoyl-ACP desaturase were greatly reduced, resulting in a marked increase in stearic acid from less than 2% to as much as 40%; oleic acid levels also decreased (Knutzon et al., 1992). In other experiments, the cloning of the genes encoding each of the soybean fatty acid desaturases enabled the cosuppression of the seed-speciﬁc desaturase, causing seed oleic acid levels to rise from 25% in nontransformed lines to 85% in transgenic soybean (Mazur et al., 1999). The cosuppressed, seedspeciﬁc, high-oleic-acid, transgenic soybean plants showed excellent agronomic properties. © 2003 CRC Press LLC
C14-C18 Monosaturates Malolyl-CoA ∆9-C14:1 ∆6-C16:1 ∆6-C18:1 ∆9-C16:1
Fatty Acid Synthase (FAS)
∆9,12- C18:2 ∆15
∆9,12,15- C18:3 -Linolenic Acid
ACP-DES ACP-DES ∆6 ACP-DES ∆6,9,12 -C18:3 γ-Linolenic Acid Elongation ∆8,11,14-C20:3 Di-Homo-γ-Linolenic Acid ∆5
∆5,8,11,14 -C20:4 Arachidonic Acid Precursors of Eicosanoids
Biomedical and Nutraceutical Uses
Prostaglandins Leukotrienes Thromboxanes
FIGURE 1.1 Generalized scheme for the biosynthesis and metabolism of lipids. Fatty acid precursors, such as pyruvate and malate, are imported into plastid for conversion to acetyl-CoA, then the FAS complex converts Acetyl-CoA and Malolyl-CoA units into C8–C18 saturated acyl-ACPs. Depending on the plant species, C14–C18 saturates may be desaturated by a variety of ACP–DES leading to synthesis of PUFAs. The arachidonic PUFA is subject to further transformations to form eicosanoid precursors that present important medical and nutraceutical applications. Abbreviations: ACP, acyl-carrier protein; ACP–DES, acyl-ACP desaturase; FAS, fatty acid synthase; PUFAs, polyunsaturated fatty acids. (Adapted from Murphy et al., 1999; Napier et al., 1999.)
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Unsaturated Fatty Acids It is well documented that the risk of coronary heart disease is positively correlated with elevated levels of high-density lipoprotein (LDL) cholesterol (Kishore and Shewmaker, 1999; MacKenzie, 1999). The C12, C14, and C16 saturated fatty acids are known to induce hypercholesterolemic effects and to increase the proportions of LDLs in the bloodstream, although they differ in their impact (MacKenzie, 1999). The intake of saturated fatty acids has been shown to be closely related to the clotting activity of platelets and their response to thrombin-induced aggregation (MacKenzie, 1999). Consequently, there has been, and continues to be, a parallel research effort to improve the quality of vegetable oils by reducing the total amount of saturated fatty acids (Kishore and Shewmaker, 1999; MacKenzie, 1999). Unsaturated fatty acids have recently attracted interest as targets for genetic manipulation (Figure 1.1) (Napier et al., 1999). Polyunsaturated fatty acids (PUFAs; long-chain fatty acids containing two or more double bonds introduced by speciﬁc desaturase enzymes) have entered the biomedical and nutraceutical areas as a result of the elucidation of their biological role in certain clinical conditions (Napier et al., 1999). Besides pharmaceutical applications, public perceptions of healthy foods and lifestyles have also brought them to the attention of the consumer. Speciﬁcally, interest is growing in 18- to 22-carbon PUFAs containing three or more double bonds, as these compounds have emerging therapeutic roles (MacKenzie, 1999; Napier et al., 1999). Current commercial production of PUFAs, largely from seed oils, marine ﬁsh, and certain mammals, is considered inadequate for the future PUFA market. However, the seeds of commonly cultivated agricultural plants contain a rather limited range of fatty acids that is truly representative of the diversity of unusual fatty acids accumulated predominantly as storage lipids by plants and fungi (Murphy, 1999). Some of these less common unsaturated fatty acids are highly valuable, although the organisms from which they originate are generally unsuitable for large-scale production and cultivation (Murphy, 1996, 1999). For example, linoleic and linolenic acid PUFAs cannot be synthesized de novo by animals; they are therefore classiﬁed as essential and must be ingested (MacKenzie, 1999; Napier et al., 1999). Furthermore, for metabolic purposes, the two families cannot be interconverted. Arachidonic acid is synthesized by the elongation of g-linolenic acid to a longer chain PUFA, a 20-carbon molecule, followed by desaturation at the D5 position (Figure 1.1). Both PUFAs are precursors of a group of short-lived regulatory molecules called the eicosanoids. The eicosanoids (prostaglandins, leukotrienes, thromboxanes) have many functions and are particularly important in reproductive function and the regulation of blood pressure (Horrobin, 1990). The ability, therefore, to produce crop plants accumulating fatty acids such as arachidonic acid and other PUFAs is an obvious biotechnological target (Figure 1.1) (Murphy, 1996, 1999; MacKenzie, 1999; Napier et al., 1999). © 2003 CRC Press LLC
Transgenic plants expressing genes that encode fatty acid desaturases have been recently generated. Thus, an animal (Caenorhabditis elegans) microsomal fatty acid desaturase was expressed in Arabidopsis, resulting in increased levels of a-linolenic acid (Spychalla et al., 1997). Moreover, these same transgenic plants desaturated exogenously applied 20-carbon PUFAs, such as arachidonic, indicating that the enzyme was a D15/D17 desaturase (e.g., w-3) with speciﬁcity for both 18- and 20-carbon fatty acids. On the other hand, a Mortierella alpina desaturase with an amino-terminal cytochrome b5-domain was shown to exhibit D5–fatty acid desaturase activity, converting exogenously supplied di-homo-g-linolenic acid (D8,11,14-C20:3); also, transgenic canola was obtained resulting in the expression of a number of unusual, D5desaturated 18-carbon PUFAs (Michaelson et al., 1998; Napier et al., 1999). g-Linolenic is a registered pharmaceutical with wide applications for antiviral and cancer therapies; unfortunately, it only accumulates in a limited number of species, including evening primrose, borage, and amaranth. In 1997, Sayanova et al. were the ﬁrst to isolate a cDNA from borage plant encoding the D6–fatty acid desaturase responsible for synthesis of this fatty acid and to introduce it into tobacco (used as a model crop). It resulted in the accumulation of g-linolenic at levels equivalent to or greater than those found in borage (Figure 1.1). The ﬁnal goal, the production of novel highvalue pharmaceutical oils and eicosanoid precursors, may be difﬁcult to achieve but will probably require transgenic oilseed producing both the D5 and D6 desaturases and other as yet undeﬁned enzymes (Napier et al., 1999).
Carbohydrates Starch is composed of two different glucan chains, amylose and amylopectin. These polymers have the same basic structure but differ in their length and degree of branching, which ultimately affects the physicochemical properties of both polysaccharides (López et al., 1994; Guzmán-Maldonado and Paredes-López, 1999). Amylose is an essentially linear polymer of glucosyl residues joined via a-1,4 glycosidic bonds, whereas amylopectin exists as a branched a-1,4; a-1,6 D-glucan polymer. The physicochemical properties of the a-1,4 glucans are based on the extent of branching and/or polymerization (López et al., 1994; Guzmán-Maldonado and Paredes-López, 1999). Starch is a very important staple in the diet of the world population and is widely used in the food and beverage industries to produce glucose and fructose syrups as sweetener and to confer functional properties for food processing (López et al., 1994; Schulman, 1999). Starch is primarily used as thickener, but also as binder, adhesive, gelling agent, ﬁlm former, and texturizer in many snacks. The relative amounts of amylose and amylopectin are what give polysaccharides their unique physical and chemical properties, which convey speciﬁc functionality and are of biotechnological importance (López et al., 1994; Guzmán-Maldonado and Paredes-López, 1999; Slattery et al., 2000). © 2003 CRC Press LLC
Therefore, it might be of value to produce polysaccharides with features that are intermediate between amylose and amylopectin, that are more highly branched, or that have a higher molecular weight (Visser and Jacobsen, 1993; Slattery et al., 2000). The basic pathways of amylose and amylopectin synthesis are well understood and genes readily cloned (Visser and Jacobsen, 1993; Schulman, 1999; Slattery et al., 2000). Thus, the potential exists to produce plant starches with a wider range of structures and properties. The targets are varied but include mutant wheat starches to mimic those from maize, phosphorylated starch (currently obtained from potato), and resistant starches for healthy diets, among others (Guzmán-Maldonado and Paredes-López, 1999; Schulman, 1999). Most starch is synthesized from sucrose, a route that involves four steps: initiation, elongation, branching, and granule formation. In maize and many other plant species, at least 13 enzymes have been identiﬁed in the starch biosynthetic pathway (Visser and Jacobsen, 1993; Schulman, 1999; Slattery et al., 2000). Of these, three enzymes are considered to be key in the synthesis of amylose and amylopectin: 1. ADP-glucose pyrophosphorylase (AGP) is involved in the initiation step and generates the glucosyl precursor ADP-glucose from glucose-1-phosphate. 2. Two distinct classes of starch synthase (involved in elongation and granule formation) are found within the plastids: those bound exclusively to the granule, known as granule-bound starch synthases (GBSSs), and others that can be found in the soluble phase or granule bound, known as starch synthases (SSs). 3. Branching enzyme (involved in branching and granule formation) is a transglycosylase involved in amylopectin synthesis, also known as starch synthase of amylopectin biosynthesis. Amylose has been shown to be the product of GBSSs. The waxy mutants, which lack GBSS, have only amylopectin but still possess soluble starch synthases, suggesting that different enzymes participate in amylose and amylopectin synthesis (Slattery et al., 2000).
Starch Quality High-amylose starches have numerous industrial applications including fried snack products to create crisp, evenly browned snacks; as thickeners, as they are strong gelling agents; and, owing to their rapid setting properties, in confectionery (López et al., 1994; Guzmán-Maldonado and Paredes-López, 1999; Schulman, 1999; Slattery et al., 2000). An added bonus of these types of starches is that they hamper the penetration of cooking oils, leading to a decrease in fat intake by the consumer (Guzmán-Maldonado © 2003 CRC Press LLC
and Paredes-López, 1999; Schulman, 1999; Slattery et al., 2000). Thus, highamylose starch is in great demand by the starch industry because of its unique functional properties. However, very few high-amylose crop varieties are commercially available. Recently, Schwall et al. (2000) achieved the goal of producing amylose-rich potatoes by simultaneously inhibiting two isoforms of starch branching enzymes (SBE A and SBE B) to below 1% of the wild-type activities through SBE A + B antisense inhibition; that is, to generate potato starch with very high amylose content, antisense SBE B lines were retransformed with an antisense SBE A construct. Starch and granule morphology and composition were noticeably altered. Normal HMW amylopectin was absent, which was also reﬂected in the strong decrease of short chains compared to wild-type starch, whereas the amylose content was increased to levels comparable to the highest commercially available maize starches, with an apparent amylose level of 90%. In normal potatoes, about 80% of the starch consists of amylopectin, while 20% is amylose. Because antisense inhibition of AGP in transgenic potatoes abolishes starch formation in tubers, an increase in the quantity of storage starch has been attempted by raising AGP activity. This possibility was investigated by transforming potatoes with a mutant E. coli AGP gene (glgC16); the bacterial strain accumulated 30% more glycogen than normal (Stark et al., 1992). The results showed a 30% increase in starch content. A different approach to increase starch concentration is the ectopic expression of inorganic pyrophosphatase (PP); thus, E. coli PP in the cytosol of potato produced some unexpected results. Both the rates of sucrose degradation and starch synthesis increased, and the transformed tubers accumulated 20 to 30% more starch than wildtype tubers (Geigenberger et al., 1998). It has previously been cited that in normal potatoes about 80% of the starch consists of amylopectin, while 20% is amylose. Thus, efforts targeted at practical alteration of starch quality, producing amylose-free crops, began with a mutagenesis program in potatoes and the successful creation of a low-amylose line, the result of a single point mutation in the GBSS gene responsible for amylose production. By means of genetic engineering, an amylose-free potato crop has also been produced, expressing an antisense RNA for the GBSS transcript. The pertinent transgenic amylopectin potato is largely free of amylose (Tramper, 2000). Plants as Bioreactors As a result of plant genetic engineering, compounds of commercial interest that were previously available only from exotic plant species, from other organisms, or in limited amounts can now be produced in domesticated crops (Goddijn and Pen, 1995). In fact, currently producing biomass by growing crop plants in the ﬁeld can, in general, compete with any other production system; the process is inexpensive and requires limited facilities to produce bulk quantities (Goddijn and Pen, 1995; Arakawa et al., 1999). © 2003 CRC Press LLC
Molecular biotechnology will enable broadening of the range of products and use of transgenic plants as a versatile renewable and low-cost source of novel high-value molecules (Goddijn and Pen, 1995; Arakawa et al., 1999; Dunwell 1999; Fischer et al., 1999; Fischer and Emans, 2000; Giddings et al., 2000). This area of novel commercial exploitation of plants is called biofarming or molecular farming and involves the crop-plant-based production of industrial or therapeutic biomolecules. In this application, the plant can be considered as a solar-powered bioreactor and an attractive alternative to conventional microbial or animal cell expression systems. Its requirements are simple and inexpensive: sunlight, mineral salts from the soil (or fertilizers), and water (Goddijn and Pen, 1995; Arakawa et al., 1999; Dunwell 1999; Fischer et al., 1999; Fischer and Emans, 2000; Giddings et al., 2000). Similarly, as traditional agriculture takes advantage of these characteristics in the largescale production of feed and foodstuff items, they can be equally exploited by plant molecular farming in the production of commercially important recombinant proteins and other biomolecules. In addition to economic beneﬁts that plants present when used as biofactories, other very important beneﬁts include reduced risk from human pathogen contamination such as mammalian viruses (human immunodeﬁciency virus and hepatitis B), bloodborne pathogens, oncogenes, and bacterial toxins (Arakawa et al., 1999; Fischer et al., 1999; Fischer and Emans, 2000; Giddings et al., 2000). The cultivation, harvesting, transport, storage, and processing of transgenic crops would also use an existing infrastructure and require relatively little capital investment, making their commercial production by molecular farming technology an exciting prospect. Vaccines The availability of recombinant biopharmaceuticals and the identiﬁcation of molecules involved in many devastating human diseases and economically important plant diseases have created demand for amounts of safe, inexpensive, recombinant proteins (Fischer et al., 1999). Large-scale production of these molecular medicines, such as recombinant antibodies and vaccines, has the potential to make new therapies and novel diagnostic tests widely available for detecting and combating these diseases (Fischer et al., 1999; Fischer and Emans, 2000). Vaccination has been one of the greatest advances in medical science, dramatically improving human life expectancy and quality (Fischer and Emans, 2000). It is the most cost-effective form of health care. In 1992, the World Health Organization (Geneva) and a consortium of philanthropic organizations presented a Children’s Vaccine Initiative. The focus of this project was to encourage the development of technology that would make vaccines available to developing countries, where they are needed most. Priority areas included lower cost vaccines that could easily be distributed in poor countries lacking refrigeration, healthcare infrastructure, and oral vaccines (Walmsley and Arntzen, 2000). © 2003 CRC Press LLC
Subunit vaccines consist of speciﬁc macromolecules that induce a protective immune response against a pathogen (Walmsley and Arntzen, 2000). Plants have now been modiﬁed on the basis of peptide epitopes of pathogens capable of invoking a protective immune response (vaccinogen) to produce vaccines against a variety of human and animal diseases (Giddings et al., 2000). Traditional vaccines utilize either killed or attenuated whole diseasecausing organisms. Plant-based vaccines allow for the use of vaccine-selected speciﬁc subunits, avoiding the risk of causing the disease, as is possible with whole organisms.
Plant Vaccines The ﬁrst demonstration of expression of a vaccinogen in plants ocurred in 1990 when Curtiss and Cardineau expressed the Streptococcus mutans surface protein antigen A (SpaA) in tobacco. After incorporation of the transgenic tobacco tissue into the diet of mice, a mucosal immune response was induced to the SpaA protein; the induced antibodies were demonstrated to be biologically active when they reacted with intact S. mutans. The most widespread of autoimmune diseases is diabetes mellitus, a condition affecting millions worldwide (Dunwell, 1999). Its treatment involves daily injections of insulin, a procedure designed to regulate the levels of glucose in the blood; if glucose levels are too high, then nonspeciﬁc protein glycosylation can occur, a process that often leads to nerve damage, blindness, and a range of other life-threatening conditions (Dunwell, 1999). It is known that oral administration of disease-speciﬁc autoantigens can prevent or delay onset of autoimmune disease symptoms. In an experiment to test transgenic potatoes that expressed either human insulin (the major autoantigen) alone or a hybrid protein in which insulin was linked to the C-terminus of the cholera toxin B subunit (LT-B), cholera toxin was used as a potent oral antigen and oral adjuvant that induces the production of antitoxin antibodies (Arakawa et al., 1998). Non-obese diabetic mice fed tuber tissue containing microgram amounts of the LT-B–insulin fusion protein showed a signiﬁcant reduction in pancreatic inﬂammation and a delay in the onset of diabetic symptoms. Reports have since followed of expression of a hepatitis antigen in tobacco and lettuce, a rabies antigen in tomato, and a cholera antigen in tobacco and potatoes (Table 1.2). Animal trials demonstrating antigenicity of plantderived vaccinogens include tobacco- and lettuce-derived hepatitis B surface antigen (HBsAg), a tobacco- and a potato-derived bacterial diarrhea antigen, a potato-derived Norwalk virus antigen, and an Arabidopsis-derived footand-mouth disease antigen (Table 1.2) (Walmsley and Arntzen, 2000). Chimeric plant viruses were proven effective as carrier proteins for vaccinogens in 1994, after rabbits raised an immune response against puriﬁed recombinant cowpea mosaic virus (CPMV) particles expressing epitopes derived from human rhinovirus 14 and human immunodeﬁciency virus-1 © 2003 CRC Press LLC
TABLE 1.2 Transgenic Plant Production of Vaccine- and Antibody-Like Biopharmaceuticals against Some Important Human Diseases Vaccine/Antibody Human immunodeﬁciency virus-1 (HIV-1) Colorectal cancer antigen E. coli heat-labile toxin B subunit Streptococcus mutans surface protein antigen A (SpaA); S. mutans adhesin Heat-labile E. coli toxin B subunit Norwalk virus capsid protein Insulin-cholera toxin B subunit fusion protein Hepatitis B surface antigen (HBsAg) Host Plant Cowpea Potential Medical Use AIDS Ref. Porta et al., 1994
Maize, tobacco Potato, tobacco
Verch et al., 1998 Tacket et al., 1998; Walmsley and Arntzen, 2000 Curtiss and Cardeneaus, 1990; Ma et al., 1994; 1995; 1998 Tacket et al., 1994; 1998; Walmsley and Arntzen, 2000 Mason et al., 1996 Arakawa et al., 1998
Dental caries; protection against oral streptococcal colonization E. coli diarrhea
Potato, tobacco Potato
Viral diarrhea Diabetes
Lettuce, lupin, tobacco
Herpes simplex virus 2 Malarial B-cell epitope Rabies virus epitopes
Soybean Tobacco Spinach
Vaginal herpes infection Malaria Rabies
Walmsley and Arntzen, 2000; Fischer and Emans, 2000; Giddings et al., 2000 Zeitlin et al., 1998 Turpen et al., 1995 Moldeska et al., 1998
Source: Adapted and modiﬁed from Fischer and Emans, 2000; Giddings et al., 2000.
(HIV-1); for both chimerae, virus particle yields were found to be in the range of 1 to 2 g/kg cowpea leaf tissue (Porta et al., 1994). Numerous reports have since veriﬁed plant viruses as effective alternative vaccinogen expression vectors (Table 1.2). Antibodies have been stimulated in mice after injection with plant virus-derived HIV-1 epitopes and rabies virus epitopes (Porta et al., 1994). Modelska et al. (1998) were the ﬁrst to detect a mucosal immune response after oral induction with a plant virus-derived vaccinogen. A recombinant alfalfa mosaic virus (AMV) was engineered to express two rabies virus epitopes. Mice were immunized intraperitoneally or orally by gastric intubation or by feeding on virus-infected spinach leaves. Interestingly, mice © 2003 CRC Press LLC
vaccinated through diet produced twice the level of anti-vaccinogen IgA than that detected in intubated animals. It appears that the additional protection against digestion afforded through delivery of the vaccinogen in plant cells (e.g., bioencapsulation within plant cell walls and membrane compartments) allows successful oral immunization. The produced levels of serum IgG and IgA proved capable of improving the clinical symptoms caused by intranasal infection with an attenuated rabies virus strain. The ﬁrst human clinical trials for a transgenic plant-derived antigen were approved by the U.S. Food and Drug Administration and performed in 1997. Transgenic potatoes constitutively expressing a synthetic bacterial diarrhea vaccinogen, the B subunit of E. coli heat labile toxin (LT-B), were orally delivered to human volunteers (Tacket et al., 1998). Thus, each participant received potato cubes from a random sample of non-transgenic control or transgenic tubers. Prior to and at multiple time points after ingestion of the potato, serum and fecal samples were taken and analyzed. A signiﬁcant rise in LT-B antibodies was displayed by 10 of the 11 test participants, whereas no LT-B-speciﬁc antibodies were detected in control volunteers. Interestingly, serum antibody levels induced by ingestion of the transgenic potatoes were comparable to those measured when participants were challenged with 106 virulent enterotoxigenic E. coli organisms (Tacket et al., 1998). Also, other human trials are currently in progress using orally-delivered, potato-derived HBsAg as booster for the commercial hepatitis B vaccine and potato-delivered Norwalk virus virus-like particles for a viral diarrhea vaccine (Thanavala et al., 1995; Tacket et al., 1998; Walmsley and Arntzen, 2000). As well as in direct therapeutic applications, plant-delivered HBsAg is also being tested for use in diagnostic systems. HBsAg has been expressed in tobacco, and the recombinant product was used successfully in the hemagglutination test routinely conducted by the Japanese Blood Center on blood samples for HBsAg-positive donors (Tsuda et al., 1998). The serological results were comparable to those achieved with the standard antigen from E. coli.
Antibodies The alternative to inducing the immune system to produce antibodies is to deliver them directly for use in passive immunization (Giddings et al., 2000). A key breakthrough in making molecular farming in plants a reality was the demonstration of functional antibody expression in tobacco leaves (Hiatt et al., 1989). The importance of this is underscored by the fact that monoclonal antibodies and recombinant antibodies are essential therapeutic and diagnostic tools used in medicine for human and animal health care, as well as the life science and biotechnology ﬁelds (Table 1.2) (Fischer et al., 1999). Advances in modern recombinant DNA technology and antibody engineering have made possible the production of novel polypeptides with desirable © 2003 CRC Press LLC
properties, smaller antibody fragments, or antibody–fusion proteins linked to enzymes, biological response modiﬁers, or toxins (Table 1.2) (Fischer and Emans, 2000). Antibodies or antibody fragments produced in plants are often referred to as plantibodies.
Plantibodies The production of a recombinant monoclonal antibody in plants was ﬁrst described by Hiatt et al. (1989). The antibody chosen for study, 6D4, was a mouse IgG1 antibody that recognizes a synthetic phosphonate ester and can catalyze the hydrolysis of certain carboxylic esters. For the creation of transgenic tobacco plants synthesizing the intact antibody, a multimer of two heavy-chain polypeptides and two light-chain polypeptides covalently linked by disulﬁde bonds, a two-step strategy was adopted. Thus, genes encoding either the heavy chain or the light chain were expressed in tobacco, followed by sexual crossing of individual plants expressing either a heavy or a light chain. From the highest overproducing F1 lines, the yield of 6D4 antibody was up to 10 g/kg of the total protein (Hiatt et al., 1989). This represents one of the highest reported levels for a recombinant produced in transgenic plants (Fischer and Emans, 2000; Giddings et al., 2000; Walmsley and Arntzen, 2000). Moreover, in F1 plants expressing the assembled 6D4 antibody, the protein is secreted into the intercellular spaces and accumulated, in cell suspension, at up to 20 mg/L (Hein et al., 1991). This compares favorably with the levels of monoclonal antibody secreted into the medium by cultured hybridoma cells. In 1994, Ma et al. reported the production of monoclonal antibody Guys 13 in transgenic tobacco; Guys 13 is a mouse IgG1 immunoglobulin that binds to the 185-kDa cell surface antigen of Streptococcus mutans, the main causative agent of dental caries in human beings. The 185-kDa antigen is streptococcal adhesin, which mediates initial attachment of the bacterium to the tooth surface. This monoclonal antibody was ﬁrst expressed in tobacco by sequentially crossing plants expressing its individual components. This permitted the production of high levels of whole recombinant Guys 13 (500 mg/g of leaf material) (Ma et al., 1995). Three years later, the same group showed that Guys 13 monoclonal plantibody afforded speciﬁc protection in human volunteers against oral streptococcal colonization for at least 4 months (Ma et al., 1998). Expression of an anticancer monoclonal antibody in plants using a tobacco mosaic virus (TMV) gene delivery system was reported by Verch et al. (1998). This well-studied antibody (CO17-1A) against a colorectal cancer antigen is an IgG1 molecule. Mature IgG1 antibody was formed; also, the authors report that trials are underway to test the tumor-suppressive activity of this anticancer plantibody (Verch et al., 1998). On the other hand, a pharmaceutical company plans to begin injecting cancer patients with doses of up to 250 mg © 2003 CRC Press LLC
of antibody-cancer drug puriﬁed from maize seeds. It is also cultivating transgenic soybean producing monoclonal antibodies against herpes simplex virus 2 (HSV-2). The plantibodies showed no detectable differences from the mammalian cell-culture-derived version in terms of their molecular weight, stability in human semen and cervical mucus over 24 hours, ability to diffuse in such mucus, and efﬁcacy in preventing vaginal HSV-2 infection in mice models (Zeitlin et al., 1998). Finally, the banana is an ideal fruit to contain edible vaccines and antibodies. Bananas are grown extensively throughout the developing world, are an inexpensive food widely consumed by infants or children, and, in contrast to potatoes, can be eaten uncooked. In the near future, banana-producing edible biopharmaceuticals such as vaccines and antibodies could be produced against a range of diseases, including polio, diphtheria, yellow fever, HIV, and certain types of viral diarrhea (Giddings et al., 2000). We predict that plants will be the premier expression system for diagnostic and therapeutic compounds. Plant expression systems have the potential to be as abundant tomorrow as prescription drugs are today. We foresee that molecular farming will provide a basket full of novel medicines for the diseases of the 21st century, just as plants were the source of medicine in Aztec, Mayan, Egyptian, and Greek times.
Milk Proteins Human milk has long been recognized as the best-balanced diet for infants, supplying high nutritional value in casein and whey proteins as well as providing a source of antimicrobial proteins such as lactoferrin and lysozyme (Arakawa et al., 1999). Casein protein (3 to 3.5 g/L human milk) makes up about 30 to 35% of the total milk protein (Arakawa et al., 1999). There are two subclasses of human casein: (1) b-casein, which is the most abundant form, representing about 70%; and (2) k-casein, which accounts for around 27% of the total casein in human milk. b-casein is a 25-kDa globular protein that exists in several isoforms depending on its phosphorylation state. Up to ﬁve phosphate groups can be attached to serine and threonine residues. The binding of calcium, magnesium, and phosphate ions facilitates formation of large casein aggregates referred to as micelles, giving milk its white color (Arakawa et al., 1999). In addition to its emulsiﬁcation properties, human b-casein has diverse biological effects such as enhancement of absorption of calcium and other divalent cations, an opiate agonist effect (bcasomorphins), immunostimulating and modulating effects, and antibacterial functions (Mitra and Zhang, 1994). It is generally accepted that human milk is superior to all other milk substitutes for growing infants; however, a large market exists for milk substitutes, as not all mothers are able to nurture their infants by breastfeeding due to adverse physical, health, psychological, or socioeconomic conditions (Arakawa et al., 1999). Infant formulae based on bovine milk or © 2003 CRC Press LLC
soybeans have been, until recently, traditional substitutes for mother’s milk (Chong et al., 1997; Glick and Pasternak, 1998). Although cow’s milk has been recognized as an excellent source of proteins, vitamins, and minerals, its consumption is often related to excessive weight gain and cow’s milk protein allergy and intolerance (CMPA/CMPI), which may affect the gastrointestinal tract, the respiratory tract, skin, and blood (Chong et al., 1997; Glick and Pasternak, 1998; Arakawa et al., 1999). CMPA is a disease of infancy and usually appears in the ﬁrst few months of life. Prenatal or early neonatal exposure to cow’s milk protein increases the risk, not only of adverse reactions to bovine milk proteins but also of development of allergies to other food, specially soybean and egg (Host et al., 1995). Soy-based infant formulae have been recommended as hypoallergenic alternatives for nonbreastfed infants; however, many infants with allergy to bovine milk are also allergic to soy proteins (Arakawa et al., 1999). Reconstitution of Human Milk Proteins in Food Plants Production of several human milk proteins in vegetables and fruits could provide a novel source of improved nutrition for children and malnourished human populations in economically emerging countries (Chong et al., 1997). The generation of individual transgenic food plants producing several milk proteins, in addition to vaccine protein antigens, can contribute to a costeffective and nutritionally superior vegetable-based diet for people in need of more complete nutrition and protection against diseases (Arakawa et al., 1998; Chong and Langridge, 2000; Giddings et al., 2000). Recently, Chong et al. (1997) succeeded in expressing b-casein, a major component of human milk proteins, in potato plants, becoming the ﬁrst reported expression of a human milk protein gene in food crops. A b-casein expression level of 0.01% of total soluble protein was detected in leaf and tuber tissue. The plant-synthesized b-casein appears to be a single peptide of approximately 24 kDa, around 1 to 1.5 kDa smaller than the human counterpart isoform, which is phosphorylated at two sites. The reason for the apparent reduction in molecular size is not clearly understood. Recently, human b-casein was introduced into tomato and its presence at the protein level was detected (Arakawa et al., 1999). Lactoferrin, one of the major whey proteins in human milk, is an ironbinding glycoprotein of around 80 kDa (Anderson et al., 1989; Chong and Langridge, 2000). Although little is known about the function of milk lactoferrin in vivo, functions ascribed to lactoferritin in vitro include promotion of cell growth, antimicrobial activity, and immune-modulating properties (Arakawa et al., 1999; Chong and Langridge, 2000). Antibacterial as well as antiviral activity of lactoferritin has been reported. Human milk lactoferrin contains a speciﬁc antimicrobial domain consisting of a loop of 18 amino acid residues. This region signiﬁcantly inhibits growth of E. coli, and, based on its iron-chelating properties, lactoferrin impedes bacterial iron utilization, causing bacteriostasis (Anderson et al., 1989; Chong and Langridge, 2000). © 2003 CRC Press LLC
Mitra and Zhang (1994) ﬁrst reported expression, in a constitutive manner, of human lactoferrin in tobacco cells. The antibacterial properties of transgenic callus extracts were tested against four different phytopathogenic bacterial strains: Xanthomonas campestris pv. phaseoli, Pseudomonas syringae pv. phaseolicola, Pseudomonas syringae pv. syringae, and Clavibacter ﬂaccumfaciens pv. ﬂaccumfaciens. In colony-forming unit reduction assays, transgenic calli containing recombinant lactoferrin exhibited substantially higher antibacterial activity than native lactoferrin. In 2000, Chong and Langridge also expressed the human milk lactoferrin gene, but in potato plants and under the control of both the auxin-inducible manopine synthase (mas) promoter and the CaMV35S tandem promoter. Auxin activation of the mas promoter increased lactoferrin expression levels in transformed tuber and leaf tissues to approximately 0.1% of total soluble plant proteins, which was signiﬁcantly greater than that driven by the CaMV35S constitutive promoter (around 0.01%). Antimicrobial activity, bacteriostatic and/or bacteriocidal, against different human pathogenic bacterial strains (E. coli Migula, Salmonella paratyphi, and Staphylococcus aureus) were detected in potato tuber tissues. Tuber extract containing more lactoferrin showed consistently stronger antimicrobial effects. This is the ﬁrst report of synthesis of biologically active human milk lactoferrin in edible crops. A cDNA encoding human a-lactalbumin has recently been introduced into tobacco plants to produce biologically functional human a-lactalbumin exhibiting an apparent molecular weight identical to human-derived a-lactalbumin (Takase and Hagiwara, 1998). When combined with galactosyltransferase, the tobacco-derived a-lactalbumin was fully active in the synthesis of lactose. Several recombinant milk proteins have been produced in the milk of different transgenic animal species (Glick and Pasternak, 1998). However, only a few human milk proteins such as human b-casein, lactoferrin, and alactalbumin have been synthesized in plants (Mitra and Zhang, 1994; Chong et al., 1997; Takase and Hagiwara, 1998; Arakawa et al., 1999; Chong and Langridge, 2000). Because human milk protein genes are available, reconstitution of human milk proteins, including the caseins (b and k) and whey proteins (a-lactalbumin, serum albumin, lactoferrin, lysozyme, and immunoglobulins), can now be achieved in transgenic food plants (Mitra and Zhang, 1994; Chong et al., 1997; Takase and Hagiwara, 1998; Arakawa et al., 1999; Chong and Langridge, 2000). Moreover, modiﬁcation of milk protein properties by protein engineering now provides the opportunity to further improve digestibility; to increase protection against microbial pathogens by production of antimicrobial peptides such as lactoferrin, isracidin, and casecidin; and to enhance the feeling of well-being during periods of stress by producing human a-casein-derived neurotropic peptides (casomorphins) (Arakawa et al., 1999; Chong and Langridge, 2000). The ultimate objective will be to reconstitute a panel of essential human milk proteins with the goal of enhancing the beneﬁcial properties of milk proteins used for both infant formulae and in dairy food products for © 2003 CRC Press LLC
simultaneous improvement in human nutrition and the prevention of autoimmune and infectious diseases, so that we can say with certainty that we will never outgrow our need for milk.
Micronutrients Essential micronutrients in the human diet include 17 minerals and 13 vitamins that are needed at minimum levels to alleviate nutritional disorders (DellaPenna, 1999; Grusak and DellaPenna, 1999; Guzmán-Maldonado and Paredes-López, 1999). Nonessential micronutrients encompass a vast group of unique organic phytochemicals that are not strictly required in the diet but when present at sufﬁcient levels are linked to the promotion of good health (Steinmetz and Potter, 1996; Bliss, 1999). On the other hand, before attempting to manipulate nutritional components in food crops, careful consideration must be given to the selection of target compounds, their efﬁcacy, and whether excessive dietary intake could have unintended negative health consequences (DellaPenna, 1999). For select mineral targets (iron, calcium, selenium, and iodine) and a limited number of vitamin targets (folate; vitamins E, B6, and A), the clinical and epidemiological evidence is clear that they play a signiﬁcant role in maintenance of optimal health and are limited in diets worldwide (DellaPenna, 1999).
Carotenoids Carotenoids comprise a group of natural pigments that are ubiquitous throughout nature. Over 600 different carotenoids with diverse chemical structures have been identiﬁed in bacteria, fungi, algae, and plants (Shewmaker et al., 1999; Mann et al., 2000). Their colors range from yellow to red, with variations of brown and purple; in addition, carotenoids as colorants take advantage of their good pH stability and their insensitivity to reducing agents such as ascorbic acid (Mann et al., 2000). Not to be outdone, humanity through the ages has learned to exploit the pleasing visual properties of carotenoid pigments by supplementing feedstocks and incorporating carotenoid pigments into cosmetics and foods (Delgado-Vargas et al., 2000; Jez and Noel, 2000). As precursors of vitamin A, they are fundamental components in our diet and play additional important roles in human health (Delgado-Vargas et al., 2000; Van den Berg et al., 2000; Ye et al., 2000). Because animals are unable to synthesize them de novo, they must obtain them by dietary means. Other outstanding industrial uses of carotenoids include pharmaceuticals and nutraceuticals. In fact, they are important nutraceutical compounds and natural lipophilic antioxidants, whose sale as food and feed supplements is estimated to be approximately U.S.$500 million, and the market is expanding (Albrecht et al., 2000). All this is due in part to the discovery that these natural products can play a role in the prevention of cancer and chronic disease © 2003 CRC Press LLC
(mainly because of their antioxidant properties) and, more recently, that they exhibit signiﬁcant tumor suppression activity as a result of speciﬁc interactions with cancer cells (Sandmann et al., 1999; Albrecht et al., 2000; Van den Berg et al., 2000). The commercial demand for carotenoids is mainly met by chemical synthesis and, to a minor extent, by extraction from natural sources or microbial fermentation (Sandmann et al., 1999; Shewmaker et al., 1999; Sandmann, 2001). Moreover, although a wide range of natural carotenoid derivatives is known to date, most of these are biosynthetic intermediates that accumulate only in trace amounts, making it very difﬁcult to extract sufﬁcient material for puriﬁcation (Albrecht et al., 2000). Some important dietary carotenoids are not abundant in the human diet. Zeaxantin, for instance, is a rare carotenoid, which together with lutein is the essential component of the macular pigment in the eye (Delgado-Vargas et al., 2000; Van den Berg et al., 2000). Low levels of intake increase the risk of age-related macular degeneration. Marigold extracts from Tagetes erecta or the dried ﬂowers themselves are well known as supplements for chicken feed to color the eggs and the chicken skin (Delgado-Vargas et al., 2000). Interestingly, marigold ﬂowers contain high concentrations of lutein as the major pigment (Delgado-Vargas and Paredes-López, 1997). During ingestion of carotenoids, the efﬁciency of their absorption depends largely on the type of food, its processing, and the amount of dietary fat or oil. Whether the presence of a carotenoid in the food matrix might facilitate its bioavailability is still not known (Van den Berg et al., 2000; Sandmann, 2001). As a result of this, the number of carotenoids available for assessing their biological function and pharmaceutical and nutraceutical potential by in vivo and in vitro assay systems is very limited. Carotenoids are a large family of C40 isoprenoid pigments. Their colorant and biological action, such as antioxidant activity, are related to the number and location of conjugated double bonds within their structure, cyclization of the ends of the molecules, and their modiﬁcation by oxygen-containing R groups such as hydroxyl, keto, and epoxi groups (Albrecht et al., 2000; Mann et al., 2000; Schmidt-Dannert et al., 2000). The ﬁrst committed step in carotenoid biosynthesis is the condensation of two geranyl-geranyl diphosphate (GGDP) molecules to form the C40 backbone, the colorless phytoene (Figure 2.2). Phytoene desaturases from bacteria can introduce four double bonds, yielding red carotenoid lycopene, whereas plants utilize two desaturase enzymes to complete this conversion. Phytoene desaturase (PDS) catalyzes the ﬁrst two desaturations (phytoene to phytoﬂuene to z-carotene), whereas the conversion of z-carotene to lycopene via neurosporene is performed by z-carotene desaturase (ZDS). The cyclization of lycopene, by lycopene cyclase, forms either a- or b-carotene, and subsequent hydroxylation reactions produce the xanthophylls, lutein, and zeaxanthin (Sandmann, 2001). Different approaches have been followed to modify the carotenoid content in plants to enhance their nutritional value: (1) modiﬁcation of carotenoid © 2003 CRC Press LLC
products in tomato, (2) increasing the amounts of preexisting carotenoids in rapeseed (Brassica napus), and (3) engineering a carotenogenic pathway in tissue that is completely devoid of carotenoids, such as rice endosperm (Table 1.3) (Shewmaker et al., 1999; Romer et al., 2000; Schmidt-Dannert et al., 2000; Ye et al., 2000). During lycopene deposition in tomato fruit ripening, the activity of phytoene synthase is the major controlling factor of the route; therefore, this enzyme should be an ideal target for the genetic manipulation of the carotenoid composition of tomato fruit (Fray et al., 1995). The constitutive highlevel expression of tomato phytoene synthase-1 in transgenic tomato has resulted in carotenoid-rich seed coats, cotyledons, and hypocotyls, but also in reduced levels of carotenoids in ripe tomato fruit due to gene silencing with the endogenous gene and dwarﬁsm due to redirection of GGDP into TABLE 1.3 Selected Essential Micronutrients for Human Diet, Their Daily Allowances, Manipulation by Plant Biotechnology, and Potential Applications Maximum Adult RDAa 1 mg REb Engineered Plant Tomato, rapeseed, rice Potential Application Provitamin A deﬁciency nutraceutical
Nutrient Vitamin A
Result Vitamin Increased levels of b-carotene
Ref. Shewmaker et al., 1999; Romer et al., 2000; Ye et al., 2000 Shintani and DellaPenna, 1998
10 mg TEc
Iron Zinc Calcium Phosphorus
15 mg 15 mg 1200 mg 1000 mg
Elevated content of atocopherol and reduced g-tocopherol content Minerals Tobacco, rice Improved Fe content — — — — Tobacco, Reduced rapeseed phytic acid levels
Vitamin E deﬁciency nutraceutical
Anemia nutraceutical — — Improvement in mineral bioavailability
Goto et al., 1998, 1999 — — Pen et al., 1993; Tramper, 2000
Recommended dietary allowances per day; values represent the highest RDA either for male or female adults, except for pregnant or lactating women. b Vitamin A activity is expressed in retinol equivalent (RE). One RE is equal to 1 mg of all-transretinol, 6 mg of all-trans-b-carotene, or 12 mg of other provitamin A carotenoids. c One TE (a-tocopherol equivalent) is equal to 1 mg (R,R,R)-a-tocopherol. Source: Adapted and modiﬁed from Shintani and DellaPenna, 1998; DellaPenna, 1999; GuzmánMaldonado and Paredes-López, 1999.
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the gibberellin pathway (Fray et al., 1995). The resultant plants were reduced in size. This work illustrates how problems arise when a balanced metabolism is disturbed (Sandmann, 2001). On the other hand, elevation of the provitamin A (b-carotene) content in transgenic tomato plants was achieved by manipulation of the desaturation activity. Romer et al. (2000) overexpressed a single carotenoid gene encoding PDS, which converts phytoene into lycopene, from Erwinia uredovora, under the control of a constitutive promoter and with the protein being targeted to the plastid by pea ribulose biphosphate carboxylase small subunit transit sequence. These researchers found that the expression of that gene in transformed tomatoes did not elevate total carotenoid levels. However, the b-carotene content increased about threefold, representing up to 45% of the total carotenoid level. The transgenic tomato fruit contained approximately 5 mg all-trans-b-carotene or 800 retinal equivalents (Table 1.3). Thus, 42% of the RDA is contained in a single provitamin A tomato fruit, as compared to 23% of the control fruit. The advantage of b-carotene instead of retinol (vitamin A) in the diet is that it is nontoxic and can be stored by the body. The alteration in carotenoid content of these transgenic plants did not affect growth and development and their phenotype was stable and reproducible over at least four generations. Also, the genetic manipulation of canola seeds to increase the carotenoid content to high levels was a tremendous success (Shewmaker et al., 1999). Overexpression of a bacterial phytoene synthase gene extended with a plastid-targeting sequence under a seed-speciﬁc promoter increased the carotenoid content of mature canola seed by up to 50-fold. In the transformant, the embryos were bright orange, as compared to the green embryos in control canola. In the transgenic seeds, concentrations of carotenoids (mainly a- and b-carotene) of more than 1 mg/g fresh weight accumulated, yielding oil with 2 mg carotenoids per g oil (Table 1.3). Other unexpected results were obtained upon transformation of tobacco with an algal b-carotene ketolase gene. Mann et al. (2000) expanded upon the metabolic framework to redirect metabolic ﬂux of the tobacco carotenoid biosynthetic pathway to produce the marine compound astaxanthin (Figure 1.2). Introducing b-carotene ketolase from unicellular algae into tobacco, astaxanthin could be synthesized using the endogenous pool of b-carotene in tobacco ﬂowers. Tissue-speciﬁc synthesis was accomplished by linking a gene promoter for ﬂower petal expression to a fused gene encompassing a transit peptide sequence for plastid localization and the algal ketolase coding sequence. Expression was high in ﬂowers as visualized by the red nectar pigmentation caused by astaxanthin and other ketocarotenoids (Mann et al., 2000). Total carotenoid levels were increased to 140% compared with the wild type. It is important to note that the astaxanthin produced in the transgenic plants had the same chirality as the natural astaxanthin found in marine organisms. In contrast, the synthetic astaxanthin that is currently used as ﬁsh feed is a mixture of stereoisomers, of which 75% have an unnatural chiral structure (Hirschberg, 1999). These results demonstrate the © 2003 CRC Press LLC
Phytoene Plant PDS Phytofluene Plant PDS ζ-Carotene ζ-ZDS Lycopene 3,4,3’,4’-Tetradehydrolycopene Desaturase 3,4,3’,4’-Tetradehydrolycopene Desaturase Desaturase-Cyclase Torulene
β-Carotene (Provitamin A)
FIGURE 1.2 Biosynthetic pathway of a- and b-carotene, their oxo derivaties, and schematic view of some the transformations of novel carotenoids obtained by using molecular breeding and an in vitro evolution approach. Production of b- carotene is universal in plants, fungi, and bacteria. Other carotenoids of biotechnological, nutraceutical, and pharmaceutical interest are lutein, zeaxanthin, and astaxanthin, including the novel and engineered carotenoids (3,4-didehydrolycopene, 3,4,3¢,4¢-tetradehydrolycopene, and torulene), which present improved antioxidant properties. Abbreviations: GGDP, geranyl-geranyl diphosphate; PDS, phytoene desaturase; z-ZDS, z-carotene desaturase. (Adapted and modiﬁed from Jez and Noel, 2000; Sandmann, 2001.)
prospect of genetically engineering carotenoid biosynthesis toward the production of naturally and commercially valuable compounds in plants. Rice, a major staple food, is usually milled to remove the oil-rich aleurone layer that turns rancid upon storage. The endosperm, the remaining edible part of rice grains, lacks several essential nutrients, such as provitamin A. In fact, rice in its milled form contains neither b-carotene nor any of its immediate precursors. Thus, predominant rice consumption promotes vitamin A deﬁciency, a serious public health problem in at least 26 countries, including highly populated areas of Asia, Africa, and Latin America (Kishore and Shewmaker, 1999; Ye et al., 2000; Potrykus, 2001). © 2003 CRC Press LLC
Immature rice endosperm synthesizes the carotenoid precusor GGDP. To convert GGDP to b-carotene, Ye et al. (2000) programmed the endosperm to carry out the necessary additional enzymatic reactions leading to the formation of cyclic carotenoids (b-carotene) in the rice endosperm. Transformation was carried out with a plasmid containing a plant phytoene synthase gene and a bacterial phytoene desaturase gene, which together should mediate the synthesis of lycopene from GGDP (Figure 1.2). Both reading frames were extended with transit sequences for targeting the endosperm plastids. One was under control of the endosperm-speciﬁc glutelin and the other under one constitutive promoter. Surprisingly, such transgenic plants did not accumulate lycopene as predicted. Instead, these plants produced essentially the same end products (b-carotene, lutein, and zeaxanthin). The authors speculate that the enzymes necessary to convert lycopene into b-carotene, lutein, and zeaxanthin are constitutively expressed in normal rice endosperm or are induced when lycopene is produced. Co-transformation with another construct that carried the third gene of interest, lycopene b-cyclase, increased the b-carotene content of the rice endosperm to a maximum level of 1.6 mg/g dry weight (Ye et al., 2000). The resulting yellow-colored endosperm, containing provitamin A (b-carotene) and other carotenoids of nutritional importance, could provide additional health beneﬁts. This type of transformed rice accumulating large levels of b-carotene is known as golden rice, and as little as 300 g of the cooked golden rice, a typical Asian diet, should provide almost the entire daily vitamin A requirement (Table 1.3). Golden rice exempliﬁes the best that agricultural biotechnology has to offer a world whose population is predicted to reach 7 billion by 2013 (Potrykus, 2001). Vitamins Tocopherols, the lipid-soluble antioxidants collectively known as vitamin E, are essential ingredients in human nutrition (Traber and Sies, 1996). Several epidemiological studies have indicated that vitamin E supplementation (100 to 400 international units [IU], or approximately 250 mg of atocopherol daily) results in decreased risk for cardiovascular disease and cancer, aids in immune function, and prevents or slows down a number of degenerative diseases associated with aging, such as cataracts, arthritis, and disorders of the nervous system caused by cumulative damage to tissues mediated by reactive oxygen species (Grusak and DellaPenna, 1999; Hirschberg, 1999). The four naturally occurring tocopherols (a-, b-, g-, and d-tocopherol) differ only in the number and position of methyl substituents on the aromatic ring (Shintani and DellaPenna, 1998). Of tocopherol species present in foods, natural single (R,R,R)-a-tocopherol is the most important to human health, has the highest vitamin E activity, and occurs as a single isomer (Grusak and DellaPenna, 1999). Although all tocopherols are absorbed equally during digestion, © 2003 CRC Press LLC
only single (R,R,R)-a-tocopherol is preferentially retained and distributed throughout the body; even though synthetic a-tocopherol is employed as a vitamin E supplement, it is a racemic mixture of eight different stereoisomers. Most of these isomers are less efﬁcacious than the (R,R,R) isomer (Traber and Sies, 1996; Grusak and DellaPenna, 1999). Tocopherols exist at different concentrations in various plant species and also vary among tissues. Whereas leaves of most common plants contain low levels of tocopherols (10 to 50 mg/g fresh weight), they can accumulate to high concentrations (500 to 2000 mg/g) in seeds (Hirschberg, 1999). However, in most seed crops, including those from which the major edible oils are derived (soybean, corn, canola, cottonseed, and oil palm), a-tocopherol is present only as a minor component because its immediate biosynthetic precursor g-tocopherol predominates; for example, in soybean oil, a- and g-tocopherol account for 7 and 70%, respectively, of the total tocopherol pool. Although other major oilseeds have similar patterns, seed oils still represent the major source of naturally derived dietary a-tocopherol (Grusak and DellaPenna, 1999). The most recent U.S. recommended daily allowance (RDA) suggests that up to 10 mg TE (a-tocopherol equivalent) be consumed every day (Table 1.3) and, because of the abundance of plant-derived components in most diets, this RDA is often met in the average human diet (Shintani and DellaPenna, 1998; DellaPenna, 1999). Substantial increases in the a-tocopherol content of the major food crops are necessary to supply the public with dietary sources of vitamin E that can approach the desired therapeutic levels and beneﬁt, because doing so is nearly impossible from the average diet, unless a concerted effort is made to ingest large quantities of speciﬁc food enriched in that vitamin (Shintani and DellaPenna, 1998; DellaPenna, 1999; Grusak and DellaPenna, 1999). g-Tocopherol is methylated to a-tocopherol in a reaction catalyzed by g-tocopherol methyltransferase (g-TMT). These observations suggest that g-TMT activity is likely limiting in the seeds of most agriculturally important oilseed crops and may be responsible for the low proportion of a-tocopherol synthesized and accumulated. As such, g-TMT is a prime molecular target for manipulation of a-tocopherol content in crops (Shintani and DellaPenna, 1998; Hirschberg, 1999). An exquisite and very signiﬁcant example of metabolic engineering in this direction has been reported by Shintani and DellaPenna (1998), who overexpressed a g-TMT cDNA in Arabidopsis seeds under control of a seed-speciﬁc carrot promoter. Untransformed Arabidopsis seeds contain around 370 ng of total tocopherols per milligram, mostly composed of g-tocopherol. In the transgenic seeds, 85 to 95% of the tocopherol pool was a-tocopherol, representing an 80-fold increase in a-tocopherol levels compared with the wild-type control (Table 1.3). Interestingly, the total seed tocopherol amount was not altered in these plants, indicating either a lack of stringent feedback regulation (by g-tocopherol) of the route in seeds or that a-tocopherol functions by the same mechanism as g-tocopherol. This qualitative change in tocopherol © 2003 CRC Press LLC
composition reﬂects a 9-fold rise in total vitamin E activity of the seed oil due to the different vitamin E potency of a- vs. g-tocopherol. Similar increases in vitamin E activity in commercially important oilseed crops could be achieved (DellaPenna, 1999; Grusak and DellaPenna, 1999).
Minerals In developing countries, cereal grains and some legumes are the primary and least expensive sources of calcium, iron, and zinc; however, their intake does not satisfy the mineral requirements of the populations of these countries (Table 1.3) (Guzmán-Maldonado et al., 2000). Guzmán-Maldonado et al. (2000) cited that the percentage of anemic subjects in developing countries (26%) was higher than that observed in Europe (10.9%) and the U.S. (8%); data revealed that anemia was predominantly caused by iron deﬁciency. They also cited that 40% of iron intake derived from legumes and cereals. Recent reports indicate that iron deﬁciency is the most prevalent micronutrient problem in the world, affecting over 2 billion people globally, many of whom depend on beans as their staple food (Goto et al., 1999; Guzmán-Maldonado et al., 2000). Some crops, such as spinach and legumes, are known for their iron content; however, these plants usually contain oxalic acid and phytate-like substances that decrease its bioavailability (Bliss, 1999). Iron in crops has been improved by increasing the iron concentration of the hydroponic culture media or soil (Bliss, 1999; Goto et al., 1999); however, this method is costly and cannot be used to target iron accumulation to a desirable part of the plant. Higher plants utilize one of two strategies for iron acquisition (Grusak and DellaPenna, 1999; Curie et al., 2001). Strategy 1 involves an obligatory reduction of ferric iron, usually as a Fe(III) compound, prior to membrane inﬂux of Fe2+; all dicotyledonous plants and non-grass monocots use this approach. Strategy 2 (used by grasses) employs ferric chelators called phytosiderophores that are released by roots and chelate ferric iron in the rhizosphere. The Fe(III)–phytosiderophore is absorbed intact via a plasmalemma transport protein. When plants using either approach are challenged with Fe-deﬁciency stress, the processes associated with one or the other strategy are upregulated in the plant root system (Grusak and DellaPenna, 1999; Curie et al., 2001). Modifying seeds to store the excess Fe chelated to peptides or in hemecontaining enzymes might further enhance the seed Fe nutritional quality by improving bioavailabity (Grusak and DellaPenna, 1999). In the ﬁrst case, one target might be the maize yellow stripe 1 (YS1) mutant, which is deﬁcient in Fe(III)–phytosiderophore uptake, suggesting that YS1 is an Fe(III)–phytosiderophore transporter. In 2001, Curie et al. showed that YS1 is a membrane protein that mediates iron uptake and whose expression in a yeast iron uptake mutant restores growth speciﬁcally on Fe(III)–phytosiderophore media. © 2003 CRC Press LLC
In the second approach, we have the ferritin, which is an iron-storage protein found in animals, plants, and bacteria (Goto et al., 1999). Recent studies show that both plants and animals use ferritin as the storage form of iron and that, when orally administered, it can provide a source of iron for treatment of rat anemia (Beard et al., 1996). These ﬁndings suggest that increasing the ferritin content of cereals by genetic engineering may help to solve the problem of dietary iron deﬁciency. To achieve this goal, Goto et al. (1999) introduced a soybean ferritin gene in rice seed, under the control of a rice seed-storage glutelin promoter, to mediate the accumulation of iron speciﬁcally in the grain. Their results indicated that soybean ferritin was overexpressed and accumulated in the rice endosperm tissue. Interestingly, transgenic rice stored up to three times (31.8 mg/g dry weight) more iron than untransformed seeds (11.2 mg/g dry weight). This achievement suggests that it may be feasible to produce ferritin rice as an iron supplement in the human diet. The iron content in a meal-size portion of ferritin rice (5.7 mg Fe per 150 g dry weight) would be sufﬁcient to supply 30 to 50% of the daily adult iron requirement (around 13 to 15 mg Fe) (Table 1.3). Also, this group constitutively expressed soybean ferritin in tobacco which resulted in a maximum iron content in transgenic leaves that was about 1.3 times that of control leaves (Goto et al., 1998). This increase seems to be low compared with the rice system; however, it is interesting to note that increments in absolute amounts of iron in these two systems are very similar (24.3 mg/g dry weight in tobacco vs. 27 mg/g dry weight in rice) (Goto et al., 1998, 1999). Although a positive correlation can be observed between exogenous ferritin and iron content, it is possible that the amount of iron accumulation is restricted by transport of iron to the ferritin molecule, rather than simply by the level of ferritin protein. Thus, it may be possible to store larger amounts of iron in the exogenous ferritin molecule by cointegrating into the target plant genome, at the same time, the ferritin gene and the gene Fe(III)–phytosiderophore transporter, such as the membrane protein of the maize yellow stripe 1 mutant (Goto et al., 1998, 1999; Curie et al., 2001). Numerous studies have led to the conclusion that phytic acid and tannins may bind proteins and some essential dietary minerals (calcium, iron, and zinc), thus making them unavailable or only partially available for absorption (Bliss, 1999; Guzmán-Maldonado et al., 2000). Zinc is essential for normal growth, appetite, and the immune function, being an essential component of more than 100 enzymes involving digestion, metabolism, and wound healing (Bliss, 1999; Guzmán-Maldonado et al., 2000). While iron deﬁciency has long been considered a major nutritional problem, zinc deﬁciency has only recently been recognized as a public health problem (Guzmán-Maldonado et al., 2000). On the other hand, approximately 60 to 65% of the phosphorus present in cereal, legume, and oilseed crops exists as phytic acid (myo-inositolhexaphosphate) which, accordingly, represents the major storage form of phosphate in plants (Tramper, 2000; Ward, 2001). However, in this form, the © 2003 CRC Press LLC
phosphate remains largely unavailable to monogastrics as these species are devoid of sufﬁcient, suitable, endogenous phosphatase activity that is capable of liberating the phosphate groups from the phytate core structure. The animal inability to degrade phytic acid has a number of nutritional consequences; phytic acid chemically complexes to zinc, iron, and calcium, preventing their assimilation by the animal (Bliss, 1999; Guzmán-Maldonado et al., 2000; Tramper, 2000; Ward, 2001). Methods to reduce the levels of these phytochemicals, such as phytic acid and tannins, should enhance the bioavailability of the micronutrients they affect (Table 1.3) (Pen et al., 1993; Bliss, 1999; DellaPenna, 1999; Grusak and DellaPenna, 1999; Tramper, 2000; Ward, 2001). One strategy to improve mineral bioavailability is to reduce phytic acid by adding phytase to the diet or to increase the level of endogenous phytase in the dietary components. Thus, an Aspergillus niger phytase gene was transferred to tobacco plants, and transgenic seeds accumulated phytase protein up to 10 g/kg total soluble protein. When samples of milled transgenic tobacco seed were mixed with standard poultry feed under conditions that stimulated the crop and stomach of the chicken, inorganic phosphate was released from the fodder (Pen et al., 1993). Also, the Dutch company Plantzyme succeeded in producing phytase in rapeseed and accumulating it in the seeds. The advantage is that the seeds can be directly added to the diet, without having to isolate the enzyme ﬁrst (Tramper, 2000).
Manipulation of Fruit Ripening A major problem in fruit marketing is premature ripening and softening during transport. These changes are part of the natural aging (senescence) process of the fruit (Gray et al., 1992). When compared to other plant organs, fruits exhibit a high metabolic activity even in their postharvest life (Giovannoni et al., 1998). It is precisely this biochemical activity that is one of the main causes of the high perishability of these commodities, resulting in short shelf life. Some postharvest problems have been solved for many commercially important plants, especially those grown in temperate climates, by harvesting before they ripen on the plant and/or storing at low temperatures or in modiﬁed or controlled atmospheres (Gomez-Lim, 1999); however, these approaches have had limited success when applied to fruits of tropical origin. For instance, some of these fruits fail to ripen properly, developing an unpleasant taste, if they are harvested at a green or immature stage (GomezLim, 1999). Fruit ripening represents a biological process unique to plant species in which developmental and hormonal signaling systems orchestrate a variety of biochemical and physiological changes which, in summation, result in the ripe stage of fruit maturation (Giovannoni et al., 1998). These changes lead to a soft, edible fruit. Some of these changes include synthesis of metabolites related to the development of ﬂavor and aroma, synthesis of pigments, degradation of chlorophyll, alterations in organic acids and cell © 2003 CRC Press LLC
wall metabolism, and softening of the fruit tissue. In so-called climacteric fruits such as tomato, cucurbit, banana, apple, mango, and many others (Giovannoni et al., 1998; Gomez-Lim, 1999), the initiation of ripening is characterized by a dramatic increase in respiration and biosynthesis of the gaseous hormone ethylene. Inhibition of ethylene biosynthesis or ethylene perception via exogenous application of inhibitors, or endogenous expression of transgenes, has been shown to have profound inhibitory effects on ethylene-mediated plant processes, including climacteric fruit ripening (Gray et al., 1992; Giovannoni et al., 1998; Grierson, 1998). Cell-wall-metabolizing enzyme polygalacturonase (PG) catalyzes the hydrolysis of polygalacturonic acid chains in unmethylated regions of pectin (Grierson, 1998). One gene appears to be responsible for the endopolygalacturonase that is synthesized de novo during tomato ripening, although three different polypeptide forms of PG are found in tomato; the difference is possibly due to carbohydrate content (Bird et al., 1988; Grierson, 1998). The inhibition of PG was ﬁrst achieved in transgenic tomato using antisense genes driven by a constitutive promoter; both PG mRNA and enzyme activity were reduced by 90% (Sheehy et al., 1988; Smith et al., 1990). Interestingly, this gene silencing was stably inherited and the PG activity had no effect on other ripening attributes such as color change and ethylene synthesis (Sheehy et al., 1988; Smith et al., 1990). Cell wall pectin, which normally decreases in molecular weight during ripening, was shown to retain a high molecular weight during ripening of fruits with a silenced PG gene (Smith et al., 1990). No signiﬁcant change in ﬁrmness could be detected, at least in some varieties, and this and other observations led various researchers to question whether PG was in fact involved in softening (Smith et al., 1990). It is now clear from biochemical studies and experiments with transgenic plants, however, that PG has a distinct effect on the textural quality of tomato (Grierson, 1998). Inhibiting its expression by antisense genes is the basis for the Flavr Savr‰ tomato and, in 1994, the U.S. Food and Drug Administration ruled that the Flavr Savr tomato is as safe as tomatoes that are bred by conventional approaches. In 1996, the genetically modiﬁed puree of that transgenic tomato was marketed in the United Kingdom. Thus, we have two different opportunities to exploit low PG tomatoes. For the fresh market, low PG makes the fruit less susceptible to cracking, splitting, and mechanical damage; therefore, fruit can be left to ripen longer on the vine before harvesting. While these same advantages may also be signiﬁcant for processing varieties, the main beneﬁt is in the longer chain pectin and perhaps the larger size of cell clumps in puree (Grierson, 1998). The plant growth hormone ethylene induces the expression of a number of genes involved in fruit ripening and senescence (Theologis, 1992). This gas is largely responsible for fruit and vegetable spoilage, thus possibilities have been explored aimed at modifying the ethylene formation or content in plants and fruits (Theologis, 1992). Ethylene is synthesized from S-adenosylmethionine via the intermediate ACC (aminocyclopropane-1-carboxylic acid). The enzyme ACC synthase catalyzes the formation of ACC. The © 2003 CRC Press LLC
second step leading to ethylene production is catalyzed by ACC oxidase, also known as the ethylene-forming enzyme (EFE) (Figure 1.3) (Theologis, 1992). Thus, two small multigene families encoding ACC synthase and (aminocyclopropane-1-carbolic acid) oxidase control the biosynthesis of ethylene (Theologis, 1992; Grierson, 1998). Treatment of plants with chemical compounds that block ethylene formation with sequestrants of it delay both fruit ripening and senescence (GomezLim, 1999). Thus, premature fruit ripening might be prevented by inhibiting the ability of the plant to synthesize ethylene. Almost complete inhibition of ACC synthase, and therefore of ethylene generation, using antisense genes inhibited the change in color and texture of tomato fruits (Oeller et al., 1991). Ripening changes could be restored by adding ethylene or ethylene-related compounds. A recent reexamination of the properties of these transgenic tomatoes has revealed, however, that they still generate sufﬁcient ethylene to induce PG gene expression (Sitrit and Bennet, 1997). Inhibiting the expression of a speciﬁc cDNA by antisense strategy in transgenic tomatoes ﬁrst identiﬁed the genes for ACC oxidase. Inhibiting ACC oxidase by 95% permitted normal development of ripening attributes of fruit attached to the plant but prevented the extreme softening, cracking, and spoilage normally associated with over-ripening so that the fruit lasted for several weeks (Picton et al., 1993). The transgenic plants synthesized a lower level of ethylene than did normal plants, and again the fruit of transgenic tomatoes had a signiﬁcantly longer shelf life. Interestingly, if the fruits were picked at the mature-green stage, they never ripened fully; adding ethylene externally did stimulate color development, but over-ripening and deterioration did not occur (Picton et al., 1993). Another approach for reducing Methionine
S-Adenosylmethionine ACC Synthase SAM Hydrolase Aminocyclopropane-1-Carboxylic Acid ACC Oxidase ACC Deaminase Ethylene
FIGURE 1.3 Plant ethylene biosynthesis and its control by genetic engineering affects fruit ripening process. Abbreviations: ACC, aminocyclopropane-1-carboxylic acid; SAM, S-adenosylmethionine. A bold and full arrow indicates normal ethylene biosynthesis pathway; a broken arrow indicates catabolism of ethylene intermediates, which have also been used to manipulate fruit ripening. (Adapted and modiﬁed from Theologis, 1992.) © 2003 CRC Press LLC
ethylene formation is to overexpress bacterial genes for S-adenosylmethionine hydrolase or ACC deaminase, thus depleting plant cells of the required precursor (Figure 1.3) (Grierson, 1998). While most reports of transgenic plants in which ethylene amounts have been lowered in an effort to delay fruit ripening have focused on tomato plants (Theologis, 1992; Picton et al., 1993; Giovannoni et al., 1998; Grierson, 1998), one research group published the generation of low-ethylene transgenic cantaloupe melon with reduced ACC oxidase gene expression by antisense technology (Ayub et al., 1996). Melons of the cantaloupe Charenties type were chosen because of their good eating quality but poor storage capability. ACC oxidase activity in fruit was virtually undetectable and ethylene production was diminished below 1% of the control level. Transgenic melons developed a functional abscission layer but remained on the plant for longer. Pigment production in the ﬂesh followed the normal ripening pattern, but the peel remained green while controls turned yellow. After 10 days of storage at 25∞C, the transgenic melons were still green and retained their shape, whereas the wild-type fruit had a shriveled yellow peel, showed signs of fungal infections, and had soft ﬂesh and a squashed shape. Applying ethylene to the transgenic fruits restored the yellowing phenotype to the peel (Ayub et al., 1996). The latter results suggest that those molecular approaches could be effective in a range of different fruits, offering new opportunities for enhancing their quality and nutritional value.
Microbial Biotechnology in Industry
Microorganisms are important for many reasons, particularly because they produce things that are of value to us (Demian, 2000a,b). These can be very large materials such as proteins, nucleic acids, carbohydrates, food polymers, or even cells, or they can be smaller molecules that are usually separated into metabolites that are essential for vegetative growth (primary) and those that are inessential (secondary) (Demian, 2000a,b). Although microbes are extremely good at producing an amazing array of valuable products, they usually produce them only in the amounts they need for their own beneﬁt; thus, they tend not to overproduce their metabolites (Demian, 2000b). By contrast, the industrial microbiologist screens for wasteful strains that overproduce and excrete a particular compound that can be isolated and marketed (Chotani et al., 2000; Demian, 2000a,b). After a desired strain has been found, a development program is initiated to improve titers by modiﬁcation of culture conditions using mutation and recombinant DNA techniques (Chotani et al., 2000; Demian, 2000b). The microbiologist is actually modifying the regulatory controls remaining in the original culture so that its inefﬁciency can be further increased and the microorganism will excrete tremendous amounts of these high-value products into the medium. © 2003 CRC Press LLC
The main reason for the use of microorganisms to produce compounds that can otherwise be isolated from plants and animals or synthesized by chemists is the ease of increasing production by environmental and genetic manipulation. Thousand-fold increases have been reported for small metabolites. Of course, the higher the speciﬁc level of production, the simpler the job of product isolation (Chotani et al., 2000; Demian, 2000b). Microbial Metabolites Primary metabolites are the small molecules of all living cells that are intermediates or end products of the pathways of intermediary metabolism, or are building blocks for essential macromolecules, or are converted into coenzymes (Glick and Pasternak, 1998; Chotani et al., 2000; Demian, 2000a,b). Primary metabolites used in the food and feed industries include alcohols, amino acids, ﬂavor nucleotides, organic acids, polyols, polysaccharides, sugars, and vitamins (Glick and Pasternak, 1998; Chotani et al., 2000; Demian, 2000a,b). Microbially produced secondary metabolites are extremely important for health and nutrition. As a group that includes antibiotics, other medicinals, toxins, biopesticides, and animal and plant growth factors, they have tremendous economic importance (Glick and Pasternak, 1998; Chotani et al., 2000; Demian, 2000a,b). Secondary metabolites have no function in the growth of the producing cultures (although, in nature, they are essential for the survival of the producing organism); they are produced by certain restricted taxonomic groups of organisms and are usually formed as mixtures of closely related members of a chemical family. Microbial Production of Small High-Value Molecules To date, molecular biotechnology research has focused largely on the production of a range of different proteins; however, recombinant DNA technology can also be used to enhance the production of a range of lowmolecular-weight compounds (Glick and Pasternak, 1998). With efﬁcient expression systems, it is relatively straightforward to clone and express a particular target protein. The expressed protein is either the ﬁnal product (e.g., a restriction enzyme) or a catalyst for a speciﬁc chemical reaction (Glick and Pasternak, 1998). Sometimes, a novel catalytic activity is introduced into a microorganism by genetic manipulation and used to produce in vivo lowmolecular-weight metabolites. In these cases, the host microorganism is engineered to become a factory for the production of useful metabolites (Glick and Pasternak, 1998; Chotani et al., 2000; Demian, 2000a,b).
Vitamins Vitamin C (L-ascorbic acid) is used on a large scale as an antioxidant in food, animal feed, beverages, pharmaceutical formulations, and cosmetic © 2003 CRC Press LLC
applications (Chotani et al., 2000). The current world market of ascorbic acid is 60,000 to 70,000 metric tons per year and generates annual revenues in excess of U.S.$500 million (Glick and Pasternak, 1998; Chotani et al., 2000). An expensive process that includes one microbial fermentation step and a number of chemical steps starting with D-glucose currently synthesizes ascorbic acid commercially. The last step in this process is the acidcatalyzed conversion of 2-keto-L-gulonic acid (2-KLG) (Chotani et al., 2000). Biochemical studies for the metabolic routes of a number of different microorganisms have shown that it may be possible to synthesize 2-KLG by an alternative pathway (Glick and Pasternak, 1998; Chotani et al., 2000). For example, some bacteria (Acetobacter, Gluconobacter, and Erwinia) can convert glucose to 2,5-diketo-D-gluconic acid (2,5-DKG) and others such as Corynebacterium, Brevibacterium, and Arthrobacter have the enzyme 2,5-DKG reductase, which converts 2,5-DKG to 2-KLG. Thus, a novel process involves the use of a genetically engineered Erwinia sp. strain containing a gene encoding 2,5-DKG reductase from Corynebacterium sp. (Anderson et al., 1985) The transformed Erwinia cells were able to convert D-glucose directly to 2-KLG. The endogenous Erwinia enzymes, localized in the inner membrane of the bacterium, converted glucose to 2,5DKG, and the cloned 2,5-DKG reductase, localized in the cytoplasm, catalyzed 2,5-DKG to 2-KLG. The engineered organism transforms glucose to 2KLG in a single-step fermentation, which can be easily converted by acid or base to L-ascorbic acid (Pramik, 1986). Using this elegant approach, Genencor International Company has produced up to 120 g of 2-KLG per liter in less than 120 hours of fermentation time in 14-L fermenters (Chotani et al., 2000; Demian, 2000b). The goal of manufacturing vitamin C directly by fermentation has remained elusive. By employing a metabolic selection strategy, Genecor International Company has now identiﬁed a 2-KLG to ascorbic acid activity in two yeast species, namely Candida blankii and Cryptococcus dimmnae. Another direct route from D-glucose to L-ascorbic acid in microalgae has been developed (Chotani et al., 2000). Additional bioengineering is required to advance toward the direct fermentation of glucose to vitamin C without the need to isolate 2-KLG. Vitamin B2 (riboﬂavin) was produced commercially for many years by both fermentation and chemical synthesis, but today fermentation is the main route. Overproducers include two yeast-like molds, Eremothecium ashbyii and Ashbya gossypii, which synthesize riboﬂavin in concentrations up to 20 g/L (Demian, 2000a). New processes using Candida sp. or recombinant Bacillus subtilis strains that produce greater than 30 g riboﬂavin per liter have been recently developed (Perkins et al., 1999; Demian, 2000a). Vitamin B12 (cyanocobalamin) is produced industrially with Propionibacterium shermanii or Pseudomonas denitriﬁcans (Demian, 2000a). Such strains make about 100,000 times more vitamin B12 than they need for their own growth. The early stage of P. shermanii fermentation is conducted under anaerobic conditions in the absence of the precursor 5,6-dimethylbenzimidazole. These conditions prevent vitamin B12 synthesis and allow for © 2003 CRC Press LLC
the accumulation of the intermediate, cobinamide (Demian, 2000a,b). The culture is then aerated and dimethylbenzimidazole is added, converting cobinamide to the vitamin. In the P. denitriﬁcans fermentation, the entire process is carried out under low oxygen. A high level of oxygen results in an oxidizing intracellular environment that represses the formation of the early enzymes in the pathway. Of major importance for the P. denitriﬁcans fermentation is the addition of betaine (Kusel et al., 1984). Production of vitamin B12 has reached levels of 150 mg/L, with a world market value of U.S.$71 million. However, while vitamin B12 overproduction is totally dependent upon betaine, the mechanism of control is unknown (Kusel et al., 1984; Demian, 2000b). D-biotin is a vitamin required for human health care and use as an animal feed additive (Masuda et al., 1995). It is industrially produced by chemical synthesis, but more economical manufacturing methods have been developed by recombinant DNA technology (Demian, 2000a). During production of biotin, feedback repression is caused by the acetyl-coenzyme A carboxylase biotin holoenzyme synthase, with biotin 5-adenylate acting as corepressor. Recently, Serratia marcescens has been engineered as a biotinhyperproducing strain. A recombinant plasmid carrying the mutated biotin operon has been constructed and is introduced into a D-biotin-producing Serratia strain that also contains an additional copy of the mutated biotin operon (Masuda et al., 1995). The strain of S. marcescens obtained and selected for resistance to biotin antimetabolites is able to produce 600 mg of D-biotin/L in the presence of high levels of sulfur and ferrous iron, in contrast to the 80 mg of D-biotin/L obtained without an additional source of these minerals. Lactic acid bacteria have very limited biosynthetic capability for the production of vitamins; however, certain exceptions have been noted. The yogurt bacterium Streptococcus thermophilus has been observed to produce folic acid, which, in fact, stimulates the growth of the other yogurt bacterium, Lactobacillus bulgaricus. L. lactis also produces substantial amounts of folic acid during fermentation (Hugenholtz and Kleerebezem, 1999). Many of the gene codings for the route of folic acid biosynthesis have been identiﬁed in the genome of this microorganism. Also, genes for riboﬂavin and biotin biosynthesis have been reported in L. lactis. This would make it possible to engineer the production of these vitamins in these food-grade bacteria, as recently reported for B. subtilis. Vitamin production processes by lactic acid bacteria would have huge advantages over the currently used processes, as they could also be implemented for in situ production, such as food fermentations (Hugenholtz and Kleerebezem, 1999). This work has just started.
Lactic Acid L-lactic acid has an ancient history of use as a food preservative and food ﬂavoring compound (Hugenholtz and Kleerebezem, 1999). Recently, lactic © 2003 CRC Press LLC
acid has received attention because it can be condensed into a biodegradable polymer that could become a major bioplastic in the future (Hugenholtz and Kleerebezem, 1999; Chotani et al., 2000; Demian, 2000b). The market for lactic acid is growing rapidly, exceeding several hundred million dollars annually (Hugenholtz and Kleerebezem, 1999; Chotani et al., 2000; Demian, 2000b). Under non-energy-limiting batch fermentation conditions, homofermentative bacteria predominantly produce lactic acid as their end product (Hugenholtz and Kleerebezem, 1999). Lactic acid yields are highest during glycolysis via the homolactic acid fermentative pathway. Although free lactic acid is preferred for most of industrial processes, anaerobic fermentation for the production of the organic acid operates optimally at pH values where the salt of the organic acid rather than free acid is formed (Hugenholtz and Kleerebezem, 1999; Chotani et al., 2000). To obtain lactic acid in its free form, the fermentation process must be carried out at or below its pKa of 3.87. An intelligent approach was recently published. By insertion of the bovine lactate dehydrogenase A (LDH-A) gene into a Kluyveromyces lactis, pyruvate ﬂux toward ethanol production was fully replaced by lactic acid production (1.19 mol lactate per mole of glucose) (Porro et al., 1999). Transferring the process to a 14-L fermenter gave a titer of 109 g/L with productivity of 0.8 g/L/h at pH 4.5. A doubling of yield as well as titer was achieved in a fermentation carried out with a strain of S. cerevisiae overexpressing the lactate-proton symporter (Chotani et al., 2000).
Amino Acids Amino acids are used extensively in the food industry as ﬂavor enhancers, antioxidants, and nutritional supplements; in agriculture, as feed additives; in medicine, in infusion solutions for postoperative treatment; and in the chemical industry, as starting materials for the manufacture of polymers and cosmetics (Glick and Pasternak, 1998). For the most part, amino acids are commercially produced either by extraction from protein hydrolysates or as fermentation products of either Corynebacterium or Brevibacterium spp., which are both nonsporulating Gram-positive soil bacteria (Glick and Pasternak, 1998). Traditionally, the productivity of these organisms has been improved by mutagenesis and subsequent screening for strains that overproduce certain amino acids (Glick and Pasternak, 1998; Demian, 2000b). However, this way of developing new strains is slow and sometimes inefﬁcient. Some preliminary progress has been made in increasing the amino acids in C. glutamicum. Thus, the synthesis of tryptophan by C. glutamicum was enhanced by introducing into wild-type C. glutamicum cells a second copy of the gene encoding anthranilate synthetase, which is the rate-limiting enzyme in the normal tryptophan biosynthetic pathway (Glick and Pasternak, 1998). On the other hand, one mutant unable to grow on a minimal medium unless anthranilic acid was added, and which did not produce tryptophan, was transformed with a copy of the anthranilate synthetase © 2003 CRC Press LLC
construct; the cloned gene did indeed restore most of the capacity of the mutant to synthesize tryptophan. However, the effect of adding this gene to wild-type C. glutamicum was much more dramatic, with the synthesis of this amino acid being increased by approximately 130%. This level of overproduction reﬂects more efﬁcient utilization of available precursor material (Glick and Pasternak, 1998). Most cereals are deﬁcient in the essential amino acid L-lysine (Shewry, 1998; Tabe and Higgins, 1998). In E. coli, the lysine route is controlled very tightly and includes three AKs that are each regulated by a different end product. In addition, after each branch point, their respective ﬁnal products inhibit the initial enzymes, and no overproduction occurs. However, in lysine fermentation in organisms such as mutants of C. glutamicum and its relatives, only a single AK exists, and it is regulated via concerted feedback inhibition by threonine and lysine (Eggeling et al., 1998). By the genetic removal of homoserine dehydrogenase, a glutamate-producing wild-type Corynebacterium was converted into a lysine-overproducing mutant that cannot grow unless methionine and threonine are added to the medium (Eggeling et al., 1998). As long as the threonine supplement is kept low, the intracellular concentration of this amino acid is limiting and feedback inhibition of AK is bypassed. Thus, E. coli and Serratia marcescens have been engineered with plasmid-bearing amino acid biosynthetic operons (Demian, 2000b). Plasmid transformation has been accomplished in C. glutamicum, so that recombinant DNA is now used to improve these commercial amino-acid-producing strains (Eggeling et al., 1998). The major manipulations have involved gene cloning to increase the levels of feedback-resistant AK and DHDPS. As a result, lysine industrial production yields 170 g/L and 0.54 moles of L-lysine per mole of glucose used. L-lysine is produced at an annual rate of 300,000 tons with a market of U.S.$600 million (Demian, 2000b).
Carotenoids The enormous progress in the cloning of carotenogenic genes offers the opportunity of modiﬁying and engineering the carotenoid pathways in either noncarotenogenic or carotenogenic microorganisms (Sandmann et al., 1999; Sandmann, 2001). Carotenoids have been successfully synthesized in noncarotenogenic yeast Candida utilis, which has systematically been genetically modiﬁed as a producer host for lycopene, b-carotene, and astaxanthin (Misawa and Shimada, 1998). The foreign bacterial carotenoid biosynthesis gene was altered according to the codon usage for C. utilis and expressed under the control of a constitutive promoter derived from the host. This engineered strain yielded around 8 mg of lycopene per gram of dry weight (Misawa and Shimada, 1998). On the other hand, Schmidt-Dannert et al. (2000) used DNA shufﬂing (also called sexual PCR) to generate diversity in both phytoene desaturases and lycopene cyclases by recombination of homologous DNA sequences. © 2003 CRC Press LLC
Phytoene desaturase gene from Erwinia herbicola and E. uredovora were subjected to one round of DNA shufﬂing. Escherichia coli engineered to produce high levels of GGDP was transformed with a library of phytoene desaturase variants and screened for their ability to synthesize carotenoids of varying colors. In contrast to the four double bonds introduced by the parental enzymes, variants synthesizing between two and six double bonds were found in this library. One gene variant that synthesizes the fully conjugated compound with six double bonds is 3,4,3¢,4¢-tetradehydrolycopene, a novel compound not known to be synthesized in nature (Figure 1.2) and yielding pink colonies. It is interesting to note that this carotenoid has greater antioxidant activity than lycopene. Lycopene cyclase does not utilize 3,4,3¢,4¢-tetradehydrolycopene as a substrate. DNA shufﬂing of the lycopene cyclase genes from E. herbicola and E. uredovora, however, resulted in the evolution of a cyclase capable of forming rings at both ends of 3,4,3¢,4¢-tetradehydrolycopene to form torulene (Figure 1.2). A related strategy employed by Albrecht et al. (2000) combined carotenogenic genes from different bacteria employing unique pathways that maintain altered product speciﬁcities in E. coli expressing the biosynthetic machinery for phytoene production. In conjunction with the four-step phytoene desaturase that yields lycopene, a ﬁve-step desaturase was used to produce 3,4-didehydrolycopene (Figure 1.2). Further, diversiﬁcation of the C40 skeleton resulted in the formation of numerous novel carotenoids, including 1-hydroxylated acyclic structures with up to 13 conjugated double bonds. Their antioxidative potential is superior to other related carotenoids. These experiments show that it is possible to use the molecular breeding approach to create novel carotenoids that have greater antioxidant activity than the carotenoids found in nature and hold the promise of creating improved nutraceuticals.
We thank Fidel Guevara-Lara from CINVESTAV-IPN for critical review of the manuscript.
Albrecht, M., S. Takaichi, S. Steiger, Z. Wang, and G. Sandmann (2000) Novel hydroxycarotenoids with improved antioxidative properties produced by gene combination in Escherichia coli, Nat. Biotechnol., 18, 843–846.
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Smith, C.J.S., C. Watson, P.C. Morris, C.R. Bird, G.B. Seymour, J.E. Gray, C. Arnold, G.A. Tucker, W. Schuch, S. Harding, and D. Grierson (1990) Inheritance and effect on ripening antisense polygalacturonase genes in transgenic tomatoes, Plant Mol. Biol., 14, 369–379. Snustad, D.P. and M.J. Simmons (2000) Principles of Genetics, John Wiley & Sons, New York. Spychalla, J.P., A.J. Kinney, and J. Browse (1997) Identiﬁcation of an animal omega3 fatty acid desaturase by heterologous expression in Arabidopsis, Proc. Natl. Acad. Sci. USA, 94, 1142–1147. Stark, D.M., K.P. Timmerman, G.F. Barry, J. Preiss, and G.M. Kishore (1992) Regulation of the amount of starch in plant tissues by ADP glucose pyrophosphorylase, Science, 258, 287–292. Steinmetz, K.A. and J.D. Potter (1996) Vegetables, fruits and cancer prevention: a review, J. Am. Diet. Assoc., 96, 1027–1039. Tabe, L. and T.J.V. Higgins (1998) Engineering plant protein composition for improved nutrition, Trends Plant Sci., 3, 282–286. Tacket, C.O., R.H. Reid, E.C. Boedeker, G. Losonsky, J.P. Nataro, H. Bhagat, and R. Edelman (1994) Enteral immunization and challenge of volunteers given enterotoxigenic E. coli CFA II encapsulated in biodegradable microspheres, Vaccine, 12, 1270–1274. Tacket, C.O., H.S. Mason, G. Losonsky, J.D. Clements, M.M Levine, and C.J. Arntzen (1998) Immunogenicity in humans of a recombinant bacterial antigen delivered in a transgenic potato, Nat. Med., 4, 607–609. Takase, K. and K. Hagiwara (1998) Expression of human alpha-lactalbumin in transgenic tobacco, J. Biochem., 123, 440–444. Thanavala, Y., Y.F. Yang, P. Lyons, H.S. Mason, and C. Arntzen (1995) Immunogenicity of transgenic plant-derived hepatitis B surface antigen, Proc. Natl. Acad. Sci. USA, 92, 3358–3361. Theologis, A. (1992) One rotten apple spoils the whole bushel: the role of ethylene in fruit ripening, Cell, 70, 181–184. Tobin, M.B., C. Gustafsson, and G.W. Huisman (2000) Directed evolution: the rational basis for irrational design, Curr. Opin. Struct. Biol., 10, 421–427. Traber, M.G. and H. Sies (1996) Vitamin E in humans: demands and delivery, Annu. Rev. Nutr., 16, 321–347. Tramper, J. (2000) Modern biotechnology: food for thought, in Food Biotechnology, S. Bielecki, J. Tramper, and J. Polak, Eds., Elsevier Science, Amsterdam, pp. 3–12. Tsuda, S., K. Yoshioka, T. Tanaka, A. Iwata, A. Yoshikawa, Y. Watanabe, and Y. Okada (1998) Application of human hepatitis B virus core antigen from transgenic tobacco plants for serological diagnosis, Vox Sanguinis, 74, 148–155. Turpen, T.H., S.J. Rein, Y. Charoenvit, S.L. Hoffman, V. Fallarme, and L.K. Grill (1995) Malarial epitopes expressed on the surface of recombinant tobacco mosaic virus, Biotechnology, 13, 53–57. Van den Berg, H., R. Faulks, H.F. Granado, J. Hirschberg, B. Olmedilla, G. Sandmann, S. Southon, and W. Stahl (2000) The potential for the improvement of carotenoid levels in foods and the likely systemic effects, J. Sci. Food Agric., 80, 880–912. Verch, T., V. Yusibov, and H. Koprowski (1998) Expression and assembly of a full length monoclonal antibody in plants using a plant virus vector, J. Immunol. Methods, 220, 69–75. Visser, R.G.F. and E. Jacobsen (1993) Towards modifying plants for altered starch content and composition, Trends Biotechnol., 11, 63–68. © 2003 CRC Press LLC
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Recent Developments in Food Biotechnology
CONTENTS Introduction Deﬁning Biotechnology Traditional Biotechnology Molecular (Modern) Biotechnology Molecular Cloning Polymerase Chain Reaction Modern Biotechnological Approaches to Traditional Processes Phage Resistance Nisin Resistance Recombinant Chymosin Other Applications Agricultural Biotechnology Modiﬁcation of Lipid Metabolism Alteration of a Major Fatty Acid Level Production of Unusual Fatty Acids Protein Modiﬁcation Carbohydrate Modiﬁcation The Gene Subtraction Approach Improving Food Quality Other Improvements The Beneﬁts of Biotechnology Transformation Techniques in Plant Biotechnology Transgenic Animals Conclusion References
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Deﬁning Biotechnology Out of the several deﬁnitions of biotechnology, perhaps one of the broadest is the use of living cells, microorganisms, or enzymes for the manufacture of chemicals, drugs, or foods or for the treatment of wastes (Jenson, 1993). A simpler approach deﬁnes biotechnology as the use of biological organisms or processes in any technological application (Riley and Hoffman, 1999). Biotechnology has had a tremendous impact on the food industry. It has provided high-quality foods that are tasty, nutritious, convenient, and safe, and it has the potential for the production of even more nutritious, palatable, and stable food (John Innes Centre, 1998). Traditional Biotechnology The roots of biotechnology can be found in the ancient processes of food and beverage fermentation. These traditional technologies are present in almost every culture in the world and have evolved over many years without losing their traditional essence. Examples of these processes include the production of some well-known foods, such as bread, wine, yogurt, and cheese. These products, like many others (such as ripened sausages [salami], pickles, sauerkraut, soy sauce, vinegar, beer, and cider), are produced using the natural processes of living organisms (e.g., fermentation) — in other words, by using biotechnology. Some relatively new developments in these traditional products include bio-yogurts, or biogurts, which contain extra bacteria (usually probiotic organisms) that are not found naturally in the original food. These probiotic bacteria most often include Lactobacillus acidophilus and Biﬁdobacterium biﬁdum. Another traditional biotechnology technique is the production of mycoprotein Quorn‘ as an alternative to meat, which was developed much more recently. Because these techniques are considered conventional, they have not caused public concern (John Innes Centre, 1998). Enzymes are also widely used in the food and beverage industries. For economic reasons they are used in a relatively crude form or in a reusable form, usually achieved by immobilization. The dairy industry uses primarily rennins and lactases; the brewing industry, proteases and amylases. Highfructose corn syrup is produced from starch by using a-amylase, amyloglucosidase, and glucose isomerase, and the beverage industry consumes a great amount of pectinases (Smith, 1988). Molecular (Modern) Biotechnology “Modern” or molecular biotechnology, in contrast with “traditional” biotechnology, also includes the use of techniques of genetic engineering — that is, techniques for altering the properties of biological organisms. This allows © 2003 CRC Press LLC
characteristics to be transferred between organisms to give new combinations of genes and improved varieties of plants or microorganisms for use in agriculture and industry (John Innes Centre, 1998). Some consumers are concerned about the safety of using the techniques of modern biology, although in countries such as Japan and the U.S. consumers remain optimistic about biotechnology. They are generally willing to purchase foods developed through these techniques, and the food and agricultural applications are as acceptable as are new medicines (Hoban, 1999). As stated above, recombinant DNA techniques have revolutionized the ﬁelds of biology, biochemistry, and biotechnology, as they have made research of genomes more possible than ever. For example, molecular cloning and the polymerase chain reaction (PCR) have been used in order to obtain the large number of DNA copies required for DNA sequencing methods (McKee and McKee, 1999). Molecular Cloning In this technique, a fragment of DNA isolated from a donor cell (e.g., bacteria, yeast, or any animal or plant cell) is incorporated into a vector (e.g., plasmids or phages), by which the gene of interest can be introduced into a host cell. The formation of the recombinant DNA molecule requires a restriction endonuclease to cut and open the vector DNA. After the sticky ends of the vector have been annealed with those of the donor DNA, a DNA ligase joins the two molecules covalently. The recombinant molecule is then inserted into bacterial cells by any suitable method such as electroporation. It is necessary that the recombinant vectors contain regulatory regions recognized by the bacterial enzymes. Polymerase Chain Reaction Polymerase chain reaction, considered one of the most signiﬁcant DNA technologies developed, is a method for amplifying very small amounts of DNA that copies part of a genome for subsequent sequencing or other analyses (Garrison and dePamphilis, 1994). It is an in vitro laboratory method to amplify a speciﬁc segment of a genome DNA by using a pair of speciﬁc primers (oligonucleotides used to start the DNA replication) to allow a section of the DNA to be repeatedly copied. Template strands are separated by heating and then cooled to allow primers to anneal to the template. The temperature is raised again to allow primer extension (template strand copying) by means of a thermostable DNA polymerase. The procedure is repeated 30 to 40 times with an exponential ampliﬁcation of the DNA concentration (Hill, 1996). With the availability of this technique, enormous quantities of genes have now been sequenced for a wide range of organisms. The genomes of several bacteria and small organisms have already been fully sequenced, and the genomic sequences of many higher organisms such as plants, animals, and humans (around 90% complete) have been published (Institute of Food Technologists, 2000). © 2003 CRC Press LLC
Modern Biotechnological Approaches to Traditional Processes Phage Resistance Bacteriophages are viruses that infect and kill bacteria, including cheese and fermented milk starter cultures, which leads to low production, low-quality products, and economic losses. The discovery that some starter strains have phage resistance mechanisms encoded on plasmids has resulted in the development of molecular cloning techniques to develop new resistant strains, especially in Lactococcus lactis. Currently, research in this subject is divided among three main areas: (1) the study of the mechanisms of infection (Valyasevi et al., 1991; Monteville et al., 1994) and resistance (McKay and Baldwin, 1984; Sing and Klaenhammer, 1990; Garvey et al., 1995, 1996); (2) the study of restriction and modiﬁcation systems in lactic acid bacteria (Sing and Klaenhammer, 1991; Su et al., 1999); and (3) the study of bacteriophage genomes (Brown et al., 1994; Djordjevic and Klaenhammer, 1997). Something similar is also being developed in the case of yogurt for Streptococcus thermophilus (Brussow et al., 1994).
Nisin Resistance Potential food-grade selectable markers from the lactococci include genes associated with carbohydrate metabolism, bacteriophage resistance, and nisin production or resistance (Froseth and McKay, 1991). Nisin is an important antimicrobial agent produced by some Lactococcus lactis subsp. lactis strains and is very effective against a variety of Gram-positive bacteria (Froseth et al., 1988). In one of the studies related to the cloning of nisin resistance, the objective was to obtain a genetically modiﬁed strain of L. lactis subsp. lactis lac(–) for potential use in accelerating the ripening rate of cheese. The lactococcal strain DRC3 was chosen as a DNA donor. This strain is a diacetyl producer and shows resistance to nisin and bacteriophage c2 aside from having a known plasmid pattern. An electroporation procedure for the plasmid-mediated genetic transformation of intact cells of L. lactis subsp. lactis LM0230 was developed. The DNA from DRC3 was isolated by alkaline lysis and puriﬁed in a CsCl–ethydium bromide gradient. Competent cells were obtained by culturing the lac(–) strain in glucose-M17 medium with 1.5% glycine and washing the harvested cells with 20% glycerol. Electroporation was performed by a single pulse at 12.5 kV/cm after mixing the cells with the puriﬁed plasmid DNA. These conditions produced a transformant that showed limited resistance to nisin. A plasmid with a molecular weight between 1 and 1.8 MDa and absent in the donor strain was observed by electrophoresis. The plasmid encoding resistance to nisin in DRC3 is very large (40 MDa) and is known as pNP40, so it was logical to assume that fragmentation occurred during the electroporation and that only a part of it could penetrate. The transformant strain proved to be suitable for use in
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starter cultures containing nisin producers and lac(+) lactococci to accelerate cheese ripening (Hernández-Sánchez, 1994). In a second paper (Froseth et al., 1988), the nisin resistance determinant and an origin of replication on pNP40 was cloned on a 7.6-kb EcoRI fragment. When self-ligated, this fragment existed as an independent replicon (pFM011) and contained a 2.6-kb EcoRI fragment encoding nisin resistance. This vector was also used to clone a 6.3-kb EcoRI fragment coding for bacteriophage insensitivity. This new plasmid, designated pFK012, conferred nisin resistance and an abortive type of phage insensitivity when introduced into L. lactis subsp. lactis LM0230. The plasmid pFK012 is then a potential food-grade vector formed only by lactococcal DNA (Hughes and McKay, 1992). A very efﬁcient new food-grade cloning system for industrial strains of Lactococcus lactis has just been developed that allows the overexpression of many technologically important cloned genes in industrial strains. This new vector (pFG200) has many advantages, including (1) easy cloning of genes into a versatile polylinker region, (2) a small and stable multicopy vector that is introduced by electroporation into Lactococcus with high efﬁciency, and (3) a selection system allowing selection and maintenance in milk, which allows genetic modiﬁcation, cloning, and overexpression of Lactococcus DNA in a food-grade manner. The plasmids generated with this vector are stably maintained in the host cells for more than 35 generations in media, including milk (Sørensen et al., 2000).
Recombinant Chymosin It is a fact that good-quality cheese can only be made with good-quality rennet, derived from the stomach of unweaned calves. Chymosin is the proteolytic enzyme in rennet that catalyzes the milk clotting activity, but it is often contaminated with other enzyme activities and microorganisms that may cause quality problems. The availability of this enzyme is limited and can be costly. The calf chymosin gene has been isolated by several research groups and cloned in microorganisms such as Escherichia coli (Beppu, 1983), Kluyveromyces marxianus, and Aspergillus niger. The K. marxianus chymosin is excreted into the fermentation broth and is easily puriﬁed. The ﬁnal product is free of other activities or microorganisms and has been on the market now for some time with excellent results (Jenson, 1993).
Other Applications Biotechnology is the method of choice, in the case of dairy industry starter cultures, to increase the efﬁciency of substrate conversion, regulate the production of ﬂavor-enhancing metabolites, and increase the ability to produce natural inhibitory substances (bacteriocins) and proteolytic enzymes (Coffey et al., 1994).
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Using traditional techniques such as selective breeding, signiﬁcant crop and animal improvements have been produced for hundreds of years. The typical crop improvement cycle takes 10 to 15 years to be completed and includes germplasm manipulations, genotype selection and stabilization, variety testing, variety increase, proprietary protection, and crop production stages. However, in recent years, a new approach, agricultural biotechnology, has emerged and is a discipline that can contribute to most of these crop improvement stages (Pauls, 1995). This powerful tool has resulted in further improvement in both crop production and crop quality. Originally, the main task of crop breeding was to achieve a high and stable yield potential. Of course, this potential is linked to other characteristics such as resistance to diseases, plagues, drought, etc. (Knorr, 1987). In a similar way, the ﬁrst projects in agricultural biotechnology were aimed at obtaining varieties with improved agronomic characteristics such as herbicide-tolerant crops and insect-protected crops (Liu, 1999). Two general strategies have been used: 1. Gene addition, in which cloning is used to alter the characteristics of a plant by providing it with one or more new genes 2. Gene subtraction, in which genetic engineering procedures are used to inactivate one or more of the genes already present in the plant (Brown, 1995) Following are some examples of the application of the gene addition approach: • Crops with herbicide tolerance. The main research is aimed at developing crops with tolerance to broad-spectrum herbicides such as RoundUp‘, Liberty‘, and imidazolinone. RoundUp is a rapidly degradable herbicide with low toxicity to animals and humans. The active ingredient in this broad-spectrum herbicide is glyphosate, which binds speciﬁcally to the enzyme 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), which is responsible for the synthesis of aromatic amino acids in plants. The Monsanto company research team has successfully expressed in different plants a gene encoding a glyphosate-insensitive EPSPS, conferring tolerance to RoundUp and providing in this way superior weed control with less total herbicide use. The bioengineered plants are known as RoundUp-ready crops, and the ﬁrst plants to be introduced on the market were soybeans, corn, cotton, and canola (Liu, 1999). • Insect-protected crops. The soil bacterium Bacillus thuringiensis produces an insecticidal protein generically known as Bt protein. The protein binds to speciﬁc receptors located on the gut membrane of © 2003 CRC Press LLC
certain insects, mainly caterpillars, interfering with the ion transport system so the insect cannot feed and eventually dies. The main advantage of this Bt protein is that it is innocuous to humans, animals, birds, and beneﬁcial insects such as honeybees. The insertion of the Bt gene in plants allows them to produce this protein and to have permanent protection against sensitive insects. This technique has been so successful that varieties of insect-protected cotton, potato, corn, and sweet corn are already present in the market. • Virus-resistant crops. Viral diseases are one of the leading causes of crop losses. China and the U.S. are the pioneers in the area of preventing such losses, having commercialized virus-resistant tobacco and tomato and later mosaic-virus-resistant squash and watermelon. • Other improved crops. A variety of corn tolerant to severely alkaline soils has been developed by Garst Seed Company, and different types of resistance to microbial pathogens have also been reported (Pauls, 1995). In addition to crops with improved agronomic traits, a limited number of genetically modiﬁed varieties have improved quality traits. The modiﬁcations are directed at the main components (oil, proteins, and carbohydrates), although modiﬁcation in vitamin content, texture, and color has also been done.
Modiﬁcation of Lipid Metabolism Vegetable oils are among the world’s most important plant products. An oil has three main quality attributes: nutritive value, oxidative stability, and functionality. No oil is considered to be optimal in all three attributes, so modiﬁcation of an oil’s composition is one of the targets of biotechnology. The two main strategies are alteration of a major fatty acid level and creation of an unusual fatty acid (Liu, 1999). Alteration of a Major Fatty Acid Level The predominant fatty acids of vegetable oils found in nature consist of just six or seven structures that have chain lengths of 16 or 18 carbons and one to three double bonds. These fatty acids are synthesized from acetylCoA by a series of reactions. The assembly of fatty acids and the introduction of the ﬁrst double bond occurs while these structures are attached to an acyl-carrier protein (ACP). The fatty acids are released from the ACP by the action of a speciﬁc thioesterase and can cross the envelope membrane of the plastid in which the reactions take place. After this, the fatty acids are reesteriﬁed to CoA. Further reactions such as desaturation to introduce additional double bonds and triacylglycerol formation are believed to occur by means of membrane-bound enzymes in the endoplasmic reticulum (Ohlrogge, 1994). © 2003 CRC Press LLC
An example of oil modiﬁcation by this technique is the development of high-oleic-acid soybean oil by DuPont through antisense suppressing and/ or cosuppression of oleate desaturase. The new oil has an oleic acid content of 80% or higher, compared with 24% in normal soybean oil (Liu, 1999). The crop looks so promising that about 50,000 acres were planted in 1998. Because it is more stable, this oil does not require hydrogenation for use in frying or spraying, which reduces processing costs and also avoids the formation of trans fatty acids, which are associated with high cholesterol levels. In addition, this new oil has a longer useful life, which is desirable in the fastfood industry (Riley and Hoffman, 1999). In the case of sunﬂower, the modiﬁed crop is known as mid-oleic sunﬂower, which has a modiﬁed fatty acid proﬁle. It was grown on 100,000 acres in the U.S. in 1998. The seed produces low saturated fat oils with 60 to 75% oleic acid, compared with 16 to 20% in normal oil. This product has the potential to replace cottonseed and partially hydrogenated soybean oils in frying and salad oils. Because the mid-oleic sunﬂower has higher yields than the standard or high-oleic varieties (77 to 89% oleic acid), this type is expected to be preferred in the future (Riley and Hoffman, 1999). The above examples also address the dietary goal of reducing saturated fatty acid intake. Vegetable oils generally contain far less saturated fatty acids than the 40 to 50% found in animal fats and are considered adequate for reducing cholesterol levels. However, most vegetable oils still contain 10 to 20% saturated fatty acids. One strategy to reduce this level of fat is based on increasing the levels of the enzyme that catalyzes the elongation of palmitoyl-ACP to stearoyl-ACP. The overexpression of 3-ketoacyl-ACPsynthase II in transgenic Brassica napus resulted in reduced levels of palmitic acid. A second strategy of reducing the activity of the acyl-ACP thioesterase has been done in soybeans by means of cosuppression. Transformation of soybeans with an additional acyl-ACP thioesterase gene resulted in a reduction of this enzyme activity and a reduction of 50% in the levels of saturated fatty acids in somatic embryos. A third approach is to transform plants with additional membrane-bound desaturases that can convert saturated fatty acids to unsaturated. This approach has been successful in the case of tobacco by introducing genes from rat or yeast in the cells (Ohlrogge, 1994). The opposite case is that of trying to increase the level of saturated fatty acids. In many cases, the purpose of this transformation is the creation of an alternative to vegetable oil hydrogenation in the manufacture of margarines and shortenings. An increased stearic acid content has been achieved in canola and soybean oils by using a strategy similar to that used in the case of high-oleic soybean (antisense expression of the stearoyl-ACP-desaturase gene). The new oil would contain more than 30% stearate, compared with about 2% in regular canola oil and around 4% in normal soybean oil. It is targeted to replace hydrogenated oils in margarine and liquid shortenings because it contains no trans derivatives (Liu, 1999). © 2003 CRC Press LLC
Production of Unusual Fatty Acids Unusual fatty acids include short-, medium-, and very-long-chain fatty acids and those having double bonds at an unusual position or carrying a hydroxy or epoxy group. The introduction of these fatty acids in conventional vegetable oils would turn them into valuable products with high prices. The strategy in this case is to transfer the genes encoding the key biosynthetic enzymes into oilseeds. A classic example of this approach is Laurical‘, a high-lauric canola developed by Calgene. Normal canola oil contains no lauric acid. By introducing the acyl-ACP thioesterase gene from the California bay laurel tree into the canola cells, a new modiﬁed oil with 38% or more laurate can be obtained. This novel oil could replace the more expensive coconut or palm kernel oils as sources of this fatty acid and as an alternative to cocoa butter (Liu, 1999; Riley and Hoffman, 1999). An interesting fatty acid modiﬁcation that reduces the need for hydrogenation and at the same time increases unsaturation in diets is the production of petroselinic acid-rich vegetable oils. Petroselinic acid is an isomer of oleic acid in which the position of the double bond has changed, resulting in a shift in the melting point from 12 to 33°C. This property means that the oils rich in this isomer are unsaturated oils that are solid at room temperature and therefore are ideal substitutes for margarines and shortenings. The strategy in this case is more complex, as apparently three genes are involved in the production of high levels of this fatty acid in the case of coriander and other Umbelliferae; however, studies are in progress (Ohlrogge, 1994).
Protein Modiﬁcation The main modiﬁcations in this case are directed to increasing the content of an essential amino acid and improving the functionality of a protein in some crops, although soybeans with improved animal nutrition that bolster the protein and amino acid content of soybean meal are near commercial introduction. The increase in the content of essential amino acids is not easy because the information for seed storage protein genes is encoded in several genes in a complex way. A possible strategy involves the transfer of a gene encoding a protein rich in methionine (in the case of legumes) or lysine (in the case of cereals) from other species. In this way, DuPont is developing transgenic soybeans by expressing a methionine-rich zein protein from corn with an 80 to 100% increase in methionine.
Carbohydrate Modiﬁcation This type of modiﬁcation can also produce interesting varieties. For example, high-sucrose beans that have a better taste (less “beany”) and greater digestibility were introduced recently, and about 25,000 acres were planted in the U.S. in 1998 (Riley and Hoffman, 1999). Starch modiﬁcation in © 2003 CRC Press LLC
potatoes is also an interesting application of plant biotechnology. One of the key enzymes in starch synthesis is ADP glucose pyrophosphorylase. This enzyme is subjected to feedback regulation but, if a bacterial gene that encodes for a feedback-insensitive enzyme is cloned in the potato, a transgenic plant with an average increase of 20 to 30% in starch content is obtained. This new potato provides french fries with more potato ﬂavor, improved texture, reduced energy content, and a less greasy taste (Stark et al., 1996).
The Gene Subtraction Approach This modiﬁcation actually does not involve the removal of a gene, but merely its activation. The main procedure to achieve this goal is antisense technology. This technique is a powerful tool that can limit or eradicate the expression of speciﬁc genes by sequence-directed targeting of messenger RNA. The two main strategies for this are antisense RNA and peptide nucleic acid (PNA) procedures. In the ﬁrst case, the gene to be cloned is ligated into the vector in reverse orientation, so when this cloned “gene” is transcribed the RNA that is synthesized is the reverse complement of the mRNA produced from the normal version of the gene. This reverse complement is known as antisense RNA, or asRNA, and is able to prevent synthesis of the product of the gene it is directed against (Brown, 1995). In the second case, PNA is a DNA mimic in which the nucleotides are attached to a pseudopeptide backbone. PNA is able to hybridize with complementary DNA or RNA and the complex is very stable, thus PNA is a good option for use in antisense technology (Good and Nielsen, 1998). Some examples of the application of this technology follow.
Improving Food Quality The ﬁrst whole-food product of modern technology to go on the market (in the U.S. in 1994) was a genetically modiﬁed tomato, the FlavrSavr‘ tomato produced by Calgene, Inc. The ﬁrst product to arrive in U.K. supermarkets (in 1996) was a tomato puree manufactured from genetically modiﬁed tomatoes (increased-pectin tomatoes) produced by Zeneca Plant Science. Tomatoes are usually harvested while they are still green and later treated with ethylene for ripening to occur. This approach makes sure that the fruits remain ﬁrm during transportation and storage, thus avoiding softening and loss of product. The tomatoes turn soft, and ﬁnally rot when the enzyme polygalacturonase breaks down the pectin that holds the cell walls together. Both of these types of genetically modiﬁed tomatoes have been produced by switching off most of the polygalacturonase production, and the tomatoes soften more slowly. Antisense technology is the strategy used in this case. Because the rate of softening has been slowed, the modiﬁed tomatoes can remain longer on the plant to develop their full ﬂavor and color but stay © 2003 CRC Press LLC
ﬁrm enough to be transported to the market. In the case of the tomato puree, the need for the addition of thickeners is minimized during its manufacture (John Innes Centre, 1998). Another example of the application of antisense technology is the prevention of enzymatic browning in potatoes. The development of brown discoloration in a wide range of fruit and vegetables reduces consumer acceptability and is thus of signiﬁcant economic importance to the farmer and to the food processor. In one study (Bachem et al., 1994), potato internode explants were transformed with Agrobacterium tumefaciens containing antisense-polyphenol oxidase (PPO) Ti plasmid constructs. It was shown that antisense inhibition of PPO gene expression abolishes discoloration after bruising of potato tubers in individual transgenic lines grown under ﬁeld conditions. Using appropriate promoters to express antisense PPO RNA, melanin formation can be speciﬁcally inhibited in potatoes. This lack of bruising sensitivity in transgenic potatoes and the absence of any apparent detrimental side effects open up the possibility of preventing enzymatic browning in a wide variety of food crops without having to use treatments such as heating or the use of antioxidants.
Other Improvements A new crop known as low-phytate or low-phytic-acid corn, providing increased availability of phosphorus, will be marketed this year. Researchers claim nutraceuticals (also called functional foods) could conceivably provide immunity to disease or improve the health characteristics of traditional food, and they can be produced by plant biotechnology. An example would be canola oil with a high b-carotene content (Riley and Hoffman, 1999).
The Beneﬁts of Biotechnology In summary, we can say that the many beneﬁts offered by biotechnology include: • Agricultural beneﬁts. It is possible to identify speciﬁc genetic characteristics, isolate them, and transfer them to valuable crop plants. • Environmental beneﬁts. The need for pesticide application is reduced because the plants have the ability to protect themselves from certain pests and diseases. Water usage, soil erosion, and greenhouse gas emissions are reduced through more sustainable farming practices. The productivity of marginal cropland is improved. • Food quality beneﬁts. Biotechnology allows for enhancements in the quality of foods, from increasing crop yields to delaying ripening for better transportability. In the future, consumers may enjoy better taste and nutrition through reduction of undesirable
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characteristics such as saturated fats in cooking oils, elimination of allergens and toxicants, and increases in vitamins and other nutrients; nutraceuticals could even help reduce the risk of chronic diseases (Monsanto, 2000). Despite the beneﬁts, the subject of biotech foods is still a very controversial one and will have to be addressed soon, as it involves a variety of complex concepts and legal principles (Korwek, 2000).
Transformation Techniques in Plant Biotechnology
Recent advances in transformation technology have resulted in the routine production of transgenic plants for an increasing number of crop species. The three main cloning vector systems for higher plants are: (1) the Ti plasmid of Agrobacterium tumefaciens, (2) plant viruses such as the caulimoviruses and gemniviruses, and (3) direct gene transfer using DNA fragments not attached to a plant cloning vector (Brown, 1995). Cereals comprise a commercially valuable group of plants that could beneﬁt from the introduction and expression of foreign genes, so research has been done directed toward the development of regulatory systems for use in cereal transformation. Among these, the reporter genes used in cereal transformation to analyze gene expression are very important. Some of the currently used reporter genes are b-glucuronidase from Escherichia coli (one of the most popular reporter genes), luciferase from the ﬁreﬂy, and anthocyanin regulators from corn. The choice depends on the particular experiment, as all three genes have advantages and disadvantages (McElroy and Brettell, 1994). Rice was the ﬁrst major monocotyledonous crop species to be transformed and regenerated. Initially, rice transformation was limited to the Oryza sativa subsp. japonica cultivars. Subsequently, a number of indica and javanica cultivars have also been transformed and regenerated into fertile transgenic plants. Most transformation studies in rice have used direct DNA uptake into protoplasts, induced by polyethylene glycol (PEG) treatment or electroporation. Also, other transformation methods have been developed that are less genotype dependent, such as microprojectile bombardment of cell suspensions and immature embryos. Agrobacterium-mediated gene delivery has also been successful in producing stable transformants. An advantage of working with rice is its relatively small genome, 4.2 ¥ 108 bp per haploid genome. This is in marked contrast to other cereals such as corn and wheat, which have 4 ¥ 109 and 1.7 ¥ 1010 bp per haploid genome, respectively (Ayres and Park, 1994). Barley has been one of the most recalcitrant crop species in regard to this technology. A prerequisite for successful transformation of barley leading to © 2003 CRC Press LLC
improved quality, tolerance, or resistance is that the desired characteristic, as stated before, must be coded by a single gene. Examples of genes for quality traits are the gene egl1 of Trichoderma reesei, coding for a thermostable b-glucanase, or a hybrid gene of Bacillus amyloliquefaciens and B. macerans, also coding for b-glucanase functional at high temperatures. This transformation has been successfully done so the gene was under germinationspeciﬁc control. The most common reporter genes used for barley are the ones for b-glucuronidase, kanamycin resistance, chloramphenicol resistance, phosphinotricin (herbicide) resistance, and luciferase. The gene transfer methods that have been successful for barley transformation are Agrobacterium infection, particle bombardment, PEG treatment of protoplasts, electroporation of protoplasts, and laser perforation. All these methods have produced stable gene expression, though particle bombardment has proven to be the best method in cereal transformation, including barley. Using these methodologies, it is possible to deliver foreign genes to barley and to regenerate fertile transgenic barley plants. The foreign genes are stably integrated into the barley genome and are inherited following the laws of Mendel (Mannonen et al., 1994). In the case of legumes, Agrobacterium infection and particle bombardment have been the most successful transformation methods. The ﬁrst method has been used in chickpeas, lentils, alfalfa, peas, and cowpeas; the second method has been preferred for peanuts, soybeans, and common beans. In most cases, a transgenic plant has been obtained (Christou, 1994).
Although many researchers had previously studied the behavior of animal cells in vitro, the ﬁrst application of such cells which led to a useful product was the production in 1949 of the polio virus. Today, many products are generated from cultured animal cells, including virus vaccines, cellular chemicals (interferons, interleukin-2, thymosin, urokinase), immunobiologicals (monoclonal antibodies), hormones (e.g., growth hormone, prolactin, ACTH), and virus predators (insecticides), among others. While it is unlikely that animal-cell-based processes for the generation of live virus vaccines will be superseded by processes based on genetically engineered bacteria, it is clear that some animal cell products such as insulin and interferons can be produced from modiﬁed bacteria (Spier, 1987). However, transgenic animals are becoming more common and useful tools because they provide an in vivo look at the capabilities and impact of foreign gene expression in a biological system. Expression of the gene of interest is controlled by DNA promoter elements that direct where and when the gene product will be expressed in the animal. For example, the expression of a transgene in the mammary gland of an animal requires the use of a promoter and regulatory © 2003 CRC Press LLC
regions of a milk protein gene (i.e., sequences that direct gene expression only in the mammary gland and only during lactation). In this particular case, most of the work has been concerned with the biological production of important and active proteins (such as pharmaceuticals) in the milk of a transgenic animal with the intent of recovering the protein of interest from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk, resulting in novel properties useful from a nutritional or technological point of view. Of the several candidates for altering the properties of milk, one of them is human lysozyme. If this protein were present in bovine milk at a signiﬁcant level, it could help to reduce the overall level of bacteria in this product and, because of its positive charge, it could interact with the negatively charged caseins to produce a reduction of the rennet clotting time. Another candidate could be k-casein. The addition of more bovine k-casein to the milk could also affect the physical properties of this food. It could increase the thermal stability of casein aggregates and act to decrease the size of the casein micelles. A smaller diameter would lead to a larger available surface area, which would result in a more consistent and ﬁrmer curd as well as an increase in cheese yield. Human lactoferrin could be also a good candidate for improving the properties of milk. Lactoferrin is the major iron-binding protein in milk and is responsible for the high bioavailability of milk iron. It also inhibits the growth of bacteria in the mammary gland and the intestines of infants. Lactoferrin can also act as an antioxidant, inhibiting the formation of toxic oxygen radicals. The presence of human lactoferrin could also enhance the association with caseins, increasing rennet gel strength and possibly cheese yield. It can be seen that transgenic livestock has a promising future (Maga and Murray, 1995).
From all of the above, it becomes clear that food biotechnology is an old tradition and a signiﬁcant challenge for the future. Many genetically modiﬁed foods are under development and soon will be on the market. It is the role of the researchers to ensure that all these new food biotechnology products are safe for the environment and to animal and human health. Finally, and according to the reports found in scientiﬁc journals and meetings proceedings, for example, the trends in research in the ﬁeld of food biotechnology include: DNA sequencing of the genomes of several organisms, developing of food-grade vectors to be used in microorganisms of food interest (lactic acid bacteria, biﬁdobacteria, yeasts, etc.), development of new biotech crops, and, of course, recombinant DNA biotechnology-derived foods as part of the continuing efforts to improve the food supply. © 2003 CRC Press LLC
Ayres, N.M. and Park, W.D. (1994) Genetic transformation of rice, Crit. Rev. Plant Sci., 13(3), 219–239. Bachem, C.W.B., Speckmann, G.J., van der Linde, P.C.G., Verheggen, F.T.M., Hunt, M.D., Steffens, J.C., and Zabeau, M. (1994) Antisense expression of polyphenol oxidase genes inhibits enzymatic browning in potato tubers, Bio/Technology, 12, 1101–1105. Beppu, T. (1983) The cloning and expression of chymosin (rennin) genes in microorganisms, Trends Biotechnol., 1(3), 85–89. Brown, J.C.S., Ward, L.J.H., and Davey, G.P. (1994) Rapid isolation and puriﬁcation of lactococcal bacteriophage DNA without the use of caesium chloride gradients, Lett. Appl.. Microbiol., 18, 292–293. Brown, T.A. (1995) Gene Cloning: An Introduction, 3rd ed., Stanley Thornes Publishers, Cheltenham, U.K., pp. 295–311. Brussow, H., Fremont, M., Bruttin, A., Sidoti, J., Constable, A., and Fryder, V. (1994) Detection and classiﬁcation of Streptococcus thermophilus bacteriophages isolated from industrial milk fermentation, Appl. Environm. Microbiol., 60, 4537–4543. Christou, P. (1994) The biotechnology of crop legumes, Euphytica, 74, 165–185. Coffey, A.G., Daly, C., and Fitzgerald, G. (1994) The impact of biotechnology on the dairy industry, Biotechnol. Adv., 12, 625–633. Djordjevic, G.M. and Klaenhammer, T.R. (1997) Genes and gene expression in Lactococcus bacteriophages, Int. Dairy J., 7, 489–508. Froseth, B.R. and McKay, L.L. (1991) Molecular characterization of the nisin resistance region of Lactococcus lactis subsp. lactis biovar diacetylactis DRC3, Appl. Environm. Microbiol., 57(3), 804–811. Froseth, B.R., Herman, R.E., and McKay, L.L. (1988) Cloning of nisin resistance determinant and replication origin on 7.6-kilobase EcoRI fragment of pNP40 from Streptococcus lactis subsp. diacetylactis DRC3, Appl. Environm. Microbiol., 54(8), 2136–2139. Garrison, S.J. and dePamphilis, C. (1994) Polymerase chain reaction for educational settings, Am. Biol. Teacher, 56(8), 476–481. Garvey, P., Fitzgerald, G.F., and Hill, C. (1995) Cloning and DNA sequence analysis of two abortive infection phage resistance determinants from the lactococcal plasmid pNP40, Appl. Environm. Microbiol., 61, 4321–4328. Garvey, P., Hill, C., and Fitzgerald, G.F. (1996) The lactococcal plasmid pNP40 encodes a third bacteriophage resistance mechanism, one which affects phage DNA penetration, Appl. Environ. Microbiol., 62, 676–679. Good, L. and Nielsen, P.E. (1998) Antisense inhibition of gene expression in bacteria by PNA targeted to mRNA, Nat. Biotechnol., 16, 355–358. Hernández-Sánchez, H. (1994) Obtainment by Electroporation of Nisin-Resistant Mutants of Lactococcus lactis subsp. lactis LM0230, Ph.D. thesis, Escuela Nacional de Ciencias Biológicas, IPN, México. Hill, W.E. (1996) The polymerase chain reaction: applications for the detection of foodborne pathogens, Crit. Rev. Food Sci. Nutr., 36(1/2), 123–173. Hoban, T.J. (1999) Consumer acceptance of biotechnology in the U.S. and Japan, Food Technol., 53(5), 50–53.
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Hughes, B.F. and McKay, L.L. (1992) Deriving phage-insensitive lactococci using a food-grade vector encoding phage and nisin resistance, J. Dairy Sci., 75, 914–923. Institute of Food Technologists (2000) IFT expert report on biotechnology and foods, Food Technol., 54(8), 124–136. Jenson, I. (1993) Biotechnology and the food supply: food ingredient products of biotechnology, Food Australia, 45(12), 568–571. John Innes Centre (1998) Biotech Bytes: Food Biotechnology, http://www.jic.bbsrc.ac.uk/exhibitions/bio-future/. Knorr, D. (1987) Food biotechnology: its organization and potential, Food Technol., 41(4), 95–100. Korwek, E.L. (2000) Labeling biotech foods: opening Pandora’s box?, Food Technol., 54(3), 38–42. Liu, K. (1999) Biotech crops: products, properties and prospects, Food Technol., 53(5), 42–49. Maga, E.A. and Murray, J.D. (1995) Mammary gland expression of transgenes and the potential for altering the properties of milk, Bio/Technology, 13, 1452–1457. Mannonen, L., Kauppinen, V., and Eneri, T.M. (1994) Recent developments in the genetic engineering of barley, Crit. Rev. Biotechnol., 14(4), 287–310. McElroy, D. and Brettell, R.I.S. (1994) Foreign gene expression in transgenic cereals, Trends Biotechnol., 12, 62–68. McKay, L.L. and Baldwin, K.A. (1984) Conjugative 40-megadalton plasmid in Streptococcus lactis subsp. diacetylactis DRC3 is associated with resistance to nisin and bacteriophage, Appl. Environ. Microbiol., 47, 68–74. McKee, T. and McKee, J.R. (1999) Biochemistry: An Introduction, 2nd ed., McGrawHill, Boston, MA, pp. 608–610. Monsanto (2000) An invitation for Dialogue: Biotechnology and the Food Industry, http:/ /www.fooddialogue.com. Monteville, M.R., Ardestani, B., and Geller, B.R. (1994) Lactococcal phages require a host cell wall carbohydrate and a plasma membrane protein for adsorption and ejection of DNA, Appl. Environm. Microbiol., 60, 3204–3211. Ohlrogge, J.B. (1994) Design of new plant products: engineering of fatty acid metabolism, Plant Physiol., 104, 821–826. Pauls, K.P. (1995) Plant biotechnology for crop improvement, Biotechnol. Adv., 13(4), 673–693. Riley, P.A. and Hoffman, L. (1999) Value-enhanced crops: biotechnology’s next stage, Agricultural Outlook, March, 18–25. Sing, W.D. and Klaenhammer, T.R. (1990) Characteristics of phage abortion conferred in lactococci by the conjugal plasmid pTR2030, J. Gen. Microbiol., 136, 1807–1815. Sing, W.D. and Klaenhammer, T.R. (1991) Characterisation of restriction-modiﬁcation plasmids from Lactococcus lactis ssp. cremoris and their effects when combined with pTR2030, J. Dairy Sci., 74, 1133–1144. Smith, J.E. (1988) Biotechnology, 2nd ed., New Studies in Biology,: Edward Arnold, London, pp. 44–59. Sørensen, K.I., Larsen, R., Kibenich, A., Junge, M.P., and Johansen, E. (2000) A foodgrade cloning system for industrial strains of Lactococcal lactis, Appl. Environm. Microbiol., 66, 1253–1258. Spier, R.E. (1987) Processes and products dependent on cultured animal cells, in Basic Biotechnology, J. Bu’lock and B. Kristiansen, Eds., Academic Press, London, pp. 509–524. © 2003 CRC Press LLC
Stark, D.M., Barry, G.F., and Kishore, G.M. (1996) Improvement of food quality traits through enhancement of starch biosynthesis, in Engineering Plants for Commercial Products and Applications, G.B. Collins and R.J. Shepherd, Eds., Annals of the New York Academy of Sciences, 792, 26–36. Su, P., Im, H., Hsieh, H., Kang’a, S., and Dunn, N.W. (1999) LlaFI, a type III restriction and modiﬁcation system in Lactococcus lactis, Appl. Environm. Microbiol., 65, 686–693. Valyasevi, R., Sandine, W.E., and Geller, B.L. (1991) A membrane protein is required for bacteriophage c2 infection of Lactococcus lactis subsp. lactis C2, J. Bacteriol., 173, 6095–6100.
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Ali Asaff-Torres and Mayra De la Torre-Martínez
CONTENTS Introduction Food Biotechnology Products Design Generalities Product/Process Development Chain Screening/Selection Biocatalysis/Bioconversion De Novo Synthesis Biological System Improvement Classical r-DNA Metabolic Engineering Laboratory Fermentation Physiology Choice of Substrate Fermentation Regime Pilot-Plant Fermentation Reactor Design Downstream Processing Screening Units Unit Design Stream-Splitting Units Fractionating Units Formulation Economic Analysis Cost Estimates Total Capital Investment Single Cell Production: An Example of Bioprocess Design Laboratory Fermentation Scale-Up
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Downstream Processing Cell Separation Thermolysis Final Design Economic Analysis Conclusions References
Most bioprocess design problems are complex, as they usually involve batch, semi-batch, continuous, and cyclic operations. Also, bioprocess performance is affected by biological variability, highly interactive unit operations, multiple processing steps and options, and complex feedstream physical properties. In some instances, design decisions are made on the basis of rules of thumb or heuristics, while in others design information exists so that a unique design solution may be obtained. Often, the design solution will represent a compromise based, for example, on a target selling price where a trade-off between yield and purity must be sought. Identifying the best design and also determining the relative sensitivity of the solution to the variables governing a system are not trivial tasks (Woodley et al., 1996). The variable, or the most important aspect to consider in synthesis and bioprocess design, is always the primary design issue; however, a unique solution does not exist, because possible solutions depend upon the intrinsic characteristics of the biological system, the selling price, consumer demand, or product speciﬁcations. This chapter discusses the steps followed during bioprocess development and synthesis, together with the most important considerations regarding technical and economic constraints and possibilities. Because literature about equipment design and cost estimation is widely available, these aspects will not be analyzed in detail in this chapter. At the end of the chapter, a case study illustrates bioprocess synthesis.
Food Biotechnology Products
Nowadays, traditional and modern biotechnology processes are used to produce a wide variety of foods and food additives. Ethanol, fermented milk products, amino acids, vitamins, enzymes, polysaccharides such as xanthan, single-cell protein, colorants, ﬂavors and fragrances, and various acids such © 2003 CRC Press LLC
as citric, lactic, acetic, butyric, and glutamic are some of the products obtained by fermentation processes. The scale of manufacture ranges from billions of gallons of ethanol produced at a cost of one to two dollars per gallon to high-value products such as ﬂavors, fragrances, and vitamins with manufacturing costs of hundreds to thousands of dollars per kilogram. The actual process employed commercially depends on a variety of factors, including economics, process and product patents, raw material availability, and manufacturing equipment. In some cases, traditional processes are employed along with technological advances — for example, in the manufacture of cheese and fermented milk products, beer, and wine. Obtaining products regulated by food and drug agencies may change the manufacturing processes proposed and may involve costly clinical trials. These processes are often not optimized from a production standpoint (Blanch and Clark, 1997).
Currently, a large amount of literature exists on systematic process synthesis and design methods for chemical processing. The same abundance of literature does not exist for bioprocess synthesis techniques (Zhou and TitchenerHooker, 1999; Steffens et al., 2000). Generally, process synthesis and design procedures, can be classiﬁed as: • Heuristic methods • Algorithmic techniques • Expert systems Heuristic methods use the rules acquired through the knowledge and experience gained from similar processes or previous designs and are most often employed for bioprocess development; such methods are not always the best for optimal design. Algorithmic techniques and expert systems are not often used for synthesis and bioprocess design, because biological systems are highly complex. Typical ﬂowsheeting packages are often inappropriate for bioprocess operations, which are complicated by the mixture of batch and semi-continuous modes of processing and by strong interactions among unit operations (Gristis and Titchener-Hooker, 1989; Bulmer et al., 1996), in addition to other obstacles such as: • Unit operations commonly used for downstream puriﬁcation generally separate components non-sharply. A non-sharp separation is one in which all of the feed components are distributed into the efﬂuent streams.
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• A relatively large range of unit operations to be used for a particular separation is available, thus making decisions difﬁcult. • Biological streams generally contain a large number of compounds. The above-mentioned situations lead to a relatively large number of alternative, feasible ﬂowsheets and, therefore, a correspondingly large search space for the synthesis algorithm (Fraga, 1996; Lienqueo et al., 1996). Synthesis problems of this nature are difﬁcult to solve using numerical optimization techniques; however, important advances in this ﬁeld have occurred in the past few years. Petrides (1994), Petrides et al. (1995), Lienqueo et al. (1996), Gregory et al. (1996), Zhou et al. (1997), and Steffens et al. (2000) have carried out interesting work using expert knowledge and exploiting differences in physical properties to select unit operations and synthesize economically favorable processes and ﬂow sheets. The success of biochemical compound manufacturing companies depends on the ability to design an efﬁcient economical optimal manufacturing process (Leser and Asenjo, 1992). New product development requires an interdisciplinary joint effort including people from several departments of a company (Figure 3.1) who will carry out research and development (at both the laboratory and pilot-plant levels), synthesis and scale-up processes, market and/or ﬁeld trials, project implementation, and production on an industrial scale. Administration Council
Development & Research Laboratory
Quality Control Laboratory
FIGURE 3.1 Relationships among departments to develop a new product. © 2003 CRC Press LLC
Product/Process Development Chain
Before starting a process or the development of a product, it is necessary to determine the feasibility of a project. The most relevant questions are (Kossen, 1994): 1. Does the product and its method of production meet the standards for health, safety, and the environment? 2. Are problems expected due to legislation or the attitude of the public? 3. Do we have an acceptable patent position? 4. Do we have the necessary tools to make the product? 5. Do we know the market (potential clients and competitors)? 6. Does the product meet the standards of potential clients? 7. Will the product be on the market in time? 8. Will the product result in an acceptable proﬁt for the company? Design involves research of process options and matching these against speciﬁc objectives. The ﬁrst step in process synthesis is to develop a block diagram with the main stages. Then, possible alternative operations are selected for each stage. Bioprocess ﬂowsheets are often synthesized in a sequential fashion, proceeding from one unit to the next until product speciﬁcations are known. Individual units are subsequently optimized to improve plant performance. Although this approach may produce economically adequate processes, alternative designs may be more proﬁtable. To avoid this problem, a systematic synthesis and design procedure that considers the overall process rather than the individual units is required (Wheelwright, 1987). To achieve high overall process efﬁciency requires optimization of the complete sequence of upstream processing operations, fermentation, subsequent downstream processing operations, and an integrated approach to design (Samsalatli and Shah, 1996). The interactions between operations can be signiﬁcant in determining overall process performance (Kelly and Hatton, 1991; Narodoslawsky, 1991; Clarkson et al., 1993; Middelberg, 1995). However, to achieve an overall process performance is not an easy task, because it will depend on the degree of knowledge of the biological system employed and ﬁnal product speciﬁcations. This knowledge comes from the research and development laboratories and pilot plant (experimental data on physiology, kinetics, etc.), while other design data are acquired from the literature or past experience. Then, synthesis and bioprocess design are developed through sequential stages that Kossen (1994) calls the product/process development chain (PDC) (Figure 3.2), which includes the following steps. © 2003 CRC Press LLC
Biological system improvement
Pilot plant fermentation
FIGURE 3.2 Product/process development chain (PDC). (Modiﬁed from Kossen, N.W.F., in Advances in Bioprocesses Engineering, E. Galindo and O. T. Ramírez, Eds., Kluwer, Dordrecht, 1994, pp. 1-11.)
Screening/Selection During this stage, a screening of the possible procedures available for obtaining the product of interest must be considered in terms of particular chemical characteristics and the costs and availability of raw materials. To illustrate, consider the case of vanillin production. Vanillin is one of the most important aromatic ﬂavor compounds used in foods, beverages, perfumes, and pharmaceuticals and is produced on a scale of more than 10,000 tons per year through chemical synthesis. Alternative biotechnology-based approaches for vanillin production are based on bioconversion of lignin, phenolic stilbenes, isoeugenol, eugenol, ferulic acid, or aromatic amino acids and on de novo biosynthesis, applying fungi, bacteria, plant cells, or genetically engineered microorganism from sugars as glucose (Priefert et al., 2001). Biocatalysis/Bioconversion Inexpensive, readily available, and renewable natural precursors, such as fatty or amino acids, can be converted to more highly valued products. Biocatalysis competes best with chemical catalysis in the following types of reactions (Krings and Berger, 1998): • Introduction of chirality • Functional of chemically inert carbons • Selective modiﬁcations of one functional group in multifunctional molecules • Resolution of racemates © 2003 CRC Press LLC
Biocatalysis is carried out by cultured plant cells or plant callus tissue, prokaryotic or eukaryotic microorganisms, or isolated enzymes as biocatalyst. De Novo Synthesis A wide range of industrial bioprocesses employ whole cells (plants, animals, or microorganisms that catabolize carbohydrates, fats, and proteins) to produce various metabolites starting from single or complex substrates, through de novo synthesis. This property is traditionally used during the production of fermented foods with their amazing number of aroma chemicals.
Biological System Improvement Once selected, the biological system subsequently will be examined to improve its product yield and performance. Major goals are enhanced production of metabolites (e.g., amino acid and antibiotic production) and improved properties of starter cultures, such as in dairy product production (e.g., reproducible growth characteristics, increased ﬂavor formation and proteolytic activities, or better autolytic properties). In other cases strains are improved to adapt them to industrial operating conditions, such as temperature, pH, mechanical stress, high substrate and/or product concentrations, etc. Methods employed for the biological system improvements are: • Classical • r-DNA • Metabolic engineering Classical This method, used for many years, consists of the application of various agents (physical or chemical mutagens), causing mutations in the genes of the microorganism and changing its cellular metabolism. After screening, those strains that better fulﬁll the objectives sought are selected; however, this process is totally random, and the probability of success in many cases is slim. A screening of strains or cellular lines with improved characteristics is also utilized. r-DNA Since the 1970s, advances in genetics and molecular biology have been impressive. Recombinant DNA techniques have allowed the creation of a great quantity of new strains. At the moment, recombinant or genetically modiﬁed microorganisms produce therapeutic and diagnostic proteins, diverse enzymes, and other types of products; however, these systems require some particular attention, such as: © 2003 CRC Press LLC
• Strain maintenance. Any microorganism is always in a constant competition with other microorganisms that share its environment. Recombinant microorganisms compete with wild strains that in time could displace the recombinant one; therefore, in recombinant strains, genes that confer resistance to antibiotics are introduced, and antibiotics are added to broth culture to maintain the strain. In many cases, though, this procedure causes additional problems in puriﬁcation processes, as the antibiotics are pollutants that must be removed. In some cases, this problem is avoided with recombinant strains that protect themselves from the conditions of their environment, such as the case of yeasts resistant to high ethanol concentrations employed during alcohol production. At high ethanol concentrations, wild yeast cannot survive. • Strain stability. The new genetic material that confers the peculiar characteristics of production and resistance to the recombinant microorganism is generally inserted in plasmid, which could get lost during processing. This phenomenon is most critical in continuous reactors with extended times of operation. It will be very important, then, to verify during the investigation stage the stability of the inserted genetic material. • Gene turn on. With the purpose of avoiding a premature metabolic collapse or to take advantage of a physiologic state of the cell, the promoter of the inserted genes can be “turned on” by means of speciﬁc compounds called inductors or by a temperature change. The inductor is added or the temperature changed at the appropriate moment. Metabolic Engineering Research programs to improve industrial microorganisms used for fermentation were initially focused on strain selection after classical mutagenesis, later followed by more direct approaches using genetic engineering. The main drawbacks of these approaches are that they are time consuming, side effects occurring in the selected or constructed strains are difﬁcult to predict and assess, and the full range of engineering possibilities cannot be exploited, due to lack of knowledge about the interrelated regulatory and metabolic processes going on in a cell (Kuipers, 1999). In order to enhance the yield and productivity of metabolite production, researchers have focused almost exclusively on enzyme ampliﬁcation or other modiﬁcations of the product pathway, but overproduction of many metabolites at high yields requires signiﬁcant redirection of ﬂux distribution in the primary metabolism, which may not readily occur following product deregulation because metabolic pathways have evolved to exhibit control architectures that resist ﬂux alterations at branch points. This problem can be addressed through the use of some general concepts of metabolic rigidity, © 2003 CRC Press LLC
including a means for identifying and removing rigid branch points within an experimental framework (Stephanopoulos and Vallino, 1991). The recent burst in activity regarding genome sequences of food-related microorganisms has opened the way for functional genomics approaches with novel analysis techniques that include transcriptome, proteome, and metabolome analysis, as well as structural genomics. DNA micro-arrays (transcriptome) (Graves, 1999), and proteome, which is an improved two-dimensional electrophoresis method, combined with matrix-assisted laser desorption/ionization time of ﬂight mass spectroscopic analysis (MALDI/TOFF) (Blackstock and Weir, 1999), are two powerful tools for genetic analysis. These methods provide information on differential gene expression, global changes in protein production level, and protein–protein interaction through the isolation of protein complexes. The huge amount of experimental data generated is gathered into large databases, the interpretation of which greatly depends on novel bioinformatics methodology that should enable linking of the databases and facilitate the classiﬁcation and interpretation of results. Eventually, researchers would like to integrate all data on the transcriptome and proteome techniques into a metabolic and regulatory model. The combination of transcriptome with proteome and metabolome research and the elucidation of structure–function relationships of biomolecules will eventually result in a true understanding of whole cell functioning (Kuipers, 1999).
Laboratory Fermentation During this stage the best conditions for cellular growth and maintenance as well as metabolites production must be found. Three ﬁelds must be researched: • Physiology • Choice of substrate • Fermentation regime Physiology An understanding of the underlying physiology of the organism can provide insight into the design of a new bioprocess, as well as the base model control of cultivation (Gregory et al., 1996). Metabolites production and microbial growth depend on nutrient availability and physical–chemical factors. Among the most important factors are temperature, pH, light, and sensitivity to mechanical stress. For plant and animal cell cultures, mechanical stress is critical, requiring in some cases immobilization of the cells. It will also be important to know the rheological changes taking place in broth cultures as a consequence of cellular metabolism because such changes can be important at the level of blended and mass transfer, in some cases setting the maximum © 2003 CRC Press LLC
benchmarks of operation. Usually oxygen availability is very important, and certain substrates and products may inhibit growth and product synthesis.
Choice of Substrate The essential elements for cellular growth are carbon, nitrogen, and, to a lesser extent, phosphorus and sulfur. Microorganisms can use a variety of carbon sources, from complex ones, such as starch and wood hydrolizates, to reﬁned sources, such as glucose syrup or sucrose. Trace metal requirements must be met with salt solutions. Complex nitrogen and carbon sources are required in some fermentation where the slow release of nutrients may be important in regulating these metabolisms (Blanch and Clark, 1997). In vegetable cell cultures, the use of speciﬁc compounds such as growth and differentiation factors may also be important. The choice of a substrate will depend on the raw material contribution to the production cost and on its inﬂuence in downstream processing operations, as well as on ﬁnal product speciﬁcations. For high-value products, the raw materials may not comprise a signiﬁcant part of the production cost, but selection of raw material may nevertheless be important in maintaining consistent product quality (Blanch and Clark, 1997). For low-value products, the cost of raw materials determines the economical success of a process such as single-cell protein production, for example.
Fermentation Regime Fermentation can occur under three regimes, the choice of which depends on the particular characteristics of the biological system: • Continuous. This fermentation regime greatly favors the overall process performance because it allows the continuous operation of other equipment, minimizing time-outs and increasing productivity. However, the application of continuous fermentation to biological processes is very limited for a number of reasons, including difﬁculty maintaining the stability of recombinant strains and keeping the equipment sterile for long periods of time, as well as the high sensibility of biological systems that can be altered by minimum interference, with subsequent loss of the stationary state. • Batch. Batch fermentation is the most used regime in bioprocesses and allows maximum substrate conversion. Given the nature of the involved operations, however, time-outs increase substantially compared to continuous operation. Also, when the production line includes continuous operation at points downstream of the reactor, auxiliary equipment must be considered, such as feeding or storing tanks.
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• Feed-batch. In systems with substrate and/or product inhibition, as well as agitation and mass transfer constraints due to changes in broth culture rheology, the feed-batch regime is a good alternative. Feed-batch allows the management of diverse dilution rates through feeding patterns (constant, exponential, pulses, etc.), thus controlling both the microbial speciﬁc growth rate and metabolic state.
Pilot-Plant Fermentation Once a new product has been developed in the laboratory, a process to produce and purify the product on a large scale is required. Historically, most processes in biologically based industries have made recourse to existing processes and relied on the use of pilot-plant facilities in which to test proposed new process sequences. Scale-up at the pilot-plant level is the ﬁrst step in the scaling process and is of vital importance for the later stages and successful bioprocess design. For each unit operation, besides their technical evaluation, it is important to carry out an economic evaluation in such a way that the selection of operation and equipment represents the best technical and economic alternatives for the overall process. This does not mean that every unitary operation and equipment required in a bioprocess should be assayed, but only those with insufﬁcient knowledge or previous experience. Generally, included in this stage are scale-up bioreactors and equipment for speciﬁc operations of separation and puriﬁcation — for example, different chromatography columns. Here, the experience and criteria of the engineering team are fundamental to decide which unit operation will be investigated at the pilot-plant scale. During this stage, it is very important to take into account the strength of interactions among unit operations employed in the bioprocesses. Some researchers suggest the use of graphic visualization of those interactions through what they call windows of operation (Woodley and Titchener-Hooker, 1996; Zhou and Titchener-Hooker, 1999). A window of operation is deﬁned as being the operational space determined by the system (chemical, physical, and biological) and process (engineering) constraints and correlations governing a particular process or operation under consideration. The constraints may be in the form of economical limitations, physical laws, or biological effects, depending upon the application (Woodley and Titchener-Hooker, 1996). The strength of this approach lies in the ability to visualize process operability in such a clear way. Windows of operation enable the engineer to visualize rapidly the variable region in which it is possible to achieve a speciﬁed level of process performance. By changing the desired speciﬁcations for the process performance, the windows of operation can be used to indicate the feasibility and ease with which the speciﬁcation can be met. The window can also be employed to investigate the effects of altering the key operating variables on the likely overall process behavior (Zhou and Titchener-Hooker, 1999).
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Reactor Design Generally, the equipment employed during the up- and downstream processing can be purchased directly from catalogs and adapted to the process requirements; however, speciﬁc reactors must be constructed for the particular features of each biological system. The design methodology of reactor systems includes exploration and optimization of the following aspects of process development (Atkinson, 1983): 1. Utilization of the extensive literature on process engineering 2. Selection of an appropriate reactor system after consideration of the process requirement 3. Effective laboratory and pilot-scale experimentation 4. Rational extrapolation of experimental data to the plant on a commercial scale 5. Development of design procedures and cost models 6. Minimization of the delay between initial concept and full-scale production Based upon the project annual production capacity, the reactor size is calculated and the design is ﬁnalized with the development of the following components: 1. Provision of aeration, blending, heating, and cooling capacity 2. Control of substrate concentration, biomass, foam, pH, etc. 3. Facilities for aseptic monitoring operations Downstream Processing The desired product of a bioprocess may be either the cells themselves or a speciﬁc metabolite of the cell. Metabolic products are either intracellular or extracellular, and the location of the product has a major impact on the puriﬁcation process. Typically, biological products are present in fermentation broth and cell culture supernatants in low concentrations. Low product concentration coupled with large amounts of interfering species can seriously complicate the task of puriﬁcation. When the desired product is an intracellular compound, cells must be mechanically disrupted. These cells are exposed to high liquid shear rates by passing them through an oriﬁce under high pressure (Blanch and Clark, 1997). For this operation homogenizers are used. The most important operating cost is the expenditure in energy. Disrupted cells release a large quantity of cell debris; therefore, the puriﬁcation process is often complex and may consist of many unit operations. Multiple separations demand more equipment and labor and generally lower the yield of the ﬁnal product (Blanch and Clark, 1997). Excessive cell © 2003 CRC Press LLC
debris breakage and micronization can, for example, increase the load on centrifugal operations, and the passage of ﬁnes to packed chromatographic columns can lead to low productivity and high media replacement costs (Siddiqui et al., 1995). Isolating the product through a series of separation and recovery steps is often referred to as downstream processing. The exact number of steps involved will depend on the original material used, the concentration and physicochemical properties of the product, and the ﬁnal purity required. The ﬁrst task in formulating a puriﬁcation strategy is to deﬁne or acknowledge the required purity of the product. The allowable ranges of impurity concentrations and the speciﬁc impurities that may be tolerated will be dictated by the end use of the product. Once the purity criteria have been established, the speciﬁc puriﬁcation procedures can be selected. In general, individual recovery operations can be grouped into different categories, depending on their general purpose (Blanch and Clark, 1997): 1. Separation of insolubles. Insoluble material includes whole cells, cell debris, pellets of aggregate protein, and undissolved nutrients. Common operations for this purpose are sedimentation, centrifugation, ﬁltration, and membrane ﬁltration. 2. Isolation and concentration. This step generally refers to the isolation of desired product from unrelated impurities. Signiﬁcant concentration is achieved in the early stages, but concentration accompanies puriﬁcation as well. This category includes extraction, adsorption, ultraﬁltration, and precipitation. 3. Primary puriﬁcation. More selective than isolation, some puriﬁcation steps can distinguish between species having very similar chemical and physical properties. Primary puriﬁcation techniques include chromatography, electrophoresis, and fractional precipitation. 4. Refolding. Although refolding is not properly a recovery operation, the production of some proteins requires this step. Recombinant bacteria or yeasts produce several pharmaceutical or therapeutic proteins, but not all of them are biologically active, as not all of them are properly folded. Proteins produced naturally by organisms that are not recombinant frequently undergo posttranslational modiﬁcations (e.g., proteolytic cleavage of precursor protein, macromolecular assembly, glycosylation). Biologically inactive recombinant proteins are typically activated in stirred tanks with buffer solutions and denaturant agents that ﬁrst unfold the proteins; then process conditions are changed to allow proper protein refolding. 5. Final puriﬁcation. This step is necessitated by the extremely high purity required of many bioproducts, particularly pharmaceuticals © 2003 CRC Press LLC
and therapeutics. After primary puriﬁcation, the product is nearly pure but may not be in proper form. Partially pure solids may still contain discolored material or solvent. Crystallization and drying are typically employed to achieve ﬁnal purity. Some types of chromatography are also utilized for protein ﬁnal puriﬁcation. Screening Units The bioprocess unit design procedure consists of a preliminary screening procedure followed by the actual design calculations, which are used to determine efﬂuent stream characteristics and cost information. The downstream processing design methodology described by Steffens et al. (2000), Leser and Asenjo (1992), and Wheelwright (1987), recommends selecting units that exploit the greatest differences in physical properties between components. Two types of tests are used to eliminate units that are not feasible for a particular separation: 1. Design constraints refer to physical limits on pieces of equipment. For example, packed-bed chromatography columns cannot process streams that contain solids. 2. Binary ratio checks allow screen units using physical properties information for the component that is to be separated. Two numbers are compared to identify candidate separation operations: a. Binary ratios. The potential driving force for the separation of any of two components is quantiﬁed by calculating the ratio of the physical property governing the separation in the unit being considered (binary ratio). b. Feasibility indices. The binary ratios are compared to feasibility indices h (given in the literature), which are design parameters for each separation technology. The indices deﬁne how large the binary ratio must be before a separation is feasible.
Unit Design When a candidate unit passes the screening tests for a particular separation, design and cost calculations for that unit can be conducted. Two different approaches are used to determine the efﬂuent stream characteristics for a separation unit depending upon the unit type: splitting or fractionating units. Stream-Splitting Units These units operate by splitting the feed into two streams with signiﬁcantly different compositions, hence achieving separation as in distillation operations. In fact, stream-splitting unit operations are designed using a similar
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concept to that used in shortcut distillation design. Initially, a list of the feed components is generated according to their physical properties that allow the separation. Two key components are selected as an adjacent pair from this list. Prior to any design calculations, the binary ratio of the two key components is calculated and compared with the feasibility index for the unit under consideration. If the binary ratio is greater than the feasibility index, design calculations are performed. To calculate the efﬂuent stream compositions, the upper key component and any components higher in the list are considered as one. Similar calculations are performed for the lower key component and the components below it. Design assumptions and mass balances are then used to calculate the efﬂuent stream compositions. For each unit, the design procedure is repeated n – 1 times at the most, where n is the number of components in the feed to the unit. Each design is performed using a different pair of adjacent components as the keys (Steffens et al., 2000). Many articles and books have been written in detail about downstream processing unit designs. Design assumptions and cost information for the most common stream-splitting unit operations are summarized below: • Settlers and centrifuges. Both solid and liquid separation units rely upon density differences between insoluble particles and the surrounding ﬂuid to operate. Sedimentation relies on gravity and settling to achieve solid–liquid separation, and is generally performed in rectangular or circular ﬂow tanks. Centrifugation, on the other hand, involves mechanical applications of centrifugal force to obtain a solid concentrate and clariﬁed supernatant. The keys are chosen from the feed-solid components, which are ranked according to settling velocity (Wheelwright, 1991). The upper key component and any components with a higher settling velocity completely distribute into the slurry stream, while the lower key component and slower components move into the less dense stream. Soluble components are assumed to uniformly distribute between the two efﬂuent streams. The slurry solid concentration, cs, is speciﬁed as a design parameter (Steffens et al., 2000). Centrifugal separation is in widespread use in the biotechnology industry, with the disk-stack centrifuge being the machine used most commonly for the separation of biological materials. Depending on the nature of the product and the mode of centrifuge operation, a wide variety of disk-stack separator types are available (Clarkson et al., 1996). Energy expenses are the principal operating costs. • Conventional ﬁlters. Conventional ﬁlters are probably the most common means for separating solids from liquids on the basis of a single physical parameter: particle size. This parameter serves to choose the key components. The small key is assumed to be completely permeable and the large key impermeable. The wash rate, © 2003 CRC Press LLC
w, and cake-solids concentration, ck, are both design parameters used to estimate the ﬁlter area, assuming constant speciﬁc cake resistance. Compared with centrifugation, ﬁltration consumes less energy and requires considerably less capital investment. The operating costs principally include energy and ﬁlter aid expenses. Because typical bacteria diameters are about 1 mm, the use of conventional ﬁlters for their separation is not recommended. • Microﬁlters. Microﬁlters are capable of retaining particles 0.1 to 10 mm in diameter — for example, whole cells or cells debris. The key components for a microﬁltration unit are chosen from the particulate components in the incoming stream. For these equipment designs, ﬂux and membrane area must be estimated. Operating costs consist of energy and membrane replacement. • Ultraﬁlters. Ultraﬁltration membranes can retain particles and macromolecules 1000 to 500,000 Da in size. The key components are chosen by molecular size and membrane retention capacity. Membrane area and ﬂux must be estimated, and operating costs are generally dominated by membrane replacement and energy costs. Fractionating Units These units generally operate batchwise and produce several fractions or cuts. Chromatography columns, where the various components are sequentially eluted, are a particularly common example of a fractionating unit (Steffens et al., 2000). Chromatography columns, which fractionate rather than split, cannot be designed using the key component technique. A bioprocess stream may contain different types of complex biological compounds that interact with each other and the column, making the system difﬁcult to model (Leser et al., 1996). For this reason, an empirical approach that estimates the efﬂuent composition for a chromatography column using physical properties differences is used: • Ion-exchange columns. Feed components are sorted according to their net charge, which is also used to calculate the binary ratios. Components with a charge opposite that of the resin are assumed to bind completely, while those with the same charge do not bind at all. The column volume is estimated by calculating a binding capacity, Bc, and residence time tr (Steffens et al., 2000). For any type of column chromatography, the main costs are the package or resin costs and the solvents employed for elution. • Gel-ﬁltration columns. Feed components are sorted by means of their molecular weights, which are also used for calculating the binary ratios (Leser et al., 1996). Column volume is determined by specifying the sample volume, Bsam (percent of column volume), and the residence time, tr .
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• Hydrophobic interaction and reverse phase columns. Hydrophobicity is the physical property used to design a hydrophobic interaction or reverse-phase chromatography column. Column volume is calculated in the same way as for ion exchange, except that the binding fraction, Fi, for each component is assumed to have a linear relationship with the hydrophobicity, f (Steffens et al., 2000). • Afﬁnity columns. Afﬁnity columns are based on highly speciﬁc interactions between the desired compound and the adsorbent. The adsorbents employed have chemical groups called ligands, which are capable of binding speciﬁcally with the solute. This type of column is often employed in protein puriﬁcation. The bases of their design are similar to those for ion exchange columns except that only one substance binds to the resin.
Formulation Once the purity requirements and speciﬁcations by food and drug agencies have been achieved, the product manufactured must be formulated in a commercial form. Other compounds, such as excipients (e.g., starch), stabilizers, emulsiﬁers, antioxidants, and a large quantity of several compounds, can be mixed with the active principle until the commercial formulation is achieved. When the product is an enzyme, it is very important to decide on the ﬁnal product formulation carefully because the biological activity, measured in international units of activity (IUA), depends directly on the ﬁnal concentration of enzyme and ﬁxes its selling price.
The analysis of the economic feasibility of a chemical or biological process is focused on two points: • Cost estimates • Total capital investment The accuracy of these estimates depends on the extent to which the process is deﬁned; many processes in the research and development phase will undergo changes before they are implemented to production plant scale. Cost estimates and capital investment in the product selling price allow calculation of different economical parameters, such as return on investment (ROI), payback time, present worth, and internal rate of return, with which it is possible to determine whether the proposed process will be proﬁtable.
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Cost Estimates The total cost of a product includes the operating costs associated with manufacturing the product along with general expenses and is referred as the cost estimate. Manufacturing costs include those elements that contribute directly to the costs of production (operating costs, ﬁxed costs, and plant overhead costs). Operating costs are a function of the production volume and include raw materials, labor, utilities, and supplies. Fixed costs relate to the physical plant and do not change with productivity levels; they include depreciation and interest, taxes, and insurance. The category denoted as plant overhead includes charges for services that are not directly attributable to the cost of the product, such as janitorial services, accounting, personnel, etc. Finally, the category denoted as general expenses includes charges for marketing, research and development, and general administration charges (Blanch and Clark, 1997). Total Capital Investment Total capital investment is the amount of capital required to construct and equip the plant (ﬁxed capital), plus the capital necessary for its operation (working capital). Fixed capital includes the cost of land, building construction, and engineering design. Working capital refers to the funds used to provide an inventory of raw materials, supplies, and cash to pay salaries, usually during the ﬁrst 3 months of plant operation. The starting point for estimating the operating and capital costs of a product is the process ﬂow sheet. The ﬂow sheet and the material and energy balances, based on a desired annual production capacity, allow establishment of a ﬂowchart indicating all liquid, gas, and solid ﬂows in the process. When these ﬂows are set, the equipment can be sized and the raw materials and utilities can be speciﬁed.
Single Cell Production: An Example of Bioprocess Design
A process for the production of a food-grade, single cell protein (SCP), developed in the Department of Biotechnology and Bioengineering of the Center for Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV-IPN), is used to evaluate the bioprocess synthesis technique (De la Torre et al., 1994). A burst of activity appeared in the ﬁeld of SCP production in the 1950s, 1960s, and early 1970s. Many companies made important improvements in the production processes and in this way reduced the operating and capital costs in order to be competitive with protein supplements of vegetable origin, such as soy. Some of these plants are operating today to produce food-grade yeast autolysates. © 2003 CRC Press LLC
From 1985 to 1991, a research team at CINVESTAV-IPN developed a highcell-density process for food-grade Candida utilis production from sugarcane molasses. A continuous, 10.5-m3, jet-loop fermentor was designed to take advantage of its intrinsic high oxygen transfer rate and energy efﬁciency. This fermentor was provided with a computer system for online data acquisition and for controlling the molasses ﬂow rate in response to inferred ethanol production rates. The operating costs were minimized because of the efﬁcient conversion of molasses into biomass and of the low water consumption in the fermentation stage. The capital costs were kept down due to the high productivity achieved in the fermentation process. An overview of the most important aspects of the process developed is presented below. Laboratory Fermentation Preliminary experiments were conducted on a bench scale for strain screening and to deﬁne culture medium and operating conditions in a 0.030-m3 stirred tank reactor (STR). Later, continuous fermentation was carried out on the same STR. It was found, for example, that under the operation conditions, assayed C/N ratios from 5 to 9.5 at dilution rates in the range of 0.14/h to 0.25/h did not affect the biomass yield based on sugars. Scale-Up Once the fermentation process conditions were established, work proceeded to the pilot-plant scale and then to the industrial scale. The most important step in this process was the choice of the critical variable that could be used as a scale-up criterion. Under carbon substrate limiting conditions, the most efﬁcient carbon substrate conversion to biomass was achieved. Under these conditions, biomass productivity was related to the oxygen transfer rate (OTR); therefore, OTR was employed as a criterion to scale-up, ﬁrst from bench to pilot plant (0.03-m3 STR to 1.00-m3 STR) and then from pilot plant to industrial reactor (1.00-m3 STR to 10.50-m3 jet-loop reactor). The biomass volumetric productivity and the biomass yield were similar for both STR reactors, 6.0 kg/m/h and 0.45 ± 0.03 kg cell/kg sugar, respectively, which indicates that OTR choice as a scale-up criterion was correct. The jet-loop reactor had a higher OTR than the STRs; therefore, the biomass volumetric productivity increased to 11.2 kg/m/h for a biomass concentration of 80 kg/m3 (8% w/v). A signiﬁcantly higher biomass yield was observed in the jet-loop reactor (0.56 kg cells/kg sugar), and yeast cells were larger than those cultivated in the STRs. Reduced yields in STRs might be a result of ethanol production under oxygen-limiting conditions. However, physiological responses of the cells continuously submitted to transient environmental conditions, such as recycling from a region of high turbulence to one of low turbulence in the jet-loop reactor, might be contributing factors in enhancing biomass yields. © 2003 CRC Press LLC
Downstream Processing Cell Separation Centrifugation tests were done in a disk-stack centrifuge (FESX 512S, AlfaLaval). The yeast cream solids concentration after two centrifugation steps was 16% (dry weight). In addition, vacuum leaf ﬁlter tests were conducted. Cotton fabric and Decalite 477 were selected as ﬁlter medium and ﬁlter aid, respectively. Trials with a rotary ﬁlter yielded a 24% solids cake. To get a 24% ﬁnal solids concentration, economic analysis showed that ﬁltration alone is a better alternative than centrifugation followed by ﬁltration of broth solids concentrations higher than 7%. Thermolysis In order to increase the availability of nutrients and to develop ﬂavor notes, yeast used as food or feed is usually broken down. Three alternatives were investigated: autolysis, autolysis-thermolysis, and thermolysis. Experiments were carried out to increase protein digestibility while minimizing available lysine and thiamin losses. The independent variables were temperature, pH, and time, and the dependent variables were protein digestibility, thiamin, and available lysine. Results are shown in Table 3.1. A two-step thermolysis was the best alternative to increase protein digestibility with no adverse effects on available lysine content and protein quality. The product has been used as a ﬂavor enhancer additive and to produce chicken-like ﬂavors.
Final Design All information acquired during the research time was employed to build and operate a demonstration industrial plant with a 10.5-m3 fermentor. A ﬂowsheet of the process developed is shown in Figure 3.3. Molasses is diluted with tap water 1:1 by volume and heated to 90°C. The settled solids
TABLE 3.1 Results of Autolysis and Thermolysis of Candida utilis Treatment None (A) Autolysis (24 h at 50°C) Autolysis-thermolysis (24 h at 50°C, 2 h at 90°C) Thermolysis (1 h at 70°C, 2 h at 85°C) Available Lysine (Relative to A) 100 93 85 98 Protein Digestibility (Relative to A) 1.00 1.53 1.76 1.85
Source: Modiﬁed from De la Torre et al., in Advances in Bioprocess Engineering, Kluwer, Dordrecht, 1994, 67-74. With permission.
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Molasses Water Dilution Tank Clarifier Continuous sterilizer
Sludge Thermolyzer Rotary filter Jet Loop reactor
FIGURE 3.3 SCP production from molasses; schematic ﬂowsheet. (Modiﬁed from De la Torre et al., in Advances in Bioprocess Engineering, Kluwer, Dordrecht, 1994. With permission.)
are separated from the molasses. Clariﬁed molasses is fed to the fermentor through sterilizing equipment. A metering pump supplies phosphoric acid while a pH controller continuously regulates the ammonia supply. The fermentor operates under gas hold-up controlled conditions to optimize oxygen transfer. The yeast is concentrated and washed using a rotary vacuum ﬁlter. The yeast cake obtained is fed to thermolyzer tanks to be liqueﬁed and increase its digestibility. Finally, the yeast is spray dried.
Economic Analysis Table 3.2 shows an economical comparison among different yeast production processes. Speichim and Vogelbusch process data were obtained from industrial facilities operating in Cuba, while CINVESTAV process data were gathered from trials at the industrial plant. The total capital investment for a new facility using the CINVESTAV process is about 80 to 85% that required for conventional processes. Raw material costs are reduced to nearly 20%, mainly as a result of a higher biomass yield on molasses and minimal use of ammonia, which is utilized instead of ammonium sulfate. The speciﬁc energy consumption of the fermentors employed is 0.39, 0.55, and 0.53 kWh kg-cell–1 for the airlift fermentor (Speichim), STR (Vogelbush), and jet-loop fermentor (CINVESTAV), respectively, so energy consumption is very similar for the STR and jet-loop fermentor. As a result of culturing at high cell density, water consumption is reduced by two thirds, but utilities cost for the CINVESTAV process is slightly higher, because the Vogelbusch and Speichim processes employ combustion gases for yeast drying, while the CINVESTAV process uses indirect heating to improve product quality. © 2003 CRC Press LLC
TABLE 3.2 Comparative Economic Data for Yeast Productiona
Total Capital Investment (Relative to A) 1.250 1.175 1.000 CapitalRelated Cost (Relative to A)b 1.250 1.175 1.000 Raw Material Cost (Relative to A) 1.226 1.229 1.000 Utilities Cost (Relative to A) 0.939 0.851 1.000 Total Production Cost (Relative to A) 1.166 1.120 1.000
Process Vogelbusch Speichim CINVESTAV
Basis: 12 ¥ 106 kg/yr. Annual capital-related cost estimated as 22% of the total capital investment. Source: Modiﬁed from De la Torre et al., in Advances in Bioprocess Engineering, Kluwer, Dordrecht, 1994, 67-74. With permission.
Bioprocess design must carefully consider the characteristics of the biological system and the desired ﬁnal product as well as chemical engineering principles to achieve the best performance of the involved process. It is very important to keep in mind that bioprocess performance depends on the interaction of all unit operations involved. A bioprocess cannot be optimized by taking into consideration each processing unit separately but must consider the whole of the processing line. This is due partially to the several alternatives for each proposed unit operation involved. Several possibilities exist for a speciﬁc bioprocess, and several processes for the production of the same bioproduct can be adequate. In this new century, functional genomics will help us understand the entire cell functioning, and this knowledge will allow redirection of cell ﬂux distribution to overproduce metabolites and thus increase bioprocess performance.
Atkinson, B. and Mavituna, F. (1983) Biochemical Engineering and Biotechnology Handbook, Macmillan Publishers, New York, pp. 593–601. Blackstock, W.P. and Weir, M.P. (1999) Proteomics: quantitative and physical mapping of cellular proteins, Trends Biotechnol., 17, 121–127. Blanch, H.W. and Clark, D.S. (1997) Biochemical Engineering, Marcel Dekker, New York, pp. 452–682. Bulmer, M., Clarkson, A.I., Titchener-Hooker, N.J., and Dunnill, P. (1996) Computerbased simulation of the recovery of intracellular enzymes and its pilot-scale veriﬁcation, Bioprocess Eng., 15, 331–337.
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Clarkson, A.I., Lefevre, P., and Titchener-Hooker, N.J. (1993) A study of process interaction between cell disruption and debris clariﬁcation stages in the recovery of yeast intracellular products, Biotechnol. Progr., 9, 462–467. Clarkson, A.I., Bulmer, M., and Titchener-Hooke, N.J. (1996) Pilot-scale veriﬁcation of a computer-based simulation for the centrifugal recovery of biological particles, Bioprocess Eng., 14, 81–89. De la Torre, M., Flores, L.B., and Chong, E. (1994) High cell density yeast production: process synthesis and scale-up, in Advances in Bioprocesses Engineering, E. Galindo and O.T. Ramírez, Eds., Kluwer, Dordrecht, pp. 67–74. Fraga, E.S. (1996) Discrete optimization using string encodings for the synthesis of complete chemical processes, in State of the Art in Global Optimization, C.A. Floudas and P.M. Pardalos, Eds., Kluwer, Dordrecht, pp. 627–651. Graves, D.J. (1999) Powerful tools for genetic analysis come of age, Trends Biotechnol., 17, 127–134. Gregory, M.E., Bulmer, M., Bogle, I.D.L., and Titchener-Hooker, N. (1996) Optimizing enzyme production by baker’s yeast in continuous culture: physiological knowledge useful for process design and control, Bioprocess Eng., 15, 239–245. Gritis, D. and Titchener-Hooker, N. (1989) Biochemical process simulation. I. Chem. E Symposium Series 114, Computer Integrated Process Engineering, 69–78. Kelly, B.D. and Hatton, T.A. (1991) The fermentation/downstream processing interface, Bioseparation, 1, 333–349. Kossen, N.W.F. (1994) Bioreactor engineering, in Advances in Bioprocesses Engineering, E. Galindo and O.T. Ramírez, Eds., Kluwer, Dordrecht, pp. 1–11. Krings, U. and Berger, R.G. (1998) Biotechnological production of ﬂavours and fragrances, Appl. Microbiol. Biotechnol., 49, 1–8. Kuipers, O.P. (1999) Genomics for food biotechnology: prospects of the use of highthroughput technologies for the improvement of food microorganisms, Curr. Opin. Biotechnol., 10, 511–516. Leser, E.W. and Asenjo, J.A. (1992) Rational design of puriﬁcation processes for recombinant proteins, J. Chromatogr.-Biomed. Appl., 584(1), 43–57. Leser, E.W., Lienqueo, M.E., and Asenjo, J.A. (1996) Implementation in an expert system of a selection rationale for puriﬁcation processes for recombinant proteins, Ann. N.Y. Acad. Sci., 782, 441–455. Lienqueo, M.E., Leser, E.W., and Asenjo, J.A. (1996) An expert-system for the selection and synthesis of multistep protein separation processes, Comp. Chem. Eng., 20(SA): S189–S194. Middelberg, A.P.J. (1995) The importance of accounting for bioprocesses interaction, Australian Biotechnol., 5, 99–103. Narodoslawsky, N. (1991) Bioprocess simulation: a system theoretical approach to biotechnology, Chem. Biochem. Eng. Q., 5, 183–187. Petrides, D.P. (1994) Biopro designer: an advanced computing environment for modeling and design of integrated biochemical process, Comp. Chem. Eng., 18(SS): S621–S625. Petrides, D.P., Calandranis, J., and Cooney, C. (1995) Computer-aided-design techniques for integrated biochemical processes, Gen. Eng. News, 15(16), 10. Priefert, H., Rabenhorst J., and Steinbüchel, A. (2001) Biotechnological production of vanillin, Appl. Microbiol. Biotechnol., 56, 296–314. Samsalatli, N.J. and Shah, N. (1996) Optimal integrated design of biochemical processes, Comput. Chem. Eng., S20, 315–320.
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Siddiqui, S.F., Bulmer, M., Ayazi Shamlou, P., and Titchener-Hooker, N.J. (1995) The effects of fermentation conditions on yeast cell debris particle size distribution during high pressure homogenization, Bioprocess Eng., 14, 1–8. Steffens, M.A., Fraga, E.S., and Bogle, I.D.L. (2000) Synthesis of bioprocesses using physical properties data, Biotechnol. Bioeng., 68(2), 218–230. Stephanopoulus, G. and Vallino, J.J. (1991) Network rigidity and metabolic engineering in metabolite overproduction, Science, 252, 1675–1681. Wheelwright, S.M. (1987) Designing downstream processing for large-scale protein puriﬁcation, Bio/Technology, 5, 789. Wheelwright, S.M. (1991) Protein Puriﬁcation: Design and Scale-Up of Downstream Processing, Hanser, Munich, pp. 76. Woodley, J.M. and Titchener-Hooker, N.J. (1996) The use of windows of operation as a bioprocess design tool, Bioprocess Eng., 14, 263–268. Zhou, H.Y. and Titchener-Hooker, N.J. (1999) Visualizing integrated bioprocess designs through windows of operation, Biotechnol. Bioeng., 65(5), 550–557. Zhou, H.Y., Holwill, I.L.J., and Titchener-Hooker, N.J. (1997) A study of the use of computer simulations for the design of integrated downstream processes, Bioprocess Eng., 16, 367–374.
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Gas Hold-Up Structure in Impeller Agitated Aerobic Bioreactors
Ashok Khare and Keshavan Niranjan
CONTENTS Introduction Gas Hold-Up Structure in Viscous Liquids Effect of Tiny Bubbles on Mass Transfer Experimental Observations on Gas Hold-Up in Impeller Agitated Systems Conclusions References
Viscous media are frequently encountered in aerobic biotechnological processes, and gas bubbling into such media is an essential feature. An efﬁcient contact between gas and liquid phases is necessary to maintain high reaction rates. The production capacity of aerobic bioreactors is often limited by the rate at which oxygen is made available to microorganisms. High viscosity of certain biological media (e.g., xanthan gum and polysaccharide fermentation) makes it more difﬁcult to transfer oxygen. It is therefore extremely important to understand and decipher the mechanism of oxygen transfer, especially in media that are highly viscous and non-Newtonian. The key variable indicating the efﬁciency of gas–liquid contacting, and in turn inﬂuencing oxygen transfer, is gas hold-up, i.e., fraction of the dispersion constituting the gas phase. This chapter examines the gas hold-up structure in impeller agitated viscous liquids, reviews the link between operating variables and gas hold-up, and presents models that can be used to analyze gas–liquid mass transfer in such systems.
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Gas Hold-Up Structure in Viscous Liquids
Gas hold-up structure in highly viscous Newtonian and non-Newtonian impeller agitated liquids (m, viscosity of Newtonian liquids; ma, apparent viscosity of non-Newtonian liquids > 0.4 Pas) is distinctively characterized by a nearly bimodal bubble size distribution (Schugerl, 1981; Nienow, 1990; Khare and Niranjan, 1995). When such liquids are aerated, large bubbles (diameter > 20 mm), some as large as the impeller, are formed along with tiny bubbles (0.1 to 5 mm in diameter). During aeration, large bubbles form rapidly due to coalescence. These bubbles quickly ascend through the liquid due to high buoyancy forces (i.e., they have relatively shorter residence times) and cannot be recirculated back into the impeller region by the liquid ﬂow. In contrast, tiny bubbles slowly ascend through the liquid, and they have comparatively longer residence times. Further, these bubbles are also recirculated back into the impeller region by the circulating liquid. Tiny bubbles are formed in all parts of the reactor. In the immediate vicinity of the rotating impeller blades turbulence is high; the greater the turbulence, the smaller are the bubbles formed. Away from the impeller, tiny bubbles are formed by the break-up of larger bubbles by local turbulence. Finally, tiny bubbles are also formed in the upper regions of the liquid when large bubbles break through the surface and disengage into the headspace. The tiny bubbles thus formed do not merely accumulate in the reactor; they constantly leave it by (1) disengaging at the surface, and (2) coalescing with other small or big bubbles. This ensures their regular turnover, which is signiﬁcant in mass transfer. Gas hold-up in impeller agitated viscous liquids can be regarded as consisting of two parts: a hold-up due to larger bubbles and a hold-up due to tiny bubbles. This fact fundamentally distinguishes hold-up structure in viscous liquids from that observed in low-viscosity aqueous solutions and forms the basis of the analysis of gas hold-up in such liquids. The simultaneous formation and disengagement of tiny bubbles also causes the holdup to vary with time after the commencement of aeration, when the rates of formation exceed the rate of disengagement. With the build-up of tiny bubbles, parity is established between the two rates, and the tiny bubble hold-up attains steady state. The time taken to attain this parity determines the rate of turnover of tiny bubbles in the reactor. In practice, the gas hold-up at a given impeller speed is determined by the difference between the height of dispersion at any time (HD) and that of clear liquid height (HC). At the onset of aeration, the immediate increase in the HC is due to large bubbles. As mentioned above, with continuous aeration the concentration of tiny bubbles in the liquid progressively increases. This causes the liquid height to increase with time and eventually attain a steady value (HDf), when the rate of generation of tiny bubbles equals the rate of their disappearance. Once a stable dispersion height is reached, the gas ﬂow © 2003 CRC Press LLC
and agitation can be simultaneously switched off. Large bubbles have been observed to leave the system within approximately 10 sec. At this moment, the dispersion height consisting of tiny bubbles and liquid (Htf) can be noted. The ﬁnal total gas hold-up (ef), the hold-up of tiny bubbles (eft), and the holdup of large bubbles (efL) can then be calculated as: ef = (HDf – HC)/HC eft = (Htf – HC)/HC efL = ef – eft (4.1) (4.2) (4.3)
The typical variation of tiny bubble hold-up with time is shown in Figure 4.1. Two alternative approaches can be taken to model the time dependency of gas hold-up. Philip et al. (1990) considered the sparged gas to divide itself into two fractions: one ﬂowing through the liquid as tiny bubbles (i.e., F) and the other as large bubbles. Further, they assumed that, initially, only tiny bubbles are formed, and no disengagement occurs. Considering these assumptions and the variation of the tiny bubble holdup (et) with time (t) (Figure 4.1), Philip et al. evaluated F, deﬁned as the ratio
0.1 VG = 0.006 ms-1 0.08 PG /V=0.17 kWm-3
VG = 0.001 ms-1
PG /V=0.2 kWm-3
0 0 10 20 30 40 50 60 70
t (min) FIGURE 4.1 Typical variation of tiny bubble hold-up with time in castor oil; CBDT impeller, vessel diameter = 0.6 m.
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of the volumetric rate of generation of tiny bubbles to the volumetric gas ﬂow rate through the system, as follows: V Ê de ˆ F =Á t˜ Ë dt ¯ t=0 QG
where (det/dt)t=0 is the initial slope of the curve as seen in Figure 4.1; V is the volume of the reactor, and QG is the volumetric gas ﬂow rate. Muller and Davidson (1992) took an alternative approach in which they assumed that the rate of disappearance of tiny bubbles at any instant is dependent on the tiny bubble inventory at that instant, and deduced the following ﬁrst-order equation: de t 1 = (e - e ) dt t ft t
where et is the hold-up of tiny bubbles at any time t during aeration. Solving Eq. (4.5) with et = 0 at t = 0, Muller and Davidson (1992) showed that the dynamic hold-up curve is as follows:
e t = e ft (1 - e - t /t )
The t in Eqs. (4.5) and (4.6) is the characteristic time constant, which can be estimated from the slope of the plot of ln(1 – et/eft) vs. t. Comparing Eqs. (4.4) and (4.5), it can be deduced that: FQG (e ft - e t ) = t V
as QG = AVG and V = AHC, where A is the cross sectional area of the reactor, VG is the superﬁcial gas velocity, and HC is the clear liquid height (i.e., before the commencement of aeration). Noting that et = 0 at t = 0, Eq. (4.7) yields: t= HC e ft A VG FA (4.8)
The numerator and denominator in Eq. (4.8) signify the volume of tiny bubbles at the steady state and the ﬂow rate of gas into the tiny bubbles, respectively; in other words, t gives an estimate of the mean residence time of tiny bubbles. © 2003 CRC Press LLC
Effect of Tiny Bubbles on Mass Transfer
Having considered the hold-up structure in viscous liquids in terms of the coexistence of tiny and large bubbles, we can now discuss its implications on mass transfer rate for the relatively simple case of a continuously stirred tank aerobic bioreactor. As described earlier, Khare and Niranjan (1995) assumed the sparged gas to be divided into two fractions: (1) a completely mixed tiny bubble fraction having lower oxygen partial pressure than the sparged air, and (2) a large bubble fraction ﬂowing through the liquid in a plug ﬂow manner with oxygen partial pressure comparable to that of the inlet air. This difference in oxygen partial pressure between the two fractions is due to the difference between their residence times. Given that the tiny bubble fraction receives a supply of oxygen continuously through the fraction of sparged air ﬂowing through it (FQG), an oxygen balance equation covering this fraction can be written as: FQG P 0Y 0 FQG P 0Y * - (kL a)t (C2 - CL )V = 0 RT RT
where P0 is the atmospheric pressure, Y0 is the mole fraction of oxygen in the inlet, Y is the mole fraction of oxygen in the tiny bubble fraction, R is the universal gas constant, T is absolute temperature, (kLa)t is the volumetric mass transfer coefﬁcient due to tiny bubbles, C* is the solubility of oxygen 2 corresponding to the tiny bubble fraction, and CL is the concentration of dissolved oxygen in the feed to the reactor. The ﬁrst two terms in Eq. (4.9) represent the rates at which oxygen ﬂows in and out of the tiny bubble fraction, respectively, and the third term represents the rate of oxygen transfer to the liquid through this fraction. The solubility of oxygen corresponding to its partial pressure in the tiny bubble fraction is given by C* = P0Y/H*, H* being the Henry’s constant. Further, 2 also following assumption (2), the solubility of oxygen corresponding to its partial pressure in the large bubble fraction is given by C* = P0Y0/H*. Con1 sidering this, Eq. (4.9) can be rearranged as: * * C1* - C2 - ND (C2 - CL ) = 0
* where C1 is the solubility of oxygen corresponding to large bubble fraction. It follows from Eq. (4.10) that:
* C2 =
C1* + N DCL 1 + ND
ND in Eqs. (4.10) and (4.11) is the dimensionless number, given by: © 2003 CRC Press LLC
( k a) VRT ND = L t * = FQG H
V FQG H* ( kL a)t RT
From Eq. (4.12) the signiﬁcance of ND can be developed. The numerator represents the mean residence time for tiny bubbles to traverse through the liquid, whereas the denominator gives a characteristic time for oxygen transfer from these bubbles. Evidently, a low value of ND indicates that the tiny bubble fraction is actively transferring oxygen. For the extreme case where ND Æ 0, Eq. (4.11) gives C* = C *1 , indicating that the solubilities and oxygen 2 partial pressures of the large and tiny bubble fractions are similar. In contrast, a high value of ND indicates greater mean residence time of the tiny bubbles in relation to the characteristic time for oxygen transfer, which enables tiny bubbles to equilibrate with the liquid. For the case where ND >> 1, Eq. (4.11) simpliﬁes to C* = C L. 2 It is now possible to estimate the ratio of the rates of oxygen transfer between large bubbles and tiny bubbles, which can be given by: * rt (kL a)t (C2 - CL ) = * rL (kL a) L (C2 - CL )
where rt and rL are the rate of oxygen transfer through tiny and large bubbles, respectively, and (kLa)L is the volumetric mass transfer coefﬁcient due to large bubbles. Substituting for C* from Eq. (4.11), Eq. (4.13) becomes: 2 rt 1 ( kL a)t = rL 1 + N D ( kL a)L
Taking a conservative estimate of (kLa)t/(kLa)L = 1 and ND = 1, Eq. (4.14) will be reduced to: rt 1 = rL 2 (4.15)
This implies that in the continuously stirred tank aerobic bioreactor considered above, half of the oxygen transfer might be taking place through tiny bubbles. This analysis reveals that tiny bubbles in viscous liquids cannot be assumed to equilibrate with the liquid phase. In fact, these bubbles can actively contribute to the oxygen transfer, although at a lower driving force. © 2003 CRC Press LLC
Experimental Observations on Gas Hold-Up in Impeller Agitated Systems This section describes the behavior of gas hold-up in low-viscosity liquids (e.g., water), high-viscosity non-Newtonian carboxymethol cellulose(CMC) solution (apparent viscosity [ma] = 0.5 to 0.8 Pas), and Newtonian castor oil (viscosity [m] = 0.76 Pas) generated by three disk impellers: the ﬂat-bladed disk turbine (DT) and two of its modiﬁed designs (available commercially), concave-bladed disk turbine (CBDT), and Scaba 6SRGT. The fundamental difference between the gas hold-up structure in low- and high-viscosity liquids lies in the bubble size distribution. In low-viscosity liquids, a statistically normal bubble size distribution is quickly established; that is, the spread of bubble size around a mean value is uniform. Moreover, the gas hold-up does not depend on the aeration time, but varies only with speciﬁc power dissipation (PG/V) and gas velocity (VG) (Figure 4.2). Further, it is also clear from this ﬁgure that gas hold-up in such liquids is independent of impeller design, an observation also reported by Saito et al. (1992) and Bakker et al. (1994).
0.10 0.09 0.08 VG=0.006 ms-1 0.07 0.06
ε f 0.05
0.04 0.03 0.02 0.01 0.00 0.0 VG=0.001 ms-1 0.5 1.0 1.5 2.0 2.5 3.0
PG/V (kWm-3) FIGURE 4.2 Variation of ﬁnal total gas hold-up (ef) with speciﬁc power dissipation (PG/V) in water; vessel diameter = 0.6 m. ( ) DT; ( ) CBDT; ( ) Scaba 6SRGT.
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In high-viscosity castor oil and CMC solution, the gas hold-up is characterized by time dependency, as well as the existence of tiny and large bubbles (Khare and Niranjan, 1999, 2002). The hold-up must therefore be analyzed by distinguishing between the contributions of tiny and large bubbles (Figures 4.3 and 4.4). It is evident that in both viscous liquids the ﬁnal steadystate values of the total gas hold-up (ef) and the hold-up of tiny bubbles (eft) increase with VG. In contrast, ef and eft exhibit different trends with respect to PG/V. For example, while ef increases with PG/V, eft either passes through a maxima (e.g., at the lower VG), or decreases progressively (e.g., at the higher VG) in the CMC solution (Figure 4.3). On the other hand, ef as well as eft pass through a maxima in castor oil (Figure 4.4). It is understood that at a particular gas velocity, increasing PG/V (i.e., increasing impeller speed) facilitates bubble break-up, which in turn would be expected to enhance the population of tiny bubbles. However, higher PG/ V/(or /speed) also induces stronger liquid circulation (hence, intense coalescence), which adversely affects the build-up of tiny bubbles. In the case of shear thinning liquids this effect is more pronounced, as stronger liquid circulation also implies reduction in effective viscosities. The net hold-up of tiny bubbles is therefore a result of the balance between these two conﬂicting effects. Thus, the plot of eft vs. PG/V can slope either upward or downward depending on which of the two effects predominates. In general, it can be noted that the eft dictates the trend of the ef, if the fractional contribution of the former is signiﬁcant to the latter (i.e., eft/ef). For example, from Figures 4.3 and 4.4, it is clear that ef and eft follow the same trend in castor oil, but not in the CMC solution (despite the fact that both liquids have comparable effective viscosities). This is due to the fact that tiny bubbles constitute only 15 to 45% of the total hold-up in the CMC solution, whereas they constitute 60 to 90% in the castor oil. It is also interesting to note from Figures 4.3 and 4.4 that, unlike water, the gas hold-up in the high-viscosity CMC solution and the castor oil is inﬂuenced by the impeller design. For example, in the CMC solution (Figure 4.3), the DT gives higher ef and eft values than its modiﬁed designs (CBDT and Scaba 6SRGT) at the lower gas velocity, whereas at the higher gas velocity these impellers give comparable hold-up values (except for the eft values produced by CBDT at the higher gas velocity, which are signiﬁcantly lower than the DT). This implies: (1) in the CMC solution, the inﬂuence of impeller type on gas hold-up is more pronounced at the lower gas velocity; and (2) the DT shows better performance over its modiﬁed designs with regard to generating gas hold-up. In the case of castor oil (Figure 4.4), the modiﬁed disk impellers (Scaba 6SRGT and CBDT) generally give higher hold-up than the DT. Further, all impellers produce comparable maximum ef and eft values, but modiﬁed disk impellers generate these values at signiﬁcantly lower PG/V levels compared to DT. In short, Scaba 6SRGT and CBDT appear to be superior to DT in castor oil. This implies that the inﬂuence of impeller design on gas hold-up is markedly
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0.055 0.050 0.045 0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0 0.5 1 1.5 2 2.5 VG=0.001 ms-1 VG=0.006 ms-1
0.015 VG=0.006 ms-1 0.012
0.000 0 0.5 1 1.5 2 2.5 3
PG/V (kWm-3) FIGURE 4.3 Variation of ﬁnal gas hold-up with speciﬁc power dissipation (PG/V) in the CMC solution; vessel diameter = 0.6 m. (a) Total gas hold-up (ef); (b) tiny bubble hold-up (eft). Keys same as in Figure 4.2.
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0.18 (a) VG=0.006 ms-1 0.15
0.09 VG=0.001 ms-1 0.06
0.14 VG=0.006 ms-1 (b)
ε ft 0.08
VG=0.001 ms-1 0.05
PG/V (kWm-3) FIGURE 4.4 Variation of ﬁnal gas hold-up with speciﬁc power dissipation (PG/V) in castor oil; vessel diameter = 0.6 m. (a) Total gas hold-up (ef); (b) tiny bubble hold-up (eft). Keys same as in Figure 4.2.
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different in high-viscosity non-Newtonian and Newtonian liquids having comparable viscosities. As mentioned, the gas hold-up in CMC solution and castor oil is time dependent (Figure 4.1). In CMC solution, the time taken for gas hold-up to reach steady state varies from 30 to 120 sec, whereas it varies between 600 and 6000 sec in castor oil (Khare and Niranjan, 1999, 2002). This implies that t in non-Newtonian liquids is much lower than in Newtonian liquids having comparable viscosities; therefore, it appears that, between the two liquids, tiny bubbles in castor oil would be expected to be relatively less effective with respect to their contribution to mass transfer. Figures 4.5 and 4.6 show the variation of t values with PG/V in the CMC solution and castor oil, respectively. It can be seen that at a given gas velocity t decreases with PG/ V in both liquids. This implies that increasing PG/V effectively enhances the contribution of tiny bubbles to the mass transfer. It is, however, essential to note that higher PG/V also lowers the hold-up of tiny bubbles (hence, the interfacial area) in both CMC solution (Figure 4.3b) and castor oil (Figure 4.4b). Under such circumstances, caution must be exercised, as decreasing area may offset the beneﬁts of lower t values. Figures 4.5 and 4.6 also
60 55 50 45 40 35 30 25 20 15 10 0 1 2 3
PG/V (kWm-3) FIGURE 4.5 Variation of characteristic time constant (t) with speciﬁc power dissipation (PG/V) in the CMC solution; vessel diameter = 0.6 m. For VG = 0.001/ms–1: ( ) DT; ( ) CBDT; ( ) Scaba 6SRGT. For VG = 0.006/ms–1: ( ) DT; (*) CBDT; ( ) Scaba 6SRGT.
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VG = 0.001 ms-1
200 VG = 0.006 ms-1 0 0 0.3 0.6 0.9 1.2 1.5
PG/V (kWm-3) FIGURE 4.6 Variation of characteristic time constant (t) with speciﬁc power dissipation (PG/V) in castor oil; vessel diameter = 0.6 m. Keys same as in Figure 4.2.
demonstrate that impeller type inﬂuences t values. For instance, DT gives the lowest t values in CMC solutions (Figure 4.5), whereas DT gives the highest t values in castor oil (Figure 4.6). Khare and Niranjan (1995) suggested that an efﬁcient impeller must not only dissipate low power, but also give high hold-up (eft and ef) and low t values. Considering this, it is clear from Figures 4.3 to 4.6 that DT shows potential for enhanced performance over its modiﬁed designs in the CMC solutions, whereas the latter impellers perform better in castor oil. Finally, the effect of scale of operation on hold-up values and trends has been found to be signiﬁcant; this is evident from Figures 4.7a and b, where hold-up in castor oil has been compared in two vessels measuring 0.3 and 0.6 m in diameter. The gas hold-up (eft and ef) passes through a maxima in the 0.6-m vessel, whereas it progressively increases with PG/V in the 0.3-m vessel. Further, these ﬁgures show that at a given PG/V, gas hold-up (ef and eft) in the 0.3-m vessel is signiﬁcantly higher than in the 0.6-m vessel. This clearly shows that gas hold-up cannot be correlated with power dissipation levels alone; geometric size and related factors must also be taken into account.
This article discusses gas hold-up structure in impeller agitated high-viscosity liquids, which are often encountered in aerobic bioreactors. The hold-up © 2003 CRC Press LLC
ε ft 0.09
0.06 VG=0.006 ms-1 0.03 (b) 0.00 0 0.3 0.6
PG/V (kWm ) FIGURE 4.7 Variation of ﬁnal gas hold-up with speciﬁc power dissipation (PG/V) in castor oil; DT impeller; vessel diameter = 0.6 m ( ) and 0.3 m ( ). (a) Total gas hold-up (ef); (b) tiny bubble hold-up (eft).
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in such liquids consists of two parts: (1) due to the large bubbles (diameter > 20 mm) and (2) due to tiny bubbles (diameter = 0.1 to 5 mm). It has been assumed, often erroneously, that the tiny bubble constituent of the gas holdup is not actively transferring oxygen, as these bubbles tend to attain equilibrium with the liquid phase due to high residence times. This chapter challenges this assumption and suggests that tiny bubbles can play a significant role in mass transfer. In addition, it has been shown that, unlike in lowviscosity liquids, gas hold-up in high-viscosity liquids is time dependent and, interestingly, need not always increase with increasing speciﬁc power dissipation levels. Impeller design and scale also play a critical role in determining hold-up, as well as its variation with operating parameters (i.e., speed and gas velocity).
Bakker, A., Smith, J.M., and Dyers, K.J. (1994) How to disperse gases in liquids, Chem. Eng., 101(12), 98–104. Khare, A.S. and Niranjan, K. (1995) Impeller agitated aerobic bioreactor: the inﬂuence of tiny bubbles on gas hold-up and mass transfer in highly viscous liquids, Chem. Eng. Sci., 50(7), 1091–1105. Khare, A.S. and Niranjan, K. (1999) An experimental investigation into the effect of impeller design on gas hold-up in a highly viscous Newtonian liquid, Chem. Eng. Sci., 54(8), 1093–1100. Khare, A.S. and Niranjan, K. (2000) Comparison of gas hold-up in impeller agitated water and high viscosity liquids, J. Chem. Eng. Jpn., 33(5), 815–817. Khare, A.S. and Niranjan, K. (2002) The effect of impeller design on gas hold-up in surfactant containing highly viscous non-Newtonian agitated liquids, Chem. Eng. Process., 41, 239-249. Muller, F.L. and Davidson, J.F. (1992) On the contribution of small bubbles to mass transfer in bubble columns containing highly viscous liquids, Chem. Eng. Sci., 47, 3525–3532. Nienow, A.W. (1990) Gas dispersion performance in fermenter operation, Chem. Eng. Progr., 8, 61–71. Philip, J., Proctor, J.M., Niranjan, K., and Davidson, J.F. (1990) Gas hold-up and liquid circulation in internal loop reactors containing highly viscous Newtonian and non-Newtonian liquids, Chem. Eng. Sci., 45, 651–664. Saito, F., Nienow, A.W., Chatwin, S., and Moore, I.P.T. (1992) Power, gas dispersion and homogenisation characteristics of Scaba and Rushton turbine impellers, J. Chem. Eng. Jpn., 25(3), 281–287. Schugerl, K. (1981) Oxygen transfer into highly viscous media, Adv. Biochem. Eng., 19, 71–174.
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Production and Partial Puriﬁcation of Glycosidases Obtained by Solid-State Fermentation of Grape Pomace Using Aspergillus niger 10
Sergio Huerta-Ochoa, María Soledad de Nicolás-Santiago, Wendy Dayanara Acosta-Hernández, Lilia Arely Prado-Barragán, Gustavo Fidel Gutiérrez-López, Blanca E. García-Almendárez, and Carlos Regalado-González
CONTENTS Introduction Materials and Methods Microorganism Solid Substrate Solid-State Culture Media Effect of C/N Ratio and Water Activity on Glycosidase Production Inoculum Production Fermentation Columns Enzyme Crude Extract Physicochemical Analyses: Carbon Dioxide (CO2) Evolution Proximal Analyses Glycosidase Activity Protease Activity Protein Partial Characterization of the Crude Extract Optimum pH and pH Stability Optimum Temperature and Temperature Stability Prepuriﬁcation Glycosidase Puriﬁcation Electrophoretic Studies (Sodium Dodecyl Sulfate–Polyacrylamide Gel Electrophoresis; SDS-PAGE)
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Results and Discussion Solid Substrate Characterization Culture Media Selection Partial Characterization of Arabinofuranosidase and Rhamnopyranosidase Partial a-L-Arabinofuranosidase Puriﬁcation Ammonium Sulfate Precipitation Anion Exchange Chromatography SDS-PAGE Electrophoresis Conclusions Acknowledgment References
Grape pomace is a waste material from winemaking comprised of residual sugars, gums, stalks, seeds, and a large number of polyphenolic and terpenoid compounds. Grape pomace can be divided in two main parts: grape pulp (rich in gums and sugars) and grape pips (rich in ﬁbers and tannins). The use of grape pomace has varied from country to country. For example, in France, current legislation makes it compulsory to transform grape pomace from cooperative wineries that send the waste to large distilleries where residual sugars are converted into a distilled liquor, which is sold as brandy. The remaining fractions are used to produce natural dyes (antocyanins), grapeseed oil, compost, and fuel for the distillery. The net result is an environmentally friendly industry with almost zero discharge, because the spillage is usually disposed of in agricultural ﬁelds or is treated to comply with environmental regulations. A review of the U.S. patent list indicates that grape pomace has been used to obtain anthocyanin pigments (Philip, 1976; Crosby et al., 1984; Shrikhande, 1984; Langston, 1985) and aroma compounds (Brillouet et al., 1989; Dupin et al., 1992; Günata et al., 1988, 1996). Hang (1988) used a technique of solidstate fermentation (SSF) in order to obtain signiﬁcant microbial production of citric acid from grape pomace. Additionally, two U.S. patents use this material for enzyme production or as part of a composting technology (Graefe, 1982a,b). The use of grape pomace has recently focused on animal feed through protein enrichment by SSF using cultures of Trichoderma viride (Bensoussani and Serrano-Carreon, 1997). The economic appraisal of microbial protein production, however, has remained uncertain because of the low cost of soybean protein. Work during the last decades has shown that grapes accumulate monoterpens, C13-norisoprenoids, and shikimate-derived metabolites mainly as odorless mono- and di-glycosidic conjugates (Voirin et al., 1990; López-Tamames et al., 1997; Charles et al., 2000; Ferreira et al., 2000). Flavor enhancement of grape juice and wine through hydrolysis of the glycosides has therefore attracted much attention (Günata et al., 1990a, 1997). Disaccharide glycosides © 2003 CRC Press LLC
can be hydrolyzed by a sequential reaction requiring, ﬁrst, a-L-rhamnopyranosidase (EC 220.127.116.11), a-L-arabinofuranosidase (EC 18.104.22.168), and b-apiofuranosidase and, second, b-glucopyranosidase (EC 22.214.171.124) (Günata et al., 1990a; Spagna et al., 1998). Such terpens are value-added materials of nearly U.S.$1000/kg in a pure form due to their high ﬂavor potency. Additionally, there are important amounts of polyphenols, namely tannins, in grape seeds. These can be transformed by fermentation or enzyme reactions by tannase into gallic acid, which is in high demand for the production of a variety of antioxidants in food and pharmaceutical industries. Gallic acid presently has a value of U.S.$200 to U.S.$500/kg. Glycosidases obtained from grapes, ﬁlamentous fungi, and yeast have been considered for research due to their involvement in the hydrolysis of glycosidic ﬂavor precursors. Thus, both grape pulp and grape pips can be used as substrates for production of microbial enzymes or value-added chemicals (terpenoids or polyphenols). In this work, one of the above-mentioned alternatives is explored for upgrading grape pomace by the production of glycosidases using SSF of grape pulp. Several reports on b-glucopyranosidase, a-L-rhamnopyranosidase, a-L-arabinofuranosidase, and b-apiosidase are available (Dupin et al., 1992; Günata et al., 1997; Guo et al., 1999); however, culture conditions for glycosidase production have only been studied using submerged cultures (Dupin et al., 1992) or puriﬁed from commercial crude enzyme preparations (Günata et al., 1997; Spagna et al., 1998, 2000). Their use in winemaking has allowed hydrolysis of monoterpenyl, arabinosyl, rhamnosyl, and apiosylglucosides. Thus, SSF can be used to produce this type of enzymes and could be of practical signiﬁcance because it may simplify materials handling. Simpliﬁcations include the use of solid substrates with minimal upstream (pasteurization and inoculation) and downstream operations (leaching by screw pressing). On the other hand, liquid fermentations require more complex unit operations (extraction of enzyme inducers, mash sterilization, stirred vat fermentation, ultraﬁltration of spent wort, and liquid waste treatment).
Materials and Methods
Microorganism Aspergillus niger, strain number 10 (ORSTOM, France) (Raimbault, 1981), was used in this study. The strain was kept in porcelain pearls at 4°C. Solid Substrate Grape (Vitis vinifera cv. Chenin Blanc) pomace obtained from the Freixenet Company (Cadereyta, Querétaro, México) was dried and separated into three parts (branches, seeds, and skins) by sieving. The skins were used as substrate for glycosidase production and were kept at room temperature until used. © 2003 CRC Press LLC
Solid-State Culture Media Solid substrate (dry skins) was supplemented by addition of other nutrients (nitrogen source and mineral salts) and an inducer (red wine, Cabernet Sauvignon 1998, Santa Rita, Chile) for glycosidase production. Initial media compositions tested are shown in Table 5.1.
Effect of C/N Ratio and Water Activity on Glycosidase Production To study the effect of C/N ratio and water activity of the culture medium on glycosidase production, a 22 factorial experiment was employed, based on the medium selected in the previous step and data reported in the literature (Table 5.2). Casein peptone was used as a nitrogen source.
Inoculum Production A porcelain pearl was inoculated into 250-mL conical ﬂasks containing 50 mL of potato dextrose agar (PDA) (Bioxon, México) before solidiﬁcation and mixed. The inoculated conical ﬂasks were incubated at 30°C for 5 days. Spores were harvested using 50 mL of sterile water added with 0.2% Tween 80 solution. Spore suspension was ﬁltered, and the concentration of spores (spores/mL) was quantiﬁed using a Newbauer chamber. The inoculum size used in the experiments was 2 ¥ 107 spores per gram of dry matter.
Fermentation Columns Small glass columns (20 g of wet material) were used for solid-state fermentation experiments, according to the methodology developed by Raimbault
TABLE 5.1 Culture Media Composition Used for Glycosidase Production by Solid-State Fermentation with Aspergillus niger 10 (NH4)2SO4 (mg/g dry substrate) 3.0 3.0 1.5 2.4 25.8 (NH4)2HPO4 (mg/g dry substrate) 1.50 1.50 0.56 0.90 9.53 Media Compound KH2PO4 MgCl26H2O Winea (mg/g dry (mL/g dry (mg/g dry substrate) substrate) substrate) 0.038 0.038 — 0.038 0.038 — 0.018 0.018 0.75 0.030 0.030 — 0.038 0.038 — Glucose (mg/g dry substrate) — — — 1.50 —
Culture Media Control Washedb Plus wine Plus glucose C/N 15.9 a
1998 Cabernet Sauvignon (Santa Rita) made in Chile. three times with hot distilled water and dried at 60°C.
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TABLE 5.2 Culture Media for the 22 Factorial Design Experiment Media Compounds Casein MgCl26H2O Peptone (mg/g dry (mg/g dry substrate) substrate) 8.73 203.9 8.73 203.9 8.73 47.26 8.73 47.26
Treatment 1 2 3 4
KH2PO4 (mg/g dry substrate) 8.73 8.73 8.73 8.73
Ethylenglycol (mL/g dry substrate) — 0.05 — 0.05
and Alazard (1980). Experiments were carried out at 30°C and a saturated airﬂow rate of 60 cm3/min.
Enzyme Crude Extract Enzyme crude extract was obtained using a modiﬁcation of the method of Roussos et al. (1992). Fermented wet samples were weighed and mixed with the same weight of 0.1 M citrate–phosphate buffer (pH 5.6). The crude extract was obtained by pressing the mixture at 2000 psi in a hydraulic press (Erkco model PH-51T, Aeroquip). The extract was ﬁltered and kept at 4°C for further puriﬁcation steps.
Physicochemical Analyses: Carbon Dioxide (CO2) Evolution Metabolic activity during the fermentation process was followed by outlet gas evolution (Saucedo-Castañeda et al., 1994) using a gas chromatograph ﬁtted on line (Cadena-Méndez et al., 1993).
Proximal Analyses Protein, ash, ether extract, moisture, C/N ratio, and crude ﬁber content were determined according to the methods of AOAC (Helrich, 1990).
Glycosidase Activity a- L -arabinofuranosidase and a- L -rhamnopyranosidase activity was determined using the method reported by Günata et al. (1997). 150 mL of enzyme extract was added to 150 mL of the proper substrate (p-nitrophenola-L-arabinofuranoside, 4 mM; p-nitrophenol-a-L-rhamnopyranoside, 4 mM), mixed and incubated for 10 min at 40°C. Enzyme reaction was stopped by adding 900 mL of 0.1-M Na2CO3 solution and cooling at room temperature. Absorbance was read at 400 nm using a Shimadzu UV-160 © 2003 CRC Press LLC
spectrophotometer and compared against a standard curve of p-nitrophenol (0.0 to 0.1 mM). An activity unit (U) was deﬁned as the amount of enzyme necessary to release 1 nmol of p-nitrophenol per minute at reaction conditions.
Protease Activity Protease activity was determined using the method reported by Perlman and Lorand (1970). Crude extract (0.5 mL) was added to 0.5 mL of 2% casein solution (pH 5.6) and incubated at 30°C for 10 min. Then 1 mL of 0.4-M TCA was added, mixed, and centrifuged at 16,800¥ g. Next, 2.5 mL of 0.1-N Na2CO3 and 0.5 mL of Folin–Ciocalteu reagent were added to 0.5 mL of the supernatant for color development. Sample absorbance was read at 660 nm and compared against a standard curve of tyrosine.
Protein Protein was quantiﬁed using the Bradford microassay (Bio-Rad) based on the Bradford (1976) method. Sample absorbance was read at 595 nm and compared against a standard curve of bovine serum albumin (BSA) in the range of 1 to 10 mg/mL.
Partial Characterization of the Crude Extract
Optimum pH and pH Stability For optimum pH determination, glycosidase activity was assayed in a pH range of 1 to 10 using different buffer solutions (Table 5.3) at 40°C for 10 min. For enzyme stability, the same pH range was tested, keeping the enzyme solutions at 4°C for 14 h, after which glycosidase activity was assayed at 40°C for 10 min. Residual enzyme activity was reported as relative activity (%).
TABLE 5.3 Buffer Solutions Used for Optimum pH and pH Stability Determination Buffer Solution HCl, 0.1 N Citrate, 0.3 M Phosphate Carbonate–bicarbonate pH Range 1–2 3–4 5–8 9.2–10
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Optimum Temperature and Temperature Stability Optimum temperature for glycosidase activity was determined by assaying enzyme activity between 15°C and 70°C for 10 min. For temperature stability, enzyme extract samples of 150 mL were kept at the temperature range tested (15 to 70°C) for 1 h, and glycosidase activity was assayed at 40°C for 10 min. Residual enzyme activity was reported as relative activity (%).
Prepuriﬁcation Precipitation by salting out was carried out using 80% saturation of ammonium sulfate under continuous agitation at 4°C and kept for 2 h. The precipitate was centrifuged at 12,000¥ g at 4°C for 15 min. The precipitate was resuspended in 10 mM citrate–phosphate buffer (pH 5.6). The resuspended precipitate was dialyzed against the same buffer at 4°C for 24 h, with the buffer solution being changed every 6 h. Glycosidase activity and protein were determined, followed by freeze drying (LabConCo equipment).
Glycosidase Puriﬁcation The lyophilized sample was resuspended in 0.3-M citrate buffer (pH 2.6). An Econo-Column (Bio-Rad) ﬁlled with DEAE-cellulose (Whatman) was equilibrated using this buffer. Two-milliliter aliquots were ﬁltered in membranes of 0.45 mm and injected onto the anion exchange column, then 4-mL fractions were collected at a ﬂow rate of 4 mL/min. From fraction 5, elution was conducted using the same buffer added with 1-M NaCl, and a linear gradient from 0 to 50% of this buffer was used up to fraction 23. From fractions 23 to 35, the gradient varied from 50 to 100%. Protein and glycosidase activity were determined for each fraction, and those having activity were mixed and dialyzed against deionized water at 4°C for 36 h.
Electrophoretic Studies (Sodium Dodecyl Sulfate–Polyacrylamide Gel Electrophoresis; SDS-PAGE) Electrophoresis under denatured conditions was carried out according to Laemmli (1970) using a Mighty Small SE 250 (Hoeffer) vertical electrophoresis chamber. Gels of 10% T (percent of acrylamide in the solution acrylamide plus bis-acrylamide) and 8% C (percent of bis-acrylamide in the monomers solution) were used for the separating gel, while the stacking gel had 4% C. Low-molecular-weight protein markers (Sigma) were used for SDS-PAGE: aprotinine (6500 Da), a-lactalbumin (14,200 Da), trypsin inhibitor (20,000 Da), trypsinogen (24,000 Da), carbonic anhydrase (29,000 Da), glyceraldehyde 3-phosphate dehydrogenase (36,000 Da), egg albumin (45,000 Da), and bovine serum albumin (66,000 Da). Injection volume was 12 mL. Protein alignment was conducted at 10 mA, while protein band separation was © 2003 CRC Press LLC
conducted at 15 mA. The rapid silver staining method (Ausubel et al., 1995) was used to identify the protein bands. Proteins were ﬁxed with a formaldehyde solution (methanol 40%, 0.5 mL of 37% [w/w] formaldehyde per liter of solution) for 10 min with constant stirring. After 5 min of washing, the gels were submerged in Na2S2O3 solution (0.2 g/L). After washing with deionized water, a 0.1% silver nitrate solution was used for protein dying for 10 min. Silver was removed by washing, and the gel was submerged in a revealing solution (3% sodium carbonate, 0.0004% Na2S2O3, and 0.5 mL of formaldehyde per liter of solution) for 1 min until bands were revealed. Five milliliters of 2.3-M citric acid per 100 mL of revealing solution were added. Finally, gels were submerged in 50 mL of drying solution (10% ethanol and 4% glycerol).
Results and Discussion
Solid Substrate Characterization Grape pomace was sundried until it reached 11% moisture, and separated into three parts (branches, seeds, and skins) by sieving (Table 5.4). Due to grape pomace compositional changes according to season, type of grapes, weather conditions, etc., no comparison with similar wastes was attempted. Skins were taken as solid substrate for solid-state fermentation experiments. Besides the presence of expected aroma precursors in the skins, solid waste had high protein and crude ﬁber content (Table 5.5).This fact allowed us to use the skins as substrate of Aspergillus niger 10 for enzyme production. This fungus has a broad enzyme synthesis spectrum and can be induced to produce glycosidases as well as proteases, hemicellulases, and lipases.
Culture Media Selection Five culture media (Table 5.1) were selected for preliminary fermentation experiments. These media were selected in order to ﬁnd a proper initial culture medium: TABLE 5.4 Results after Grape Pomace Sieving
Part Seeds Skins Branches Percent of Dry Matter 58.46 32.01 9.53
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TABLE 5.5 Proximal Analysis of Skins Separated from Grape Pomace and Results from Other Reports Parameter Protein Ash Ether extract Moisture C/N ratio Crude ﬁber Percent Wet Basis This Work Literaturea 14.6 12–17 6.31 3–13 2.25 5–10 11.0 nr 20.6 nr 31.5 22–35
aBensousani and Serrano-Carreón (1988). Note: nr = not reported.
1. Control: Solid waste without any treatment 2. Washed: Solid waste washed three times with hot distilled water to remove possible unwanted compounds 3. Plus wine: Solid waste supplemented with commercial wine to add aroma precursors that may induce glycosidase production 4. Plus glucose: Solid waste supplemented with glucose to induce high microbial growth 5. C/N ratio, 15.9: Solid waste supplemented with ammonium sulfate and ammonium phosphate to modify the initial C/N ratio of 20 Microbial growth and enzyme production were quantiﬁed by measuring CO2 evolution during fermentation and glycosidase activity, respectively, in the crude extract (a-L-rhamnopyranosidase and a-L-arabinofuranosidase). Results of enzyme activity after 65 h of fermentation are shown in Figure 5.1.
Enzyme activity (U/mL)
Effect of media supplementation on enzyme production by Aspergillus niger 10 growth on grape skins as solid substrate.
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Under the fermentation conditions tested, the main activity produced was arabinofuranosidase. Control and C/N 15.9 culture media showed important enzyme production. The highest arabinofuranosidase and rhamnopyranosidase activities were produced when the fungus grew in the C/N 15.9 culture medium. Washed culture media presented some arabinofuranosidase activity and little rhamnopyranosidase activity, probably due to nutrient removal during the washings. On the other hand, the plus-wine and plus-glucose culture media were the poorest for enzyme production, probably due to the presence of simple carbon sources, which inhibited enzyme synthesis (SolisPereira et al., 1996). Thus, the C/N 15.9 medium was chosen for further enzyme production. Moreover, the maximum speciﬁc respiration rate, m(CO2), for this media was also the largest: 0.312/h (Table 5.6), indicating that A. niger invaded the solid substrate much faster with this culture media. Although enzyme production is not completely associated with fungal growth, the magnitude of the m(CO2) for this culture media is in agreement with the glycosidase production obtained. Kinetic studies of microbial growth (CO2 evolution) and enzyme production using a culture medium with a C/N ratio of 15.9 were conducted over 34 h. From kinetic data (Figure 5.2) it was observed that enzyme production started after 22 h of fermentation, when microbial growth was at the end of the exponential phase. During the beginning of the stationary phase, arabinofuranosidase activity increased up to 170 U/mL in 4 h (between 22 and 26 h of fermentation time). During the next 8 h, the arabinofuranosidase production rate slowed to about 75%; however, the activity after 32 h of fermentation increased to a value of 250 U/mL. From data reported in the literature (Dupin et al., 1992) and preliminary experimental results (data not shown), we decided to change the nitrogen source from inorganic to organic (peptone) to improve enzyme production. The effect of water activity (aw) and C/N ratio on glycosidase production was also studied. It was observed that the highest arabinofuranosidase activity was obtained using a C/N ratio of 10 and aw value of 0.979 (Table 5.7) after 29 h of fermentation. Figure 5.3 shows the kinetic data of microbial growth (CO2 evolution) and enzyme production using the above-mentioned conditions for 44 h of
TABLE 5.6 Maximum Speciﬁc Respiration Rate Based on CO2 Evolution Kinetics Culture Media Control Washed Plus wine Plus glucose C/N ratio 15.9 m (CO2) (hr1) 0.254 0.165 0.117 0.134 0.312
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Enzyme activity (U/ml)
100 0.5 50
Fermentation time (h)
Rhamnopyranosidase Arabinofuranosidase CO2
Kinetics of CO2 evolution and enzyme production during solid-state fermentation (SSF) of grape skins using Aspergillus niger 10 and supplementing with C/N 15.9 culture media.
TABLE 5.7 Effect of Water Activity (aw) and C/N Ratio on Enzyme Production C/N Ratio 10 15
aw = 0.985
Arabinofuranosidase Activity (U/mL) aw = 0.979 aw = 0.975 243 — — 45
aw = 0.968
fermentation time. An enzyme production pattern similar to that found in Figure 5.2 was observed until 34 h of fermentation. After this fermentation time, arabinofuranosidase production increased almost three times in the next 6 h, up to 670 U/mL in 44 h. However, rhamnopyranosidase production was not improved by the changes tested in the culture medium.
Partial Characterization of Arabinofuranosidase and Rhamnopyranosidase This study was conducted primarily to monitor the effect of pH and temperature on arabinofuranosidase and rhamnopyranosidase stability and activity in the crude extract. The crude extract was obtained from SSF experiments using a culture medium with C/N = 10 after 30 h of fermentation. © 2003 CRC Press LLC
Enzyme activity (U/mL)
0.0 0 10 20 30 40
Fermentation time (h)
Rhamnopyranosidase Arabinofuranosidase CO2
Kinetics of CO2 evolution and enzyme production during solid-state fermentation (SSF) of grape skins using Aspergillus niger 10 and supplementing with C/N 10 culture media with aw of 0.979.
Arabinofuranosidase activity was the highest at pH 3.0 (Figure 5.4a). This optimum pH is close to the pH of wine (3.8) used in this work. Günata et al. (1997) puriﬁed from a commercial enzyme preparation an arabinofuranosidase with an optimum pH of 4.0 (Klerzyme 200, Gist-Brocades, France). Spagna et al. (1998) also reported an optimum pH of 4.0 for a-L-arabinofuranosidase puriﬁed from another commercial preparation (AR 2000, Gist-Brocades, France). This enzyme showed only a relative activity of 40% at pH 3.3. Arabinofuranosidase activity showed high stability at pH 3.0 (Figure 5.4b) where no relative activity was lost after 14 h of incubation at 4°C; however, arabinofuranosidase activity had poor stability at higher pH values. At pH 3.5, the activity decreased to about 50% that shown without 14 h incubation, and it further decreased at higher pH values (Figure 5.4b). At pH 3.0 the stability was good (100% activity retention), but the activity relative to that at pH 9.0 was insigniﬁcant. Results on pH for optimum activity and stability for rhamnopyranosidase are shown in Figures 5.4a and b. The activity was low ( 9.0. The effect of increasing IS was to increase protein solubilization (from 0.10 to 0.20 M), but further increases in IS resulted in more reduction of protein activity recovery after back extraction. The puriﬁcation achieved using this process is shown in Table 9.7. Precipitation of the crude extract with acetone or ammonium sulfate or both gave lower puriﬁcation results; for instance, after treatment with both precipitants, the puriﬁcation factor was 11 with a yield of 44%. Therefore, reverse micelles can be used for the initial stage of peroxidase puriﬁcation from Brussels sprouts, a process that gave 5.5 times better puriﬁcation factor and double activity yield than the precipitation methods.
Puriﬁcation using CTAB reverse micelles produced a turnip peroxidase extract having a high speciﬁc activity (173 U/mg) suitable for the analysis of glucose, cholesterol, etc., where high purity is not important. A two-step forward extraction with reverse micelles of 0.20-M CTAB in isooctane/hexanol (90:10), followed by back extraction, produced a Brussels sprouts peroxidase with a 5.5-fold puriﬁcation factor and twice the activity yield when compared to acetone and ammonium-sulfate precipitations. Similarly, turnip root peroxidase puriﬁcation using 0.20-M CTAB reverse micelles in isooctane/pentanol (90:10) produced about 8 times the puriﬁcation factor © 2003 CRC Press LLC
95 pH 6.0, then pH 8.0
Activity recovery (%)
75 55 35 15 -5 0.05
pH 6.0, then pH 9.0 pH 6.0, then pH 10.0
Ionic Strength (adjusted with KCl; M) FIGURE 9.4 Effect of pH and ionic strength on reverse micellar peroxidase puriﬁcation from Brussels sprouts using a two-stage forward extraction with 0.20 M CTAB in isooctane/hexanol (90:10). The ﬁrst forward-transfer step was conducted at the pH shown on the left side of the legend, while the second step pH used is shown on the right side of the legend; mean values of triplicate runs with SEM within 5%.
TABLE 9.7 Summary of the Puriﬁcation Stages of Brussels Sprouts Peroxidase Using CTAB Reverse Micelles in Isooctane/Hexanol (90:10)a Puriﬁcation Stage DCEb PPc
a Average b
Protein (mg) 3.20 0.045
Activity (U) 1.40 1.20
Speciﬁc Activity (U/mg) 0.45 26
Puriﬁcation Factor 1.00 61
Yield (%) 100 85
of three replicates, with SE within 5% of the mean. Dialyzed crude extract from Brussels sprouts. c Puriﬁed peroxidase after back extraction. Forward transfer: 0.2 M KCl, pH 6.0 (ﬁrst stage), pH 9.0 (second stage). Back extraction: aqueous solution containing 2.0 M KCl, pH 4.0.
achieved with ammonium sulfate with similar activity yield. Therefore, reverse micelles can be advantegously used for the initial stage of peroxidase puriﬁcation from these sources. Peroxidase isoenzymes can have different amino acid and carbohydrate composition, leading to pI in a wide pH range. This complicates reverse micellar extraction because of weak electrostatic interactions at low or high pH suitable for extraction with anionic or cationic surfactants without denaturation. The effect of K+ on forward transfer was important in achieving better activity recovery after back extraction for radish and Brussels sprouts extracts. For turnip roots, the use of Na+ instead of K+ to adjust IS during forward transfer allowed 50% better puriﬁcation factor, but about 20% less activity yield. © 2003 CRC Press LLC
Because liquid–liquid extraction techniques can be applied to reverse micellar extraction, the advantages and limitations of this technology apply to the large-scale recovery of bioproducts. Adequate extraction equipment must be selected, bearing in mind the objectives of high capacity, high mass transfer rate, and low axial mixing. Typical extraction equipment meeting these requirements are mixer-settler units, centrifugal extractors, and column contactors. Fundamental studies over a wide range of process conditions must be conducted to support the scaling-up of a reverse micellar extraction. This technology has yet to prove its value on a pilot-scale basis over a complete recovery cycle before it can be ranked as a genuine alternative method of protein puriﬁcation.
Adachi, M., Harada, M., Shioi, A., Takahashi, H., and Katoh, S. (1996) Selective separation of concanavalin A in a reverse micellar system, in Proc. of ISEC ’96, D.C. Shallcross, R. Paimin, and L.M. Prvcic, Eds., The University of Melbourne, pp. 1393–1398. Adachi, M., Yamazaki, M., Harada, M., Shioi, A., and Katch, S. (1997) Bioafﬁnity separation of trypsin using trypsin inhibitor immobilized in reverse micelles composed of a nonionic surfactant, Biotechnol. Bioeng., 53, 406–408. Adachi, M., Shibata, K., Shioi, A., Harada, M., and Katoh, S. (1998) Selective separation of trypsin from pancreatin using bioafﬁnity in reverse micellar system composed of a nonionic surfactant, Biotechnol. Bioeng., 58, 649–653. Almeida, F.C.L., Valente, A.P., and Chaimovich, H. (1998) Stability and activity modulation of chymotrypsins in AOT reversed micelles by protein-interface interaction: interaction of a-chymotrypsin with a negative interface leads to a cooperative breakage of a salt bridge that keeps the catalytic active conformation (Ile16-Asp194), Biotechnol. Bioeng., 59, 360–363. AOAC (1990) Ofﬁcial Methods of Analysis, Vol. 1, 15th ed., K. Helrich, Ed., AOAC, Arlington, VA, pp. 47–48. Aveyard, R., Binks, B.P., and Fletcher, P.D.I. (1989) Interfacial tensions and aggregate structure in C12E5/oil/water microemulsion systems, Langmuir, 5, 1210–1217. Bartsotas, P., Poppenborg, L.H., and Stuckey, D.C. (2000) Emulsion formation and stability during reversed micelle extraction, J. Chem. Technol. Biotechnol., 75, 738–744. Bausch, T.E., Plucinsky, P.K., and Nitsch, W. (1992) Kinetics of the re-extraction of hydrophilic solutes out of AOT-reversed micelles, J. Colloid Interface Sci., 150, 226–234. Bernardi, R. and Palmiere, S. (1996) Isolation of a trypsin-like activity from larvae of corn borer (Ostrinia nubilatis) using reverse micelles, Biotechnol. Lett., 18, 663–666. Bosetti, A., Tassinari, R., and Battistel, E. (1997) Partial puriﬁcation of glutaryl-7-ACA acylase from crude cellular lysate by reverse micelles, Appl. Biochem. Biotech., 66, 173–183.
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Brandani, S., Brandani, V., DiGiacomo, G., and Spera, L. (1996a) Effect of nonlinear equilibrium on the mass transfer rate of a-amylase extraction by reversed micelles, Process Biochem., 31, 249–252. Brandani, V., Di Giacomo, G., and Spera, L. (1996b) Recovery of a-amylase extracted by reverse micelles, Process Biochem., 31, 125–128. Campa, A. (1991) Biological roles of plant peroxidase: known and potential function, in Peroxidases in Chemistry and Biology, L. Everse, K.E. Everse, and M.B. Grisham, Eds., CRC Press, Boca Raton, FL, pp. 26–43. Cardoso, M.M., Barradas, M.J., Kroner, K.H., and Crespo, J.G. (1999) Amino acid solubilization in cationic reversed micelles: factors affecting amino acid and water transfer, J. Chem. Technol. Biotechnol., 74, 801–811. Carlson, A. and Nagarajan, R. (1992) Release and recovery of porcine pepsin and bovine chymosin from reverse micelles: a new technique based on isopropyl alcohol addition, Biotechnol. Progress, 8, 85–90. Chang, Q.L. and Chen, J.Y. (1996) Liquid–liquid reversed micellar extraction for isolating enzymes: studies on the puriﬁcation of a-amylase, Process Biochem., 31, 371–375. Chang, Q.L., Chen, J.Y., Zhang, X.F., and Zhao, N.M. (1997) Effect of the cosolvent type on the extraction of a-amylase with reversed micelles: circular dichroism study, Enzyme Microb. Technol., 20, 87–92. Chaudhuri, J.B. and Spirovska, G. (1994) Recovery of proteins from reversed micelles using a novel ion-exchange material, Biotechnol. Technol., 8, 909–914. Chen, S.J., Evans, D.F., Ninham, B.W., Mitchel, D.J., Blum, F.D., and Pickup, S. (1986) Curvature as a determinant of microstructure and microemulsions, J. Phys. Chem., 90, 842–847. Childs, R.E. and Bardsley, W.G. (1975) The steady-state kinetics of peroxidase with 2,2¢-azino-di-(3-ethyl-benzthiazoline-6-sulphonic acid) as chromogen, Biochem. J., 145, 93–103. Clark, S., Fletcher, P.D.I., and Ye, X. (1990) Interdroplet exchange rates of water-inoil and oil-in-water microemulsion droplets stabilized by pentaoxyethylene monododecyl ether, Langmuir, 6, 1301–1309. Clementi, F. and Palade, G.E. (1969) Intestinal capillaries I. Permeability to peroxidase and ferritin, J. Cell Biol., 41, 33–58. Contreras-Padilla, M. and Yahia, E.M. (1998) Changes in capsaicinoids during development, maturation, and senescence of chile peppers in relation with peroxidase activity, J. Agric. Food Chem., 46, 2075–2079. Dahuron, L. and Cussler, D.L. (1988) Protein extractions with hollow ﬁbres, AICHE J., 34, 130–136. Dekker, M. (1990) Enzyme Recovery Using Reversed Micelles, Ph.D. thesis, Wagenigen Agricultural University, Holland. Dekker, M., Van’t Riet, K., Van Der Pol, J.J., Baltussen, J.W.A., Hilhorst, R., and Bijsterbosch, B.H. (1991) Effect of temperature on the reversed micellar extraction of enzymes, Chem. Eng. J., 46, B69–74. Duarte-Vázquez, M.A., García-Almendárez, B., Regalado, C., and Whitaker, J.R. (2000) Puriﬁcation and partial characterization of three turnip (Brassica napus L. var. esculenta D.C.) peroxidases, J. Agric. Food Chem., 48, 1574–1579. Dunford, H.B. (1991) Horseradish peroxidase: structure and kinetic properties, in Peroxidases in Chemistry and Biology, Vol. II, J. Everse, K.E. Everse, and M.B. Grisham, Eds., CRC Press, Boca Raton, FL, pp. 1–24.
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Dungan, S.R., Bausch, T., Hatton, T.A., Plucinsky, P., and Nitsch, W. (1991) Interfacial transport processes in the reversed micellar extraction of proteins, J. Colloid Interface Sci., 145, 33–50. Eastoe, J., Robinson, B.H., Steytler, D.C., and Thorn-Lesson, D. (1991) Structural studies of microemulsions stabilised by aerosol-OT, Adv. Colloid Interface Sci., 36, 1–31. Eicke, H.F. and Kvita, P. (1984) Reverse micelles and aqueous microphases, in Reverse Micelles: Biological and Technological Relevance of Amphiphilic Structures in Apolar Media, P.L. Luisi and B.E. Straub, Eds., Plenum Press, New York, pp. 21–35. Fletcher, P.D.I., Howe, A.M., and Robinson, B.H. (1987) The kinetics of solubilisate exchange between water droplets of a water-in-oil microemulsion, J. Chem. Soc. Faraday Trans. I, 83, 985–1006. Goklen, K.E. and Hatton, T.A. (1985) Protein extraction using reverse micelles, Biotechnol. Prog., 1, 69–74. Goto, M., Ono, T., Nakashio, F., and Hatton, T.A. (1997) Design of surfactants suitable for protein extraction by reverse micelles, Biotechnol. Bioeng., 54, 26–32. Gupta, R.B., Han, C.J., and Johnston, K.P. (1994) Recovery of proteins and amino acids from reverse micelles by dehydration with molecular sieves, Biotechnol. Bioeng., 44, 830–836. Hashimoto, Y., Ono, T., Goto, M., and Hatton, A. (1998) Protein refolding by reversed micelles utilizing solid–liquid extraction technique, Biotechnol. Bioeng., 57, 620–623. Hatton, T.A. (1989) Reversed micellar extraction of proteins, in Surfactant Based Separation Processes, J.F. Scamehorn and J.H. Harwell, Eds., Marcel Dekker, New York, pp. 55–90. Hayes, D.G. (1997) Mechanism of protein extraction from the solid state by waterin-oil microemulsions, Biotechnol. Bioeng., 53, 583–593. Hayes, D.G. and Marchio, C. (1998) Expulsion of proteins from water-in-oil microemulsions by treatment with cosurfactant, Biotechnol. Bioeng., 59, 557–566. Hentsch, M., Menoud, P., Steiner, L., Flaschel, E., and Renken, A. (1992) Optimization of the surfactant (AOT) concentration in a reverse micellar extraction process, Biotechnol. Technol., 6, 359–364. Hossain, M.J., Takeyama, T., Hayashi, Y., Kawanishi, T., Shimizu, N., and Nakamura, R. (1999) Enzymatic activity of Chromobacterium viscosum lipase in an AOT/ Tween 85 mixed reverse micellar system, J. Chem. Technol. Biotechnol., 74, 423–428. Hu, Y. and Gulari, E. (1996a) Extraction of aminoglycoside antibiotics with reverse micelles, J. Chem. Technol. Biotechnol., 65, 45–48. Hu, Y. and Gulari, E. (1996b) Protein extraction using the sodium bis(2-ethylhexyl)phosphate (NaDEHP) reverse micellar system, Biotechnol. Bioeng., 50, 203–206. Huang, S.Y. and Lee, Y.C. (1994) Separation and puriﬁcation of horseradish peroxidase from Armoracia rusticana root using reversed micellar extraction, Bioseparation, 4, 1–5. Huang, W., Li, X., Zhou, J., and Gu, T. (1996) Activity and conformation of bovine pancreatic ribonuclease A in reverse micelles formed by dodecylammonium butyrate and water in cyclohexane, Colloids Surfaces B. Biointerfaces, 7, 23–29. Jarudilokkul, S., Poppenborg, L.H., and Stuckey, D.C. (1999) Backward extraction of reverse micellar encapsulated proteins using a counterionic surfactant, Biotechnol. Bioeng., 62, 593–601. © 2003 CRC Press LLC
Jarudilokkul, S., Paulsen, E., and Stuckey, D.C. (2000a) The effect of demulsiﬁers on lysozyme extraction from hen egg white using reverse micelles, Bioseparation, 9, 81–91. Jarudilokkul, S., Paulsen, E., and Stuckey, D.C. (2000b) Lysozyme extraction from egg white using reverse micelles in a Graesser contactor: mass transfer characterization, Biotechnol. Bioeng., 69, 618–626. Kadam, K.L. (1986) Reverse micelles as a bioseparation tool, Enzyme Microbiol. Technol., 8, 266–273. Kawakami, L.E. and Dungan, S.R. (1996) Solubilization properties of a-lactalbumin and b-lactoglobulin in AOT-isooctane reversed micelles, Langmuir, 12, 4073–4083. Krei, G.A. and Hustedt, H. (1992) Extraction of enzymes by reverse micelles, Chem. Eng. Sci., 47, 99–111. Krell, H.W. (1991) Peroxidase: an important enzyme for diagnostic test kits, in Biochemical, Molecular and Physiological Aspects of Plant Peroxidases, J. Lobarzewsky, H. Greppin, C. Penel, and T. Gaspar, Eds., University M. Curie, Lublin, Poland; University of Geneva, Geneva, Switzerland, pp. 469–478. Krieger, N., Taipa, M.A., Aires-Barros, M.R., Melo, E.H.M., Lima-Filho, J.L., and Cabral, J.M.S. (1997) Puriﬁcation of the Penicillium citrinum lipase using AOT reverse micelles, J. Chem. Technol. Biotechnol., 69, 77–85. Lazarova, Z. and Tonova, K. (1999) Integrated reversed micellar extraction and stripping a-amylase, Biotechnol. Bioeng., 63, 583–592. Leodidis, E.B. and Hatton, T.A. (1989) Speciﬁc ion effects in electrical double layers: selective solubilization of cations in aerosol-OT reversed micelles, Langmuir, 5, 741–753. Leser, M.E., Mrkoci, K., and Luisi, P.L. (1993) Reverse micelles in protein separation: the use of silica for the back-transfer process, Biotechnol. Bioeng., 41, 489–492. Lu, Q., Li, K., Zhang, M., and Shi, T. (1998) Study of centrifugal extractor for protein extraction using reverse micellar solutions, Sep. Sci. Technol., 33, 2397–2409. Luisi, P.L. and Magid, L.J. (1986) Solubilization of enzymes and nucleic acids in hydrocarbon micellar solutions, Crit. Rev. Biochem., 20, 409–474. Luisi, P.L. and Steinmann-Hofmann, B. (1987) Activity and conformation of enzymes, in reverse micellar solutions, in Methods in Enzymology, Vol. 136, K. Mosbach, Ed., Academic Press, New York, pp. 188–219. Luisi, P.L., Bonner, F.J., Pellegrini, A., Wiget, P., and Wolf, R. (1979) Micellar solubilization of proteins in aprotic solvents and their spectroscopic characterization, Helv. Chim. Acta., 62, 740–753. Lye, G.J., Asenjo, J.A., and Pyle, D.L. (1996) Reverse micellar mass-transfer processes: spray column extraction of lysozyme, AICHE J., 42, 713–726. Maitra, A.N. (1984) Determination of size parameters of water–aerosol OT-oil reverse micelles from their nuclear magnetic resonance data, J. Phys. Chem., 88, 5122–5125. McGuire, R.G. (1992) Reporting of objective color measurements, Hort. Sci., 27, 1254–1255. Melo, E.P., Costa, S.M.B., and Cabral, J.M.S. (1996) Denaturation of a recombinant cutinase from Fusarium solani in AOT-isooctane reverse micelles: a steady-state ﬂuorescence study, Photochem. Photobiol., 63, 169–175. Melo, E.P., Carvalho, C.M.L., Aires-Barros, M.R., Costa, S.M.B., and Cabral, J.M.S. (1998) Deactivation and conformational changes of cutinase in reverse micelles, Biotechnol. Bioeng., 58, 380–386. © 2003 CRC Press LLC
Muñoz de Chávez, M., Chávez Villasana, A., Roldán Amaro, J.A., Ledesma Solano, J.A., Mendoza Martínez, E., Pérez-Gil Romo, F., Hernández Cordero, S.L., and Chaparro Flores, A.G. (1996) Tablas de Valor Nutritivo de los Alimentos de Mayor Consumo en Latinoamérica, PAX, México, pp. 74, 91, 96. Nagayama, K., Nishimura, R., Doi, T., and Imai, M. (1999) Enhanced recovery and catalytic activity of Rhizopus delemar lipase in an AOT microemulsion with guanidine hydrochloride, J. Chem. Technol. Biotechnol., 74, 227–230. Naoe, K., Nishino, M., Ohsa, T., Kawagoe, M., and Imai, M. (1999) Protein extraction using sugar ester reverse micelles, J. Chem. Technol. Biotechnol., 74, 221–226. Nishii, Y., Nii, S., Takahashi, K., and Takeuchi, H. (1999) Extraction of proteins by reversed micellar solution in a packed column, J. Chem. Eng. Jpn., 32, 211–216. Nitsch, W. and Plucinsky, P. (1990) Two phase kinetics of the solubilization in reverse micelles, J. Colloid Interface Sci., 136, 338–351. Ono, T., Goto, M., Nakashio, F., and Hatton, T.A. (1996) Extraction behavior of hemoglobin using reverse micelles by dioleyl phosphoric acid, Biotechnol. Prog., 12, 793–800. Orlich, B. and Schomaecker, R. (1999) Enzymatic reduction of a less water-soluble ketone in reverse micelles with NADH regeneration, Biotechnol. Bioeng., 65, 357–372. Orlich, B., Berger, H., Lade, M., and Shoemäcker, R. (2000) Stability and activity of alcohol dehydrogenases in W/O-microemulsions: enantioselective reduction including cofactor regeneration, Biotechnol. Bioeng., 70, 638–646. Paradkar, V.M. and Dordick, J.S. (1993) Afﬁnity-based reverse micellar extraction and separation (ARMES): a facile technique for the puriﬁcation of peroxidase from soybean hulls, Biotechnol. Prog., 9, 199–203. Paul, K.G. and Stigbrand, T. (1970) Four isoperoxidases from horseradish root, Acta Chem. Scand., 24, 3607 3617. Pérez-Arvizu, O., García, B.E., and Regalado, C. (1999) Puriﬁcation of peroxidase from waste frozen vegetable processing companies using reverse micelles, in Food for Health in the Paciﬁc Rim, J.R. Whitaker, N.F. Haard, C.F. Shoemaker, and R.P. Singh, Eds., Food & Nutrition Press, Trumbull, CT, pp. 206–215. Pires, M.J. and Cabral, J.M.S. (1993) Liquid–liquid extraction of a recombinant protein with a reverse micelle phase, Biotechnol. Prog., 9, 647–650. Plucinsky, P. and Nitsch, W. (1989) Two phase kinetics of the solubilization in reverse micelles extraction of lysozyme, Ber. Bunsenges. Phys. Chem., 93, 994–997. Rariy, R.V., Bec, N., Klyachko, N.L., and Levashov, A.V. (1998) Thermobarostability of a-chymotrypsin in reversed micelles of aerosol OT in octane solvated by water-glycerol mixtures, Biotechnol. Bioeng., 57, 552–556. Regalado, C., Asenjo, J.A., and Pyle, D.L. (1994) Protein extraction by reverse micelles. Studies on the recovery of horseradish peroxidase, Biotechnol. Bioeng., 44, 674–681. Regalado, C., Asenjo, J.A., and Pyle, D.L. (1996) Studies on the puriﬁcation of peroxidase from horseradish roots using reverse micelles, Enzyme Microb. Technol., 18, 332–339. Regalado, C., Pérez-Arvizu, O., García-Almendárez, B.E., and Whitaker, J.R. (1999) Puriﬁcation and properties of two acidic peroxidases from Brussels sprouts (Brassica oleraceae L.), J. Food Biochem., 23, 435–450. Robinson, B.H., Toprakcioglu, C., Dore, J.C., and Chieux, P. (1984) Small-angle neutron-scattering study of microemulsions stabilized by aerosol-OT. Part 1. Solvent and concentration variation, J. Chem. Soc. Faraday Trans. I, 80, 13–27. © 2003 CRC Press LLC
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Improving Biogeneration of Aroma Compounds by In Situ Product Removal
CONTENTS Introduction Downstream Processes for Aroma Compounds In Situ Product Removal of Aroma Compounds Extraction into Another Phase Liquid–Liquid Extraction Solid-Phase Extraction Gas Extraction Membrane Separation Hollow-Fiber Membranes Pervaporation Conclusion Acknowledgments References
Consumer preferences for natural food additives has led to an increasing demand for natural aroma compounds; however, the poor productivity of these novel biotechnological processes often prevents industrial application. Several works concerning the production of ﬂavors by microorganisms have shown that the production rate decreases when the product concentration exceeds a toxicity threshold beyond which growth and/or production are inhibited. Attention is now being directed to extractive fermentation, which is regarded as a promising fermentation technology because it is capable of relieving end-product inhibition and increasing productivity. Also, it can recover fermentation products in situ and hence
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simplify downstream processing. Reports on in situ aroma extraction from the culture broth are mostly focused on two general techniques: (1) extraction into another phase, and (2) membrane separation. This chapter presents a review of the literature concerning the experimental results, primarily from the last decade, on attempts to improve biogeneration of aroma compounds by in situ product removal.
Downstream Processes for Aroma Compounds
Flavor and aroma chemicals used in the food, cosmetic, and pharmaceutical industries are products of great commercial signiﬁcance. Increased production of processed foods is accompanied by an increasing demand for ﬂavors; however, chemical synthesis, now used to produce aroma compounds, is markedly declining because of new regulations concerning food additives and because of consumer dislike of synthetic compounds. Existing technologies for ﬂavor and aroma production by microorganisms or enzymatic synthesis offer an alternative to chemical synthesis (Gatﬁeld, 1988). The industrial exploitation of microorganisms (bacteria, yeast, and fungi) for the production of ﬂavors is in fact an extension of the biotechnology used for traditional processes. Most of the aroma compounds (terpenes, esters, ketones, lactones, alcohols, and aldehydes) are secondary metabolites. Secondary metabolites are compounds that cells do not require for growth. They are present at low concentrations during the logarithmic growth phase, but appear in large quantities during the stationary phase. In many cases, such compounds result from detoxiﬁcation processes developed by the cell to contend with unfavorable environmental conditions (e.g., when a high concentration of nutrients or metabolites is present). In aroma biosynthesis, the success of the process depends on the individual stages, namely strain screening, improvement of selected strains, process design, and downstream processing (Belin et al., 1992). Downstream processing is the general term used to describe separation processes for the recovery of biological products at some stage in their production (Liddell, 1994). The current industrial standard in aroma biotechnology is batch fermentation with off-line distillation (or extraction), which requires the handling of large volumes of dilute aqueous solutions (Berger, 1995). The concentration of product in the bioreaction mixture is a major factor in production costs. Data on product concentration and retail price for a wide range of biological products show a strong inverse correlation between these two parameters (Humphrey, 1994). For example, for a production level of 1 g/L, annual depreciation based on a 5-year amortization period, and no discount rate, the breakeven price for a hypothetical ﬂavor decreases from $1240/kg for production of 1000 kg/yr to around $300/kg for 10,000 kg/y, and to $202/kg for 100,000 kg/yr (Welsh, 1994). However, © 2003 CRC Press LLC
in many cases, the biosynthesis of aroma compounds is far from reaching such high concentrations. Several factors may limit process productivity (Berger, 1995): 1. Loss of product via the waste airstream due to volatility 2. Biochemical instability of the product in the presence of the producing cell 3. Inhibition phenomena 4. Nonstationary product concentration in conventional batch processes In any of these cases, it is necessary to perform in situ extraction of the aroma compound from the culture broth. The objective of in situ product removal (ISPR), also known as extractive fermentation, is to remove the product as it is formed from the vicinity of the producing cell (and the reaction medium). While ISPR is considered primarily for the improvement of existing processes, in some cases, where product–cell interference is intensive, ISPR is essential for carrying out the envisaged process (Freeman et al., 1993).
In Situ Product Removal of Aroma Compounds Reports on in situ aroma extraction from the culture broth are focused on two general techniques: extraction into another phase and membrane separation. Most of the work reported in this area deals with the use of ISPR for the production of ethanol (and other related compounds such as butanol and acetone), organic acids (lactic and propionic), lactones (6-pentyl-apyrone and g-decalactone), ketones, and some aldehydes. An interesting point is that the use of various ISPR techniques has been evaluated for most of the aroma compounds cited here. That such evaluations have been performed by many different research groups reﬂects the tremendous effort required to ﬁnd the best alternative for each case. It is possible to ﬁnd general trends for the suitability of each ISPR technique, and in this review the emphasis will be on how ISPR has improved aroma productivity (in relation to conventional fermentation) and, when applicable, on the long-term performance of the process.
Extraction into Another Phase Liquid–Liquid Extraction The principle of liquid–liquid extraction involves an efﬁcient contact of two liquid phases: a feed phase and an extracting solvent. The phases are put in © 2003 CRC Press LLC
contact by dispersing one liquid as a drop dispersion into the second liquid, which remains as a continuous phase. The chemical properties of the solvent are chosen to facilitate selective uptake of the desired product from the feed phase and to ensure rapid disengagement of the two phases after contact (Weatherley, 1996). Organic solvents used as an in situ extractive system can lead to physical, morphological, and biochemical changes of the microorganism. It is accepted that low-polarity/high-molecular-weight solvents are less toxic for cells (Brink and Tramper, 1985). In general, solvent biocompatibility (low or no toxic effect) can be related to the logarithm of the partition coefﬁcient of the solvent in a standard octane–water (1:1 v/v) system, called log P. It has been reported that minor toxic effects of solvents are found for log P > 4 (Laane et al., 1987; Yabannavar and Wang, 1991). The use of anionexchange liquid membranes for the extraction of carboxylic acids and amino acids provides high permeability of the products while preventing the extractant from contaminating the product and the feed (Eyal and Bressler, 1993). The most recent work on ISPR by a two-phase, liquid–liquid process is brieﬂy discussed below. Ethanol production from lactose by liquid–liquid extractive fermentation has been reported (Jones et al., 1993). The authors reported a critical log P value for ethanol production by Candida pseudotropicalis of 5.2, and the use of Adol 85 F (log P = 8) showed a 60% improvement in lactose yield and ethanol production, as well as a 75% higher volumetric productivity when compared with control cultures. Like other weak organic acids, propionic acid is known to inhibit cell growth, substrate consumption, and acid production as a result of its antimicrobial activity (Lueck, 1980). The maximal propionic acid volumetric productivity (3 g/L/h) can be obtained if propionic acid concentration is maintained below 3 g/L in a Propionibacterium thoenii fermentation (Gu et al., 1998). In situ product removal of lactic acid has been studied extensively. The use of secondary and tertiary amines as extraction solvents for lactic acid by Lactobacillus delbrueckii has been reported (Honda et al., 1995). The use of Alamine 336 and oleyl alcohol as extractant and back-extractant, respectively, improved the total lactic acid productivity by 40% in relation to the control cultures. Planas et al. (1996) reported the use of an aqueous two-phase system (ATPS) of extractive fermentation for enhanced production of lactic acid by Lactobacillus lactis. The productivity of lactic acid increased from 2.5 to 3 mM/h when a copolymer of ethylene oxide and propylene oxide was used with hydroxypropyl starch as the ATPS. When the ATPS was replaced with a fresh top phase, the productivity reached a value of 4.7 mM/h, which is still far from the 7 mM/h required for commercial fermentation (Planas et al., 1997). The use of a poly(ethyleneimine)/hydroxyethyl cellulose ATPS extractive fermentation allowed only slight improvements in the lactic acid productivity in the continous cultivation of Lactococcus lactis (Kwon et al., 1996).
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The authors reported maximal productivities of 9.3 and 10.2 g/L/h for the control (D = 0.54 h) and ATPS extraction (D = 0.3 h), respectively. Production of 6-pentyl-a-pyrone (6PP) by Trichoderma spp. has been limited by product inhibition, as biomass growth inhibition occurs at 6PP concentrations as low as 100 mg/L (Prapulla et al., 1992). The potential of a polyethylene glycol/phosphate ATPS for the recovery of 6PP has been reported (Rito-Palomares et al., 2000). The use of octane as an extraction solvent in combination with a surface culture of Trichoderma viride over wooden rods yielded 432 mg/L of 6PP (Tekin et al., 1995); however, the main bottleneck of this methodology is the scaling-up of the process. The production of ketones in aqueous-decane two-phase systems by microencapsulated spores of Penicillium roquefortii was investigated (Park et al., 2000). The biotransformation of hexanoic and octanoic acid or their alkyl esters to 2 alkanones was only possible in the decane-based two-phase system. Indeed, the bioconversion of fatty acids into methyl ketones by spores of Penicillium roquefortii was improved by the use of an isoparafﬁn solvent, Hydrosol IP 230 (Creuly et al., 1992). The batch-fed bioconversion in the Hydrosol (log P = 7)-water system resulted in 21, 73, and 57 g/L of solvent for 2-pentanone, 2-heptanone, and 2-nonanone, respectively. The main advantage of product recovery by solvent extraction is the low cost, as no infrastructure investment is required. Solvent can be recycled and downstream costs are reduced as the product is concentrated in the solvent (Mathys et al., 1999); however, the use of organic solvents may exert toxic effects on the producing cells. This is of crucial importance in the production of ethanol, butanol, acetone, and organic acids, as the better the extraction solvents (afﬁnity) are, unfortunately, the more toxic they are to the cells. Therefore, the results obtained are modest when liquid–liquid extraction is used for the recovery of hydrophilic products. This also could be the primary reason for improved performance in liquid–liquid extraction when hydrophobic aroma compounds are extracted, as in the case of methyl ketones and 6PP. Solid-Phase Extraction This process can be deﬁned as the adsorption of a product in an inorganic or organic polymeric solid. Synthetic adsorbents are organic polymeric materials with a hydrophobic surface, which can be exploited in downstream processes. They are now widely used in the ﬁeld of biotechnology and for natural product isolation from laboratory scale to industrial production. The use of synthetic adsorbents has several advantages over conventional solvent-extraction processes (Takayanagi et al., 1996): 1. They are nontoxic for the cells. 2. Synthetic adsorbents have a molecular sieving function based on their pore structure. © 2003 CRC Press LLC
3. Adsorbents can be used either in a contained form (column) or in batch mode (in suspension). 4. Adsorbents can be used repeatedly, as the adsorbed compounds can be eluted under mild conditions. The use of adsorbents in ISPR during aroma production has been widely reported. With the objective of harvesting aroma compounds from fermentation broths, Krings et al. (1993) evaluated 31 different adsorptive materials and their adsorption properties. The best adsorbent materials turned out to be activated carbons, which adsorbed more than 86% of the compounds (model solution of 12 aroma compounds at 33 ppm each); however, these adsorbents exhibited poor desorption capabilities when common organic solvents were use as desorbents. The most suitable adsorbents were found to be styrene-divinybenzene resine and zeolite, which exhibited adsoption rates similar to activated carbons and better desorption properties. Very few examples can be found regarding the use of adsorbents for improving aroma compound production, probably because ISPR by adsorption of aromas has attained only modest results; however, the development of new synthetic resins in the area of chromatography could lead to the appearance of better materials with higher selectivity, high achievable loadings, and long-term performance. Production of 6-pentyl a-pyrone by Trichoderma viride was improved using Amberlite XAD-2 (Prapulla et al., 1992). When this adsorbent was added (13.3 g/L) at the beginning of the fermentation, 6PP production reached 248 ppm, 2.75 times greater than the control. When a packed column with an anion-exchange resin (Amberlite IRA-400) was used to improve lactic acid production by Lactobacillus delbreuckii, the lactic acid yield was improved from 0.83 to 0.93 g/g substrate, while the productivity went from 0.31 to 1.67 g/L/h in relation to conventional batch mode (Srivastava et al., 1992). Evaluation of the most suitable adsorbents for extractive fermentation of benzaldehyde and g-decalactone resulted in the selection of Lewatit 1064 (styrene-divinyl-benzene resin) due to the high achievable loading, even at a low solute concentration (Krings and Berger, 1995). Adsorption of gdecalactone for the bioconversion of methyl ricinoleate by Sporidiobolus salmonicolor has been studied (Souchon et al., 1998). However, under the experimental conditions tested, the production of g-decalactone was not improved when polystyrene-type polymers (Porapaq Q, Chromosorb 105, or Resin SM4) were used, probably because methyl ricinoleate (precursor) was also adsorbed. In another example, the production of thiopenes by hairy root cultures of Tagetes petula increased up to 40% when XAD-7 was used in an elicitation–extractive fermentation system (Buitelaar et al., 1993).
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Gas Extraction Supercritical ﬂuids — ﬂuids at temperatures and pressures slightly above the critical points (e.g., 31°C and 7.38 MPa for CO2) — exhibit unique combined properties: liquid-like density (and hence solvent power), high compressibility, very low viscosity, and high diffusivity. The ﬁrst two properties make the solvent power of supercritical ﬂuids easy to control by changing pressure and/or temperature, while low viscosity and high diffusivity markedly enhance mass transport phenomena and hence extraction process kinetics (Jarzebski and Malinowski, 1995). Extraction of aroma compounds with supercritical carbon dioxide offers an ideal process for general use in the food and ﬂavor industries because of the high physiological compatibility of CO2, the absence of residues in the ﬁnal product, low temperature of separation, and protection from oxidizing atmospheres during processing. A highly relevant feature of this technique is the ability to separate and select speciﬁc aroma fractions within individual products and thus provide a high degree of control over ﬁnal product speciﬁcation (Carbonell, 1991). Despite the fact that this technique requires high-cost investment, the use of supercritical CO2 in extractive fermentation has been reported. The production of ethanol by Saccharomyces cerevisiae and Saccharomyces rouxii under hyperbaric conditions has been evaluated (L’Italien et al., 1989). Using high cell concentrations it was possible to obtain a volumetric productivity of 10.9 g/L/h under 7-MPa pressure of CO2. This performance can be further improved by combining a high-cell-concentration fermentation under atmospheric pressure (which gave a maximum volumetric productivity of 29.7 g/L/h) with a period of hyperbaric conditions for the rapid recovery of ethanol by supercritical CO2. Direct supercritical CO2 extraction of 2-phenylethyl alcohol from a culture of Kluyveromyces marxianus was not possible due to the drastic effect of CO2 on yeast viability (Fabre et al., 1999). Indeed, the use of a cell-broth separation step (ultraﬁltration) coupled with two depressurization steps at the outﬂow of the extraction vessel resulted in 97% of aroma extracted with a mass purity of 91%. Aroma extraction with supercritical ﬂuids (mainly CO2) seems to be restricted to the recovery of these compounds from plant extracts. Its use in a fermentation process (one aroma compound at the time) is still far from being economically feasible.
Membrane Separation Membrane separation covers a wide range of different processes, ranging from microﬁltration to electrodialysis. The common factor linking this wide range of separation operations is the physical arrangement of the process. Separation occurs between two ﬂuids that are separated by a thin physical
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barrier, or interface. This interface constitutes the membrane and allows some materials to pass through the membrane while others are retained (Bell and Cousins, 1994). Typical membrane processes applied for downstreaming of aroma compounds are hollow-ﬁber membranes and pervaporation. Hollow-Fiber Membranes Hollow-ﬁbers are capillary tubes installed in a shell-and-tube arrangement and provide a very high surface area for a given volume unit. Because of the small pore of the ﬁbers (35%) phosphatidylcholine containing phospholipid as a liposomeforming ingredient. Typical liposome stabilizers are animal and plant sterols, triglycerides, diglycerides, monoglycerides, phenolics, and sucrose esters of long-chain fatty acids. Mechansho et al. (1998) reported the use of cholesterol-stabilized, lecithin-based liposomes for encapsulating divalent mineral salts and vitamins to prevent discoloration, off ﬂavor, and astringency.
The Mesophases of an Emulsiﬁer–Water–Oil Ternary System In addition to La, Hi, Hii, and cubic phases, water–oil–emulsiﬁer systems also form microemulsions. Microemulsions are single, thermodynamically stable, optically isotropic, and nonviscous liquid solutions. Microemulsions have three or more components: a hydrophobic component, a hydrophilic component, and at least one emulsiﬁer. Microemulsions can form spontaneously without mechanical energy input, with a droplet size of less than 0.1 mm. Two types of microemulsions exist. In oil/water (O/W) microemulsions (L1), oil (O) is inside the droplets, while water (W) is inside the droplet in W/O microemulsions (L2). Microemulsions have been widely used for non-food applications, such as cosmetics, drycleaning ﬂuids, paint technology, tertiary oil recovery, precious metal recovery, photochemical and polymerization reactions, advanced fuel technology, drug delivery, and biomedical applications (Paul and Mouliuk, 1997; Shad and Shechter, 1977). The biggest challenge for food application is the limited type of oils and emulsiﬁers allowed. The surfactants and cosolvents also must be food grade and of good sensory quality. Triglycerides, especially long-chain triglycerides (mainly C16 and C18), are much more © 2003 CRC Press LLC
difﬁcult to solubilize into microemulsions than hydrocarbons or alkyl esters. Thus, they tend to form liquid crystalline mesophases (Alander and Warenheim, 1989a,b). For more hydrophilic oils, the regions for both the L1 and L2 phases are extended (Tokuoka et al, 1993).
O/W Microemulsion (L 1) Structure Triglycerides containing unsaturated or short-chain fatty acids have better tendency to form L1 phase compared to triglycerides with saturated or longchain fatty acids. Water-soluble co-solvents, such as sucrose and alcohol, when used alone or in combination, can destabilize the liquid crystalline mesophases and promote the formation of the L1 phase, as conﬁrmed by xray diffraction and Polaroid microscopy. Sucrose enhances the formation of the L1 phase while destroying the L2 phase (Joubran et al. 1994). Application The ﬁrst food O/W microemulsion patent was based on using a high amount of alcohol (>25%) as a co-solvent and 1 to 30% of high hydrophile–lipophile balance (HLB) emulsiﬁers (Wolf and Hauskotta, 1989). The alcohol phase includes ethanol, propylene glycol, glycerol, sugar, and sugar alcohol. The microemulsion concentrate is stable at 70 to 75°F for at least a year and can be added up to 0.2% in beverages or up to 50% in salad dressings. Chung et al. (1994a,b) reported L1-type microemulsions for delivering a small amount of hydrophobic spearmint mouthwash and fragrance oil without using alcohol. Gaonkar (1994) showed L1 phases comprising only those oils that cannot be formed into a microemulsion in a matrix of water and alcohol speciﬁcally for aromatized coffee oil and oil-soluble egg ﬂavor. Chmiel et al. (1996) reported a soluble preconcentrated emulsion of hydrolyzed coffee oil. Upon heating of the food product above the melting point of the hydrolyzed fat, the preconcentrated emulsion spontaneously forms an L1 phase for rapid release of the ﬂavor. The advantages of these procedure are that there are no unappealing oil slicks on the coffee surface and no exceedingly high levels of emulsiﬁer, because the hydrolyzed coffee oil contains 75 to 85% free fatty acids, which behave as emulsiﬁers. Chmiel et al. (1997) further extended the same utility from hydrolyzed coffee oil to hydrolyzed fat. Up to 2% can be used in frozen or chilled food products to release ﬂavor upon heating. Merabet (1999) reported that an L1 phase has microwave absorption characteristics that make it highly suitable as a crisping and browning agent when added onto the surface of a food product. This L1 phase absorbs microwave energy in a thin layer at the surface of the food product and thus heats the © 2003 CRC Press LLC
dispersed oil droplets up to about 200°C or higher, while the water continuous phase quickly evaporates. This characteristic gives a high microwave heating rate and a small microwave penetration depth. Logan and Porzio (2000) reported the method of making ﬂavored vinegar by diluting a ﬂavored L1 phase concentrate containing 25 to 70% vinegar and a relatively high level of ethanol at 5 to 35%.
W/O Microemulsion (L 2) Structure A few triglyceride-based L2 systems have been reported in the food science literature (Friberg and Ridhay, 1971; Schwab et al., 1983; Hernqvist, 1986; Engstroem, 1990; El-Nokaly et al., 1991; Dunn et al., 1992, 1993). Lindstrom et al. (1981) characterized small areas of L2 systems containing triglycerides, monoglycerides, and an aqueous phase. Joubran et al. (1993) reported that triglycerides easily formed an L2 phase at a 3:1 ratio of ethoxylated monoglycerides to monoglycerides. The L2 phase can be used for protecting fat from oxidation. Ascorbic acid and tocopherol can inhibit oxidation of oil in an L2 system (Moberger et al., 1987; Jakobsson and Sivik, 1994). Another use of the L2 phase is as a novel reaction medium, eliminating the insolubility problem frequently encountered with triglycerides and other lipophilic substances. It has been used for enzymatic preparation of monoglycerides (Stamatis et al., 1993), for lipase-catalyzed transesteriﬁcation of unsaturated lipids with stearic acid (Osterberg et al., 1989), for lipase-catalyzed interesteriﬁcation of butterfat (Piard et al, 1994), for lipase-catalyzed hydrolysis of oils (Chandrasekharam and Basu, 1994), and for phospholipase-catalyzed synthesis of phosphatidylcholine with w-3 fatty acids (Na et al., 1990). Other uses of the L2 phase include controlling the availability of water for microbiological activities (Pfammatter et al., 1992), freezing (Garti et al., 2000), and as a separation matrix for proteins (Ayala et al., 1992; Hayes, 1997). Dungan (1994) used the L2 phase for separating various fractions of whey protein by changing pH and salt concentration during protein extraction from the L2 phase.
Application The L2 phase can be used to disperse water-soluble nutrients, vitamins, ﬂavor, and ﬂavor precursors in oils. El-Nokaly (1991b) reported an L2 phase containing up to 90% oil and 9% mono- and di-unsaturated C18 ester of diand triglycerol as a delivering system for up to 5% water-soluble nutrients and ﬂavors in foods. Leser (1995) showed an L2 phase formed from up to 33% water, up to 30% phospholipid, and diacylglyceride esters from nitrate, © 2003 CRC Press LLC
tartrate, lactylates, and up to 30% monoglycerides. Alander et al. (1996) used L2 microemulsion to deliver up to 2% of nanometer-sized water droplets uniformly into a chocolate matrix to make it heat resistant. Merabet (2000) took advantage of the high microwave absorption of super-cooled water in the L2 phase to thaw frozen foods uniformly. The microemulsion has been applied either onto or into the frozen food, so that it absorbs microwave energy to create a uniform blanket of heat that thaws the frozen food evenly. The L2 phase is based upon water, medium-chain triglycerides, and diglycerol monooleate. Typically, water of up to 15% of total weight of the L2 phase is supercooled to at least –8°C (Garti, 2000). Thus, water also has a higher dielectric loss factor at frozen temperatures (Senatra et al, 1985).
More food companies are learning from the emulsiﬁer-based mesophase technologies developed in the chemical and pharmaceutical industries. The regulations on emulsiﬁer usage level and the sensory quality of emulsiﬁers have limited the applicability of those different mesophases in commercial food production. A new generation of emulsiﬁers with better ﬂavor quality is needed before this technology can be widely applied.
Alander, J. and Warenheim, T. (1989a) Model microemulsions containing vegetable oils. Part I. Nonionic surfactant systems, JAOCS, 66(11), 1656–1660. Alander, J. and Warenheim, T. (1989b) Model microemulsions containing vegetable oils. Part II. Ionic surfactant systems, JAOCS, 66(11), 1661–1665. Alander, J., Warnheim, T., and Luhti, E. (1996) Heat-Resistant Chocolate Composition and Process for the Preparation Thereof, U.S. Patent No. 5486376. Ayala, C.A., Kamat, S., and Beckman, A.J. (1992) Protein extraction and activity in reverse micelles of a nonionic detergent, Biotechnol. Bioeng., 39(8), 806–814. Bergenstahl, B. (1997) Physicochemical aspects of an emulsiﬁer functionality, in Food Emulsiﬁers and Their Applications, H.L. Hasenhuettl and R.W. Hartel, Eds., Chapman & Hall, New York, pp. 149–172. Bos, M., Nylander, T., Arnebrant, T., and Clark, D. (1997) Protein/emulsiﬁer interactions, in Food Emulsiﬁers and Their Applications, H.L. Hasenhuettl and R.W. Hartel, Eds., Chapman & Hall, New York, pp. 127. Chandrasekharam, C.V. and Basu, A.K. (1994) Lipase catalyzed hydrolysis of oils in microemulsion medium, J. Oil Technologists’ Association of India, 26(2), 53–58. Chmiel, O., Traitler, H., and Voepel, K. (1997) Food Microemulsion Formulations, U.S. Patent No. 5674549. Chmiel, O., Traitler, H., Watzke, H., and Westfall, S.A. (1996) Coffee Aroma Emulsion Formulation, U.S. Patent No. 5576044. © 2003 CRC Press LLC
Chung, S.L., Tan, C.T., Tuhill, I.M., and Scharpf, L.G. (1994a) Transparent Oil-in-Water Microemulsion Flavor Concentrate, U.S. Patent No. 5320863. Chung, S.L., Tan, C.T., Tuhill, I.M., and Scharpf, L.G. (1994b) Transparent Oil-in-Water Microemulsion Flavor or Fragrance Concentrate, Process for Preparing Same, Mouthwash or Perfume Composition Containing Said Transparent Microemulsion Concentrate, and Process for Preparing Same, U.S. Patent No. 5283056. Crowe, J.H., Crowe, L.M., Carpenter, J.F., and Aurell-Wistrom, C., (1987) Stabilization of dry phospholipid bilayers and proteins by sugars, Biochem. J., 242, 1. Dungan, S. (1994) Puriﬁcation of milk proteins using reversed micelle systems, California Dairy Beat, 2, 3–5. Dunn, R.O., Schwab, A., and Bagby, M. (1992) Physical property and phase studies of nonaqueous triglyceride/unsaturated long chain fatty alcohol/methanol systems, J. Dispersion Sci. Technol., 13(1), 77–93. Dunn, R.O., Schwab, A., and Bagby, M. (1993) Solubilization and related phenomena in nonaqueous triglyceride/unsaturated long chain fatty alcohol/alcohol/ methanol systems, J. Dispersion Sci. Technol., 14(1), 1–16. Eliasson, A. and Larsson, K., Eds. (1993) Cereals in Breadmaking: A Molecular Colloidal Approach, Marcel Dekker, New York, pp. 11–16. El-Nokaly, M. (1992) Food Products Containing Reduced Calorie, Fiber Containing Fat Substitute, U.S. Patent No. 5106644. El-Nokaly, M. (1993) Encapsulated Cosmetic Compositions, U.S. Patent No. 5215757. El-Nokaly, M. (1997) Encapsulated Materials, U.S. Patent No. 5599555. El-Nokaly, M., Hiler, G.D., and McGrady, J. (1991a) Food Microemulsion, U.S. Patent No. 5045337. El-Nokaly, M., Hiler, G., and McGrady, J. (1991b) Solubilization of water and watersoluble compounds in triglycerides, in Microemulsions and Emulsions in Foods, A. Magda and C. Donald, Eds., ACS Symposium Series #448, American Chemical Society, Washington, D.C., pp. 26–43. Engstroem, L. (1990) Aggregation and structural changes in the L2-phase in the system of water/soybean oil/sunﬂower oil monoglycerides, J. Dispersion Sci. Technol., 11(5), 479–489. Engström, S. and Larsson K. (1999) Microemulsions in foods, in Handbook of Microemulsion Science and Technology, K.L. Mittal and P. Kumar, Eds., Marcel Dekker, New York. Friberg, E. and Rydhag, L. (1971) Solubilization of triglycerides by hydrotropic interactions: liquid crystalline phases, JAOCS, 48, 113–115. Gaonkar, A.G. (1994) Microemulsion of Oil and Water, U.S. Patent No. 5376397. Garti, N. (2000) The properties of water in W/O microemulsion at subzero temperatures, 8th International Symposium on the Properties of Water (ISOPOW) meeting abstract, Zichron Yaakov, Israel, Sept. 16-21, p. 17. Garti, N., Clement, V., Fanun, M., and Leser, M.E. (2000) Some characteristics of sugar ester nonionic microemulsions in view of possible food applications, J. Agri. Food Chem., 48, 3945–3956. Gripon, J.C. (1986) Acceleration of cheese ripening with liposome-entrapped proteinase, Biotech Lett., 8, 241–246. Hayes, D.G. (1997) Mechanism of protein extraction from the solid state by waterin-oil microemulsions, 53(6), 583–593. Haynes, L.C., Levine, H., and Finley, J.W. (1991) Liposome Composition for the Stabilization of Oxidizable Substances, U.S. Patent No. 5015483.
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Haynes, L.C., Levine, H., and Finley, J.W. (1992a) Method and Liposome Composition for the Stabilization of Oxidizable Substances, U.S. Patent No. 5139803. Haynes, L.C., Levine, H., Otterburn, M.S., and Mathewson, P. (1992b) Microwave Browning Composition, U.S. Patent No. 5089278. Hernqvist, L. (1986) An electron microscopy study of the L2 phase (microemulsion) in a ternary system: triglyceride/monoglyceride/water, in Food Emulsion and Foam, E. Dickinson, Ed., The Royal Society of Chemistry, Leeds, England, pp. 158–169. Jakobsson, M. and Sivik, B. (1994) Oxidative stability of ﬁsh oil included in a microemulsion, J. Dispersion Sci. Technol., 15(5), 611–619. Joubran, R.F., Cornell, D., and Parris, N. (1993) Microemulsion of triglycerides and nonionic surfactant: effect of temperature and aqueous phase composition, Colloids Surf., 80, 153–160. Joubran, R.F., Parris, N., Lu, D., and Trevino, S. (1994) Synergetic effect of sucrose and ethanol on formation of triglyceride microemulsions, J. Dispersion Sci. Technol., 15(6), 687–704. Kirby, C.J., Brooker, B.E., and Law, B.A. (1987) Accelerated ripening of cheese using liposome-encapsulated enzyme, Int. J. Food Sci. Technol., 22, 355–375. Koide, K. and Karel, M. (1987) Encapsulation and stimulated release of enzymes using lecithin liposomes, Int. J. Food Sci. Technol., 22, 707–723. Krog, N. and Lauridsen, J.B. (1976) Food emulsiﬁers, in Food Emulsions, S. Friberg, Ed., Marcel Dekker, New York, pp. 67–114. Lauridsen, J.B. (1976) Food emulsiﬁers: surface activity, edibility, manufacture, composition, and application, J. Am. Oil Chem. Soc., 53, 795–802. Law, B.A. and King, J.S. (1985) Use of liposomes for proteinase addition to cheddar cheese, J. Dairy Res., 52, 183–188. Lengerich, B., Haynes, L.C., Levine, H., Otterburn, M.S., Mathewson, P., and Finley, J. (1991) Extrusion Baking of Cookies Having Liposome Encapsulated Ingredients, U.S. Patent No. 4999208. Leser, M.E. (1995) Thermodynamically Stable Transparent Edible Water-in-Oil Microemulsion Comprises Oil, Oil-Soluble Surfactants and Polar Ingredients, European Patent No. 657104. Lindstrom, M., Lusberg-Wahren, H., and Larsson, K. (1981) Aqueous lipid phase of relevance to intestinal fat digestion and absorption, Lipids, 16(10), 749–754. Logan, S.S. and Porzio, M.A. (2000) Flavored Oil-in-Vinegar Microemulsion Concentrates, Method for Preparing the Same, and Flavored Vinegars Prepared from the Same, U.S. Patent No. 6077559. Mechansho, H., Mellican, R.I., and Trinh, T. (1998) Use of Bilayer Forming Emulsiﬁers in Nutritional Compositions Comprising Divalent Mineral Salts to Minimize Off-Tastes and Interactions with Other Dietary Components, U.S. Patent No. 5707670. Merabet, M. (1999) Edible Micro-Emulsion and Method of Preparing a Food Product Treated with the Micro-Emulsion, U.S. Patent No. 5891490. Merabet, M. (2000) Microwave Thawing Using Micro-Emulsions, U.S. Patent No. 6149954. Miller, M., Akashe, A., and Das, D. (2000) Mesophase-Stabilized Emulsions and Dispersions for Use in Low-Fat and Fat-Free Food Products, U.S. Patent No. 6068876. Moberger, L., Larson, K., Buchheim, W., and Timmen, H. (1987) A study of fat oxidation in a microemulsion system, J. Dispersion Sci. Technol., 8(3), 207–215. © 2003 CRC Press LLC
Na, A., Eriksson, S.G., Osterberg, E., and Holmberg, K. (1990) Synthesis of phosphotidylcholine with w-3 fatty acids by phopholipase A2 in microemulsion, JAOCS, 67(11), 766–770. Osterberg, E., Blomstrom, A.C., and Holmberg, K. (1989) Lipase-catalyzed transesteriﬁcation of unsaturated lipids in a microemulsion, JAOCS, 66(9), 1330–1333. Paul, B.K. and Mouliuk, S.P. (1997) Microemulsions: an overview, J. Disp. Sci., 18(4), 301–367. Pfammatter, N., Hochkoeppler, A., and Luisi, P.L. (1992) Solubilization and growth of Candida pseudotropicalis in water-in-oil microemulsions, Biotech. Bioeng., 10(1), 167–172. Piard, J.C., El Soda, M., Alkhalaf, W., Rousseau, M., Desmazeaud, M., Vassal, L., Safari, M., and Kermasha, S. (1994) Interesteriﬁcation of butterfat by commercial microbial lipase in a cosurfactant-free microemulsion systems, JAOCS, 71(9), 969–973. Schwab, A., Nielsen, H., Brooks, D., and Ryde, E. (1983) Triglycerides/aqueous ethanol/1-butanol microemulsions, J. Dispersion Sci. Technol., 4(1), 1–17. Senatra, D., Guarini, G.T., Gabrielli, G., and Zoppi, M. (1985) Thermal and dielectric behavior of free and interfacial water in water-in-oil microemulsions, in Macroand Microemulsions: Theory and Practice, D.O. Shah, Ed., ACS Symposium Series #272, American Chemical Society, Washington, D.C., pp. 133–148. Shad, D.O. and Shechter, R.S., Eds. (1977) Improved Oil Recovery by Surfactant and Polymer Flooding, Academic Press, New York. Silva, R.S., Fierro, J., Buccino, J., and Jodlbauer, H. (1992) Food Composition and Method, U.S. Patent No. 5120561. Stamatis, H., Xenakis, A., Menge, U., and Kolisis, F.N. (1993) Kinetic study of lipasecatalyzed esteriﬁcation reactions in water-in-oil microemulsions, Biotechnol. Bioeng., 42(8), 931–937. Tokuoka, Y., Uchiyama, H., and Abe, M. (1993) Phase diagrams of surfactant/water/ synthetic perfume ternary systems, Colloid Polymer Sci., 272, 317–323. Wolf, P.A. and Hauskotta, M.J. (1989) Microemulsion of Oil in Water and Alcohol, U.S. Patent No. 4835002. Yi, O.S., Meyer, A.S., and Frankel, E.N. (1997) Antioxidant activity of grape extracts in a lecithin liposome system, JAOCS, 74(10), 1301–1307.
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Drying of Biotechnological Products: Current Status and New Developments
Arun S. Mujumdar
CONTENTS Introduction Effect of Drying on Bioproduct Quality Commonly Used Dryers Some Emerging Drying Technologies Closing Remarks References
Drying, by deﬁnition, involves removal of a liquid (generally water, but in many bioprocessing applications it could be an organic solvent or an aqueous mixture) from a solid, semi-solid, or liquid material to produce a solid product by supplying thermal energy to cause a phase change, which converts the liquid to vapor. In the exceptional case of freeze-drying, the liquid is ﬁrst solidiﬁed and then sublimed. Bioproducts are produced by microbial action and are related to living organisms. Bioproducts are a subset of a broader generic deﬁnition of biomaterials which includes wood, coal, biomass, foods (biopolymers), vegetables, fruits, etc. This overview is limited to such bioproducts as whole cells (e.g., baker’s yeast, bacteria, vaccines), fermented foods (e.g., yogurt, cheese), synthetic products of both low molecular weight (e.g., amino acids, citric acid) and high molecular weight (e.g., antibiotics, xanthene), and enzymes. All of these products are characterized by their high thermal sensitivity; they are damaged or denatured and inactivated by exposure to certain
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temperatures speciﬁc to the product. Some are inactivated by mechanical stress or damage caused to the cell walls during the drying operation. These products are often produced in smaller qualities in batch mode. Further, they are typically high-value products so that the cost of drying is often secondary to quality constraints. It is therefore not unusual to use more expensive drying techniques (e.g., freeze-drying, vacuum-drying etc.) even when less expensive techniques such as heat-pump drying could be applied successfully. Of course, some biotechnology products are produced in bulk in continuous operation using conventional drying technologies such as spray- or ﬂuid-bed drying. The activity of water in a bioproduct is determined by the state of water in it. So-called free water represents the intracellular water in which nutrients needed by the living cells are in solution. Bound water is built into cells or the biopolymer structures. It is more strongly held to the solid matrix and is also resistant to freezing. The ratio of the vapor pressure expected by the water in the product to the equilibrium vapor pressure of pure water at the same temperature is referred to as the water activity. For safe storage, the objective of a drying process is to reduce the product moisture content so as to lower its activity below a threshold value safe for storage.
Effect of Drying on Bioproduct Quality
As noted earlier, quality of the dried product is of paramount concern in selecting a dryer and its operating conditions for thermolabile bioproducts. Numerous and varied undesirable changes can occur in the product during drying; in the worst case scenario, one may obtain a dry but totally inactivated product. Table 13.1 summarizes such changes for various biomaterials and their effects on product quality. Various indices are used to quantify changes in quality as a result of drying, and their choice clearly must depend on the product, but it is beyond the scope of this brief overview to discuss this important issue. Brieﬂy, typical quality criteria may be as follows: • For food biopolymers, criteria include color, texture, organoleptic properties, nutritional value (vitamin content), taste, and ﬂavor. • For “live” products (e.g., bacteria, yeast) or products such as enzymes or proteins that are thermally destabilized or inactivated, quality index A may be represented in terms of the degradation kinetics: dA = f (Ci Xi ) where the Ci represent moisture or temperdt
ature and Xi are process variables.
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TABLE 13.1 Possible Quality Changes during Bio-Material Drying Material Yeast Bacteria Molds Enzymes Vitamins Proteins, fats, carbohydrates, antibiotics Other Change Type Biochemical Biochemical Biochemical Enzymatic Enzymatic Chemical Physical/chemical/ biochemical Effect Atrophy of cells Atrophy of cells Atrophy of cells Loss of activity Loss of activity Loss of activity, nutritive contents Solubility, rehydration, loss of aroma, shrinkage
A usual simpliﬁcation that works satisfactorily within engineering accuracy involves assumption of ﬁrst-order kinetics: dA = – kd A dt where kd = k• exp (–DE/RT); R = 8.314 J/mol/K; DE = activation energy (J/ mol); T = absolute temperature (K). Small changes in temperature can have a dramatic effect on the degradation rate constant, kd, which can increase three- to eight-fold over a temperature rise of 60 to 80 K. Kudra and Strumillo (1998) have given values of kd for selected bioproducts. As an example of the diverse quality criteria employed in practice for a bioproduct, Table 13.2 lists quality indices often used to deﬁne suitability of dried proteins or protein-containing compounds. Not all of these criteria are used for a given product, however. For biomaterials such as various foods, fruits, and vegetables, numerous other quality parameters apply (refer to Krokida and Maroulis  for a detailed review). Physical properties such as shrinkage, pufﬁng, porosity, and texture are also important in these applications.
TABLE 13.2 Quality Changes: Drying of Protein-Containing Compounds Quality Indices Nitrogen solubility index (NSI) Protein dispersibility index (PDI) Water dispersed protein (WDP) Water-soluble protein (WSP) Nitrogen solubility curve (NSC) Protein precipitate curve (PPC) Note: For fruits, vegetables, and other foods, other criteria apply, including color, texture, taste, ﬂavor, nutrition, organoleptic properties, etc.
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Commonly Used Dryers
Often, the wet bioproduct to be dried is in the form of a wet solid, sludge, ﬁlter cake, a suspension, or solution. Mujumdar (1995) and Devahastin (2000) presented a classiﬁcation scheme for the numerous dryer types and their selection criteria in a general way, and the reader is referred to these basic references for details. Sufﬁce it to say here that the choice of dryers for bioproducts is constrained mainly by the ability of the dryer to handle the material physically, while the choice of the operating conditions is determined by the thermal sensitivity of the material. Table 13.3 lists some of the conventional dryers, as well as some emerging drying techniques for heatsensitive bioproducts, many of which are already commercialized but not commonly offered by vendors yet. Table 13.4 summarizes the key restrictive criteria that determine suitability of a given drying technology for biotechnology products. Note that aside from heat, such products may be damaged by the presence of oxygen. Some products may have to be stabilized by additives such as sugars or salts, as in the case of drying of some enzymes. Certain cryoprotective chemicals are used when freeze-drying live cells to avoid rupture of the cell walls. The rate of drying may have a direct or indirect effect on the quality as well as on the physical handling of the product. Spray-drying and freeze-drying are some of the most common drying technologies used for drying of bioproducts, although ﬂuid-bed, batch- and continuous-tray, spin-ﬂash, and vacuum dryers are also common. Pilosof and Terebiznik (2000) reviewed the literature on the drying of enzymes using spray- and freeze-drying. In recent years, we have seen a considerable rise in applications of enzymes as industrial catalysts, as pharmaceutical products, as clinical diagnostic chemicals, and in molecular biology. Because most enzymes are not stable in water, dehydration is used to stabilize them. Multistage drying systems (e.g., spray dryer to remove surface moisture followed by a ﬂuidized or vibrated bed to remove internal moisture at milder drying conditions over an extended period) are often used to speed up the overall drying process while maintaining product quality. Low-pressure ﬂuid-bed drying can be used to achieve drying of particulate solids at lower temperatures, although it is not a commonly used process. Freeze-drying (lyophilization) is used extensively in the industry to dry ultra-heat-sensitive biomaterials (e.g., some pharmaceuticals). Some $200 billion worth of pharmaceutical products are freeze-dried worldwide each year. It is a very expensive dehydration process, justiﬁed by the high value of the product. Liapis and Bruttini (1995) have given an excellent account of freeze-drying technologies.
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TABLE 13.3 Commonly Used Dryers and Emerging Drying Technologies Suitable for Biotech Products
Dryers for Biotech Products
Spray dryer Spray–ﬂuid bed (two-stage) Freeze dryer Vacuum tray Continuous tray dryer Drum dryer/vacuum Indirect vacuum Plate or turbo dryer
Heat-pump dryers (below/above freezing point) Intermittent batch dryer Vacuum ﬂuid-bed dryer Low-pressure spray dryer with ultrasonic atomizer Sorption dryer Pulse combustion dryer Cyclic pressure/vacuum dryer High electric ﬁeld (HEF) dryer Superheated steam dryer at low pressures
Some Emerging Drying Technologies
Numerous new drying techniques proposed and tested over the past decade have potential for application to biotech products. Extensive discussion of the basic principles, advantages, and limitations of each of these is beyond the scope of this brief review. Table 13.3 lists some such emerging technologies that have potential for bioproducts. For detailed discussion of most of these, the reader is referred to Mujumdar (1995), Mujumdar and Suvachittanont (1999, 2000), and Kudra and Mujumdar (2001), among others. For recent advances in heat-pump drying (HPD) of heat-sensitive products, the reader is referred to Chua et al. (2000), who provide a comprehensive overview of the numerous variants possible, including those involving multistage heat pumps; multistage dryers; drying below the freezing point; HPD with supplemental heat input by conduction, radiation, or dielectric (MW or RF) ﬁelds; and use of cyclical variation of the drying air temperature for batch drying of heat-sensitive products. Figure 13.1 is a schematic of the
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TABLE 13.4 Choice of Dryers and Drying Conditions for Biotech Products Depending on Speciﬁc Constraints Restrictive Criterion When Drying Biotech Products Highly heat-sensitive; thermally inactivated; or damaged Possible Dryer/Drying Conditions Dehumidiﬁed air drying (heat pump or adsorption dehumidiﬁer) at low temperatures Vacuum drying with indirect heat supply Intermittent batch drying Freeze drying Convective drying in N2 or CO2 Vacuum drying Freeze drying Addition of sugars, maltodextrin, salts, etc. to stabilize some enzymes Control of pH change during drying Use of gentle drying (e.g., packed bed or continuous tray as opposed to ﬂuid bed) Better drying of some products in one type of dryer than others (e.g., yeast in spouted bed vs. ﬂuid bed)
Damaged by oxidation
Product subject to destabilization (e.g., enzymes) Product affected by physical processing
wide assortment of HPDs possible; not all of these have been tested at laboratory or pilot scales. Several of these are of interest when drying highly heat-sensitive biotechnology products, as they are more cost effective than freeze dryers. The two-stage heat-pump dryer designed by Alves-Filho and Strommen (1996), in which the ﬁrst stage is a ﬂuid-bed freezer/freeze dryer at atmospheric pressure and the second stage is a ﬂuid-bed dryer operated with dehumidiﬁed air but above the freezing point, can successfully compete with the freeze dryer for certain products. It yields dried product properties that resemble those obtained by the much more expensive freeze-drying process. When the biomaterial to be dried is very sticky due to the presence of proteins, fats, or sugars, special drying techniques may be necessary. Such problems must be solved on an individual product basis, however. Sadykov et al. (1997) have proposed an interesting technique to dry bioactive materials. It involves cycling the operating pressure in a batch mode. Heat is supplied convectively at atmospheric pressure for a certain length of time and then the moisture is ﬂashed off in a subsequent cycle when vacuum is applied to the chamber for a given, but different, length of time. This process may be repeated several times. Some French researchers recently proposed a similar idea, suggesting that heat be supplied indirectly by conduction through the chamber walls. For heat-sensitive products, intermittent application of high-pressure and lowpressure environments can achieve drying at low product temperatures. The process must be operated in batch mode, however. More research and development are suggested to test this interesting new concept. © 2003 CRC Press LLC
Multiple Stages of HP Multistage Dryer (any convective type)
Batch or Continuous Operation
Operating Temperature and Pressure
With/Without Auxiliary Heat Input – Continuous or Intermittent -Conduction -Radiation -Microwave or RF
Cyclic or Interrupted HP Heat Input
Coupled with Conventional Dryer
FIGURE 13.1 A classiﬁcation scheme for heat-pump dryers.
An early Russian doctoral thesis also had a similar idea but implemented it differently, placing the drying material in a cylindrical chamber for which volume (and pressure) could be altered cyclically at a desired frequency (or cycle time) by a tight-ﬁtting reciprocating piston. The idea of pulse combustion drying has been proposed and revisited several times over the past two decades with limited success. In principle, even highly heat-sensitive products such as vitamins, enzymes, and yeasts can be dried by direct injection into the highly turbulent pulse combustor exhaust tailpipe; despite the ultra-high temperatures of the exhaust, the rapid heat and mass transfer rates and ﬁne atomization of the feed (slurry or dilute paste) by the highly turbulent ﬂow allow drying in a fraction of a second and without thermal degradation. The process has not been a commercial success yet, possibly due to problems of noise, scale-up, and capital cost. Energy consumption for thermal dehydration depends to a great extent on the dryer or drying system chosen and on the wet feed and properties of the dried product to some extent. Sometimes a lower thermal efﬁciency dryer is chosen for a given application, as the alternative higher efﬁciency dryers yield a lower quality product. For example, ﬁsh meal dried in a direct rotary dryer gives a 10% better yield of salmon weight than that dried in a thermally more efﬁcient steam tube rotary dryer (Flesland et al., 2000). The drying technique used can affect such properties of ﬁsh meal as protein digestibility, feed utilization, and the growth rate of the ﬁsh that eat it. Indirect drying, both atmospheric and vacuum, is found to produce a ﬁbrous © 2003 CRC Press LLC
TABLE 13.5 Comparison of Speciﬁc Energy Consumption of Various Drying Technologies Drying Technology (with Indirect Rotary Dryer as Predryer) Superheated steam dryer Indirectly heated steam dryer Hot-air rotary dryer Mechanical vapor recompression (MVR) evaporator Three-stage evaporator MVR-superheated steam dryer Speciﬁc Energy Consumption (kWh/kg) 0.75 1.00 1.15 0.04 0.30 0.20 0.20
product with low ﬂowability and hence a homogeneously extruded feed product. It is interesting to compare the energy consumption ﬁgures for commercialscale drying of ﬁsh meal provided by Flesland et al. (2000), who considered the case of a 42 T/h unit for drying ﬁsh meal. About 19.2 T/h of water is removed in the evaporator. Table 13.5 gives the estimated speciﬁc energy consumption ﬁgures (kWh/kg water removed) for alternative drying technologies. If a superheated steam dryer could replace the entire drying capacity without use of a predryer, the potential energy savings is about 50%, but with a loss of quality. Depending on whether or not a predryer is used, different process layouts yield different speciﬁc energy consumption ﬁgures, as listed below (in kWh/kg water evaporated): With Predryer Hot-air drying/three-stage evaporation Hot-air drying/mechanical vapor recompression (MVR) Superheated steam drying/MVR Superheated steam drying/ MVR + vapor reuse) MVR dryer/MVR evaporator 0.82 0.65 0.58 0.47 0.46 Without Predryer 0.83 0.66 0.55 0.44 0.33
Superheated steam drying is a concept that has been around for over a century, although commercial products appeared on the market only two decades ago for such products as pulp, waste sludge, hog fuel in the paper industry, beet pulp, etc. For heat-sensitive materials that are damaged in an atmosphere containing oxygen, superheated drying is possible only at low operating pressures. This technique has been shown by the author to be successful for drying of silkworm cocoons. The resulting silk is also found to be stronger and brighter. More recent laboratory studies have focused on drying of vegetables but the results are tentative. No work has been reported on biotech product drying to date. Due to the fact that most biotech products are made in small quantities and in batch mode, it is unlikely that superheated steam drying will be a major contender in this application area. © 2003 CRC Press LLC
Nitrogen could be used to provide the inert medium when oxygen must be excluded. For batch drying, intermittent supply of energy is an especially interesting concept if the bulk of the drying takes place in the falling rate period. Jumah et al. (1996) explained the principle with application to a novel intermittently spouted and intermittently heated spouted-bed dryer for grains. It was shown that appreciable reductions in energy and air consumption could be made while enhancing product quality due to the lower product temperature attained, as well as reduced mechanical handling of the grain due to intermittent spouting. This idea has been extended to ﬂuidized beds as well. Again, no direct biotech applications have been reported, but the concept is fundamentally sound and is expected to ﬁnd new applications. When a product to be dried is used in a mixture, one of the components of the mixture can be used as a carrier. This drying method is known as contact-sorption drying. The carrier can have different roles: • If the product is a liquid suspension, the particulate-form carrier disperses it, thus providing a large interfacial area for evaporation of the moisture while producing a granulated product. • The presence of the carrier effectively reduces the hygroscopicity of the material. • Dispersion of the liquid on a “dry” substrate makes the mixture easier to handle (e.g., ﬂuidize, convey, feed), thus permitting the use of a number of conventional dryers.
Sorption dryers of various designs have been reported in the literature, ranging from single- or multistage ﬂuidized bed dryers to cocurrent spray dryers in which the carrier is dispersed in the zone with the drying air in the atomizer zone. High electric ﬁeld (HEF) drying is a relatively new application for a wellknown technique. Kulacki (1982) discussed the fundamental principles of electrohydrodynamics and the effect of electrical ﬁeld on heat and mass transfer. In the HEF technique, wet materials can be dried at ambient temperature and pressure (or at lower temperatures and pressures) using an AC high electric ﬁeld (Hashinaga et al., 1999). Unlike MW or RF, heat is not generated in the material, so no loss of color, nutrients, or texture occurs during drying. The apparatus is very simple, consisting of point and plate electrodes. The main cost is that of electrical power consumption. Bajgai and Hashinaga (2000) showed the high quality attained in HEF drying in a ﬁeld of 430 KV/m of chopped spinach. The drying rates were very low, but the dried product quality was very high. Although not tested for biotech products, this technique could have potential for drying smaller batches of materials. Further research is needed to evaluate and compare the technoeconomics of this technique with competing drying methods. © 2003 CRC Press LLC
Intermittent Drying for Batch Dryers
On / Off
Time-varying Energy input (concurrent or sequential)
Cyclic variation of temperature or pressure or gas velocity
FIGURE 13.2 A schematic of the various types of intermittent batch dryer operations. Time-varying energy input may be by convection, conduction, radiation, or MW or RF; several heat transfer modes can be used concurrently.
Figure 13.2 summarizes some other ideas for intermittent or cyclically operated batch dryers. This ﬁgure is not all inclusive; rather, it is intended to give the reader ideas to consider when developing new drying systems for a new product or even when designing a new facility for an existing product line. Chua et al. (2000) and Chou et al. (2001) demonstrated experimentally and by mathematical modeling the superior performance of intermittent drying of heat-sensitive fruits in terms of the quality parameters such as color and ascorbic acid content. The drying time may be increased marginally.
Clearly, it is impossible to provide an overview all of the emerging drying technologies that are relevant to drying of the diverse and ever increasing numbers of bioproducts. The reader is referred to recent books by Mujumdar (2000) and Mujumdar and Suvachittanont (1999, 2000), as well as Kudra and Mujumdar (2001) for detailed discussions of some of the new technologies listed in this chapter. Descriptions of conventional drying technologies as well as most of the new ones can also be found in Mujumdar (1995) and Strumillo and Kudra (1998). The proceedings of the biennial International Drying Symposium (IDS) series initiated in 1978 at McGill University in Canada by the author now represent a gold mine of technical literature providing the latest information on emerging drying techniques and research © 2003 CRC Press LLC
and development. Researchers interested in drying will ﬁnd these proceedings invaluable for their work. The website (www.geocities.com/ drying_guru/) gives ready access to all the key sources of literature and their availability. The proceedings of the recently initiated Inter-American Drying Conferences (1997–), Asia-Oceania Drying Conferences (1999–), and Nordic Dewatering Conferences (2001–) provide additional sources of new technical information on drying technology. For archival information on the subject, Drying Technology — An International Journal (Marcel Dekker) remains the premier periodical for both academic and industrial practitioners.
Alves-Filho, O. and Strommen, I. (1996) Performance and improvements in heat pump dryers, Drying ’96, C. Strumillo and Z. Pakowski, Eds., Krakow, Poland, pp. 405–415. Bajgai, T.R. and Hashinaga, F. (2000) High electric ﬁeld drying of spinach, in Proc. 12th Int. Drying Symp., Amsterdam, The Netherlands. Cao, C.W. and Liu, X.D. (2000) Experimental study on impinging stream drying of particulate materials, in Proc. 12th Int. Drying Symp., Amsterdam, The Netherlands. Chou, S.K., Chua, K.J., Mujumdar, A.S., Ho, J.C., and Hawlader, M.N.A. (2001) On intermittent drying of an agricultural product, Trans. Instn. Chem. Eng. (London) (in press). Chua, K.J., Mujumdar, A.S., Chou, S.K., Hawlader, M.N.A., and Ho, J.C. (2000) Convective drying of banana, guava and potato pieces: effect of cyclical variations of air temperature or drying kinetics and color change, Drying Technol., 18(5), 907–936. Devahastin, S. (2000) Mujumdar’s Practical Guide to Industrial Drying Technology, Exergex, Canada (email@example.com). Flesland, O., Hostmark, O., Samuelsen, T.A., and Oterhals, A. (2000) Selecting drying technology for production of ﬁsh meal, in Proc. IDS ’2000, P.J.A.M. Kerkhof, W.J. Coumans, and G.D. Mooiweer, Eds., Elsevier, Amsterdam. Jumah, R., Mujumdar, A.S., and Raghavan, G.S.V. (1996) Batch drying of corn in a novel rotating jet spouted bed, Can. J. Chem. Eng., 74, 479–486. Krokida, M. and Maroulis, Z. (2000) Quality changes during drying of food materials, Develop. Drying, II, 149–195. Kudra, T. and Mujumdar, A.S. (1989) Impingement stream drying for particles and pastes, Drying Technol., 7(2), 219–266. Kudra, T. and Mujumdar, A.S. (2001) Advanced Drying Technologies, Marcel Dekker, New York, 472 pp. Kulacki, F.A. (1982) Electrohydrodynamically enhanced heat transfer, in Advances in Transport Processes, A.S. Mujumdar and R.A. Mashelkar, Eds., Wiley, New York. Mujumdar, A.S., Ed. (1995) Handbook of Industrial Drying, 2nd ed., Marcel Dekker, New York, 1440 pp. Mujumdar, A.S., Ed. (2000) Drying Technology in Agriculture and Food Science, SicPub, New York, and Oxford/IBH, New Delhi.
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Mujumdar, A.S. and Suvachittanont, S., Eds. (1999) Developments in Drying. Vol. I. Food Dehydration, Kasetsart University, Bangkok, Thailand. Mujumdar, A.S. and Suvachittanont, S., Eds. (2000) Developments in Drying. Vol II. Drying of Food and Agro-Products, Kasetsart University, Bangkok, Thailand. Pilosof, A.M.R. and Terebiznik, V.R. (2000) Spray and freeze drying of enzymes, in Developments in Drying. Vol. II, A.S. Mujumdar and S. Suvachittanont, Eds., Kasetsart University, Bangkok, Thailand, pp. 71–94. Sadykov, R.A., Pobedimsky, D.G., and Bakhtiyarov, F.R. (1998) Drying of bioactive products: inactivation kinetics, Drying Technol., 15(10), 2401–2420. Strumillo, C. and Kudra, T. (1998) Thermal Processing of Bioproducts, Gordon and Breach, London.
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An Update on Some Key Alternative Food Processing Technologies: Microwave, Pulsed Electric Field, High Hydrostatic Pressure, Irradiation, and Ultrasound
José J. Rodríguez, Gustavo V. Barbosa-Cánovas, Gustavo Fidel GutiérrezLópez, Lidia Dorantes-Alvárez, Hye Won Yeom, and Q. Howard Zhang
CONTENTS Introduction Microwave Processing Description of the Technology Equipment and Engineering Principles Effects of Microwaves on Microbial Inactivation Current Limitations and Status Pulsed Electric Fields Technology Description of the Technology Equipment and Engineering Principles Effects of Pulsed Electric Fields on Microbial Inactivation Current Limitations and Status High Hydrostatic Pressure Processing Description of the Technology Equipment and Engineering Principles Effects of High Hydrostatic Pressure on Microbial Inactivation Current Status and Limitations Food Irradiation Description of the Technology Equipment and Engineering Principles Effects of Irradiation on Microbial Inactivation Current Status and Limitations Ultrasound Processing Description of the Technology
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Equipment and Engineering Principles Effects of Ultrasound on Microbial Inactivation Current Status and Limitations Final Remarks References
For decades, various technologies have been used to preserve the quality and microbial safety of foods. Traditional preservation methods involve the use of heat (commercial sterilization, pasteurization, and blanching), preservatives (antimicrobials), and changes in the microorganism’s environment, such as pH (fermentation), water availability (dehydration, concentration), or temperature (cooling and freezing). These technologies involve one or more of these mechanisms: (1) preventing access of microbes to foods, (2) inactivating microbes, or (3) slowing the growth of microbes (Gould, 1995). Heat is by far the most widely utilized technology to inactivate microbes in foods. Despite the effectiveness of traditional technologies from a microbial safety standpoint, such technologies may also cause nutritional and sensorial food deterioration. Although food fortiﬁcation can overcome certain nutritional degradation attributes, sensorial attributes such as ﬂavor, aroma, texture, and appearance are difﬁcult to retain in traditional heat treatments. The consumer’s increasing demand for fresher, more natural foods has promoted the search for new food preservation technologies that are capable of inactivating foodborne pathogens while minimizing deterioration in food quality, from both nutritional and sensory points of view. New technologies base their antimicrobial action on physicochemical principles that tend to reduce food quality deterioration during processing. In traditional thermal processing, heat is transferred to food by conduction or convection. This energy not only inactivates microorganisms by disrupting the chemical bonds of cellular components such as nucleic acids, structural proteins, and enzymes (Farkas, 1997) but also affects desirable food components that are responsible for ﬂavor, aroma, texture, and appearance. Most new technologies are considered nonthermal, as their action does not imply food temperature increases (e.g., pulsed electric ﬁeld, high hydrostatic pressure, irradiation, and ultrasound). Those new technologies that do imply food temperature increases (e.g., microwave heating, radiofrequency, and ohmic heating) use more efﬁcient heat transfer modes than do traditional thermal techniques, thus allowing shorter heating times and minimizing food quality deterioration. The purpose of this chapter is to describe ﬁve of these new technologies (microwave heating, pulsed electric ﬁeld, high pressure, irradiation, and ultrasound), to explain their mechanisms of action on microbial inactivation, and to illustrate their interaction with food systems. © 2003 CRC Press LLC
Description of the Technology Microwave heating refers to the use of electromagnetic waves of certain frequencies to generate heat in a material. The Federal Communications Commission (FCC) regulates the use of electromagnetic waves in the U.S. (Curnutte, 1980). The FCC has allocated four frequencies for food processing and medical applications; however, industrial food microwaves use only 2450 and 915 MHz.
Equipment and Engineering Principles In microwave heating, continuous electromagnetic waves are produced in the magnetron and transmitted through a hollow metallic tube into a resonant cavity where the food is processed (Van Zante, 1973). In contrast with conventional thermal processing, where heat is applied to the outside of the food, microwave processing involves heat generated from within the food through molecular vibrations (Cross and Fung, 1982). Foods are heated because of molecular friction caused by alternating polarization of molecules promoted by a time-varying electric ﬁeld resulting from electromagnetic wave propagation. Foods absorb microwave energy in the form of orientational and ionic polarization (Decareau and Peterson, 1986). Orientational polarization results from dipolar molecules, such as water, which tend to align according to the applied electric ﬁeld. The electric ﬁeld oscillates at 2450 or 915 million times per second (MHz), making the dipolar molecules rotate, thus promoting molecular friction, which in turn results in heat dissipation (Curnutte, 1980). Ionic polarization occurs when dissolved salts are present. Positive and negative ions tend to migrate to opposite-charged regions, colliding with other ions and converting kinetic energy into heat. Dipole rotation is more important than ionic polarization as a microwave heating mechanism (Decareau and Peterson, 1986). The rate of heat generation per unit volume P0 (watts/cm3) absorbed by a substance can be expressed as (Lewis, 1987): P0 = 55.61 ¥ 10 -14 f Ef2 e" where f (Hz) is the frequency of radiation, Ef (V/cm) is the ﬁeld strength, and e¢¢ is the dielectric loss factor. Therefore, the amount of power absorbed by a food will increase as the frequency, ﬁeld strength, and loss factor increase. The power generated from a microwave operating at 2450 MHz is more than twice the power generated by a microwave operating at 915 MHz, provided the electric ﬁeld and dielectric properties of the heated food are © 2003 CRC Press LLC
about the same. The ﬁeld strength is another effective way to increase the rate of heating, although it is limited by the voltage breakdown strength in the microwave cavity (Lund, 1975). Typically, the ﬁeld strength, frequency, and load of materials are ﬁxed in a microwave system; only the dielectric properties of the food will affect the amount of radiation absorbed (Lewis, 1987). Moisture and salt content are the two main food components that most affect food dielectric properties (Swami and Mudgett, 1981). Another important parameter in microwave heating is the microwave penetration depth. This distance is deﬁned as (Fryer, 1997): d= l 2 p e r tan d
where d is the depth at which the intensity decays to 36.8% of its surface value, l is the wavelength of the microwave radiation, and er is the relative permittivity. The degree of microwave penetration depth increases with wavelength. The shorter the wavelength (higher frequencies), the greater the extent to which the energy will be absorbed at the surface of the product instead of within the product (Mudgett, 1989).
Effects of Microwaves on Microbial Inactivation Microwave heating offers the opportunity of shortening the time required in conventional heat treatments to achieve the desired food-processing temperature (Datta and Hu, 1992; Meredith, 1998); therefore, microwave pasteurization or sterilization can potentially improve the product quality of traditional heat processes (Stenstrom, 1974). Conventional high-temperature/short-time (HTST) processing of solid foods is limited by slow heat transfer via conduction which increases the time required to transfer the heat to the cold spot, causing overheating at the surface of the solid (Ramesh, 1999). Microwave heating offers the possibility of overcoming such limitations of conduction and convection heating modes (Meredith, 1998). Heatsensitive nutrients such as vitamins and ﬂavor constituents can be retained better through rapid heating than by conventional heat treatments (Mudgett and Schwartzberg, 1982); however, little conclusive evidence exists for any real ﬂavor differences between many conventionally and microwave-heated foods (Lorenz and Decareau, 1976). Most studies have concluded that microwave energy inactivates microbes via conventional thermal mechanisms, including thermal irreversible denaturation of enzymes, proteins, and nucleic acids (Heddleson and Doores, 1994). A few studies have proposed athermal microwave mechanisms, such as production of toxic compounds that could lead to microbial inactivation (Dreyfuss and Chipley, 1980; Khalil and Villota, 1986, 1989). It is very unlikely that microwaves could induce the production of toxic compounds via an © 2003 CRC Press LLC
“athermal” mechanism, as their quantum energy is signiﬁcantly lower than the energy required to break covalent bonds (Pomeranz and Meloan, 1987; Mudgett, 1989). As in traditional heat processing, the time–temperature history of the coldest point determines the microbiological safety of the microwave process. The accumulated microbial lethality (F0) in a given period of time (t) can be calculated as (Datta and Hu, 1992): t T -TR z
F0 = Ú 10
where T is the temperature at a speciﬁc location, TR is the reference temperature, and z is the temperature dependency of the reaction rate. In microwave heating, it is more difﬁcult to determine the location and history of the coldest point than in traditional heat processing. Complex models must be used and validated to locate the point of lowest integrated time history (Burfoot et al., 1988; Ramaswamy and Pillet-Will, 1992; Fleishman, 1996). Heat migration occurs from the initial, hottest locations in the interior to the surface, making it much more difﬁcult to utilize simpler procedures such as the Ball formula (Ball and Olson, 1957). Commercial software that models electromagnetic and heat transfer is available to assess process parameters in a more efﬁcient manner (Dibben, 2000).
Current Limitations and Status Applications of microwave heating are found for most of the heat treatment operations in the food-processing industries. While tempering and bacon cooking account for hundreds of operating systems, most of the remaining applications are single consumer installations (Schiffmann, 2001). Sterilization using microwaves has been investigated for many years, but commercial introduction has only occurred in the past few years in Europe and Japan. Microwave pasteurization and sterilization promise fast heat processing, which should lead to small quality changes due to thermal treatments according to the HTST principle (Ohlsson, 2000). However, it turns out that, in order to fulﬁll these quality advantages, microwave heating requires achieving heating uniformity (Burfoot et al., 1988). The nonuniformity of microwave heating can be attributed to several factors, such as localized microwave absorption due to heterogeneity of dielectric properties and heat capacity among food components, variations in ﬁeld intensity, and differences in shape, size, and placement of food (Schiffmann, 1986, 1990; Ruello, 1987; Stanford, 1990; Keefer and Ball, 1992). Besides heating uniformity, the cost of microwave processing is another limitation to its industrial use on a larger scale. The most likely future for microwave food processing, then, is the continued development of unique single systems that overcome these © 2003 CRC Press LLC
limitations. Examples of such systems are sausage and breakfast cereal precookers. Also, Dorantes et al. (2000) suggested that some commodities such as avocado might be blanched using microwaves. Avocado develops an offﬂavor when heated conventionally. When heated with microwaves, quality (including ﬂavor) is improved, as rapid heating is achieved. It must be pointed out that the dielectric properties of avocado favor uniform heating. The rate of heating when using microwaves is the main advantage of microwave blanching. When heated rapidly, the quality of fruits and vegetables, such as ﬂavor, texture, color, and vitamin content, is better kept. Thus, blanching by microwaving could be used as a pretreatment prior to canning, evaporating, frying, or freezing.
Pulsed Electric Fields Technology
Description of the Technology Pulsed electric ﬁeld (PEF) processing is based on the application of short pulses of high voltage (typically 20 to 80 kV/cm) to food placed between two electrodes. PEF is considered a nonthermal process, as foods are treated at room temperature or below for only a few microseconds, minimizing the energy loss caused by heating (Barbosa-Cánovas et al., 1999).
Equipment and Engineering Principles In PEF technology, the energy derived from a high-voltage power supply is stored in one or several capacitors and discharged through a food material to generate the necessary electric ﬁeld (Barbosa-Cánovas et al., 1999). The energy stored in one capacitor (Q [J/m3]) is given by: Q = 0.5 C0 V 2 where C0 is the capacitance, and V is the charging voltage. The energy stored in the capacitors can be discharged almost instantaneously (in a millionth of a second) at very high levels of power (VegaMercado et al., 1999). The discharge occurs in a treatment chamber in which the food is placed or circulates through a small gap between two electrodes (Barbosa-Cánovas et al., 1999). When a trigger signal is activated, a highvoltage switch is closed and the charge stored in the capacitor ﬂows through the food in the treatment chamber (Zhang et al., 1995; Barsotti et al., 1999). In order to avoid undesirable thermal effects, cold water is recirculated through the electrodes to dissipate the heat generated by the electric current passing through the food (Barbosa-Cánovas et al., 1999). © 2003 CRC Press LLC
Varying arrangements of capacitors, inductors, and resistors produce different types of pulses. Pulse polarity can be constant or alternating, and pulse waveform can be of exponential decay or square shape, among others (Figure 14.1) (Barbosa-Cánovas et al., 1999; Barsotti et al., 1999). Square pulses are more effective on microbial inactivation, as they maintain a peak voltage for a longer period than do exponential decay pulses. Exponential decay pulses have a long tail with a low electric ﬁeld, during which excess heat is generated in the food without bactericidal effects (Zhang et al., 1995). In the case of a square wave pulse, the electrical energy W¢ (J) dissipated in one pulse is given by: E2 V t r
where E is the electric ﬁeld (V/cm), V is the volume of food between the two electrodes (cm3), t is the pulse duration (sec), and r is the electrical resistivity of the food sample (ohms cm).
Pulse shape Voltage
Power Supply Capacitors Bank Resistor Shaping Inductors Capacitors Bank Resistor Switch Switch
Time Exponential decay pulse
Pulse shape Voltage
Power Supply PEF Chamber
Time Squarewave pulse
FIGURE 14.1 Representative pulse-wave shapes and corresponding schematic electric diagrams. © 2003 CRC Press LLC
Treatment chambers are designed to hold the food between two electrodes during PEF application. The chambers can be designed to work in static or continuous modes. In the static mode, food is held between two parallel electrodes, whereas in the continuous mode food circulates between the electrodes during PEF treatment. Two designs more commonly used in a continuous mode are coaxial (cylindrical or conical) and coﬁeld (Bushnell et al., 1991; Zhang et al., 1995; Yin et al., 1997; Barbosa-Cánovas et al., 1999). Static chambers are suitable for laboratory use, whereas continuous chambers are required for large-scale operations (Qin et al., 1995, 1998; Zhang et al., 1995). Laboratory-scale PEF systems have been developed by the University of Guelph (Ho et al., 1995), Washington State University (Qin et al., 1996; Barbosa-Cánovas et al., 1997), Ohio State University (Sensoy et al., 1997; Reina et al., 1998), and PurePulse Technologies, Inc. (McDonald et al., 2000) (Figures 14.2 and 14.3). Electric ﬁeld strength and treatment time are the two most important parameters inﬂuencing microbial inactivation (Jeyamkondan et al., 1999). In a coaxial treatment chamber design, the electric ﬁeld changes with position and is given by Eco = V R r ln 1 R2
where r is the position in which the electric ﬁeld is measured, and R1 and R2 are the radii of the inner and outer electrodes surfaces, respectively (Zhang et al., 1995). The treatment time depends on the pulse width and the number of treatment pulses. The number of pulses (n) is given by: fV n= ˙ V where f is the pulse repetition rate (Hz), V is the chamber volume (cm3), and ˙ [ V ] is the treated volume rate (cm3/sec) (Zhang et al., 1995). The ﬂow velocity proﬁle of food is important in the determination of total treatment time, as it affects the residence time of food in the PEF chamber. Therefore, ﬂow rate and viscosity of food should be considered in the determination of PEF processing conditions (Ruhlman, 1999).
Effects of Pulsed Electric Fields on Microbial Inactivation Pulsed electric ﬁeld (PEF) technology is used in the areas of genetic engineering and biotechnology to promote cell membrane reversible electroporation and cell electrofusion, respectively (Chang et al., 1992). The same principle is applied to PEF technology for microbial inactivation during food
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FIGURE 14.2 Pilot plant PEF generator (OSU-2C): bipolar, 50 kV/20 kW.
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FIGURE 14.3 OSU-2C PEF pilot plant treatment chambers and tubular heat exchangers.
processing. The duration and intensity of the treatment are increased to make the membrane disruption an irreversible phenomenon (Jeyamkondan et al., 1999). Electroporation is the most widely accepted concept used to describe the phenomenon of cell membrane discharge (reversible electroporation) and cell membrane breakdown (irreversible electroporation) with the application of short pulses. Sale and Hamilton (1967a) conducted one of the earliest works in PEF microbial inactivation. These authors reported the lethal effect on several © 2003 CRC Press LLC
bacteria and yeasts of high voltages up to 25 kV/cm applied as direct current pulses. They concluded that the products from electrolysis did not cause the inactivation nor did temperature. Their explanation for microbial inactivation was that the electric ﬁelds possibly caused an irreversible loss in the function of the membrane as a semipermeable barrier between the bacterial cell and its environment (Hamilton and Sale, 1967). According to this theory, electric ﬁelds induce a transmembrane electrical potential (TMP) that results from membrane polarization. Membrane polarization results from the fact that the cell membrane has a dielectric constant much lower than most food products; thus, the applied electric ﬁeld induces the accumulation of negative and positive charges within the cell at areas closest to the cathode and anode, respectively (Zimmermann, 1986). When the TMP reaches a critical value of about 1 V (called the breakdown TMP), membrane integrity is lost and cells lyse (Hamilton and Sale, 1967). The exact mechanism of electroporation is still not clear (Ho and Mittal, 1996). One proposed mechanism states that the membrane could be assumed as an ideal (linear) elastic material. Accumulated charges with different signs located at both sides of the membrane create electromechanical forces that lead to elastic strain forces on the membrane. When large compressions exceed the elastic mechanical region, the membrane is permanently ruptured and cells lyse (Croweley, 1973; Coster and Zimmermann 1975). Another theory suggests that large populations of pores are already present in cells when the electric ﬁeld strength is zero (Weaver and Powel, 1989). Pores increase in size under the effect of a transmembrane electric ﬁeld due to changes in both the lipid bilayer and the protein channels (Tsong, 1991). Ultimately, cell inactivation is caused by the osmotic imbalance across the cell membrane induced by the electroporation (Tsong, 1990). It has also been suggested that the structure and dynamics of electroporation may vary from cell type to cell type (Chang et al., 1992). Researchers agree that electroporation is a sequential and complex phenomenon where more than one pathway could occur simultaneously. The PEF sensitivity of microorganisms varies with the type of microorganism. Vegetative cells are more sensitive than spores (Yonemoto et al., 1993; Vega-Mercado et al., 1996a; Raso et al., 1998a); however, depending on experimental conditions, the irreversible inactivation of bacterial spores after PEF treatment has been reported (Marquez et al., 1997). The size of microorganisms also affects the effectiveness of microbial inactivation induced by PEF. Larger yeast cells are easier to inactivate than smaller bacteria (Qin et al., 1998; Aronsson et al., 2001). Up to a 7-log reduction of yeast cells in PEFtreated orange juice was reported by Yeom et al. (2000). Gram-positive bacteria are more resistant than Gram-negative types (Mackey et al., 1994), and cells harvested in the logarithmic growth phase are more sensitive than those in the stationary growth phase (Hülsheger et al., 1983; Pothakamury et al., 1996). Microbial inactivation by PEF also depends on the treatment medium. Lowering the medium pH increased the effectiveness against Escherichia © 2003 CRC Press LLC
coli (Vega-Mercado et al., 1996b). The combined effect of PEF and pH was attributed to a possible decrease in the cytoplasmic pH enhanced by an increase in the transfer rate of protons through the membranes when electroporated (Vega-Mercado et al., 1996b). Lowering the ionic strength also increased PEF inactivation of E. coli (Vega-Mercado et al., 1996b). The reduced inactivation rate on high ionic strength solutions was attributed to the stability of the cell membrane when exposed to a medium with several ions (Vega-Mercado et al., 1996b). Conductivity of the medium has been shown to inﬂuence the antimicrobial effectiveness of PEF on Lactobacillus brevis (Jayaham et al., 1992). As the conductivity of the medium was increased, the resistance of the treatment chamber was reduced, which in turn reduced the treatment time by reducing the pulse width. Consequently, the higher the conductivity of the treated medium, the lower the microbial inactivation attained (Jayaham et al., 1992). Treatment temperature has been shown to increase inactivation of Escherichia coli (Zhang et al., 1995), Lactobacillus brevis (Jayaham et al., 1992), and Listeria monocytogenes (Reina et al., 1998). The combined effect of PEF and temperature has been attributed to a decrease in the electrical breakdown potential of the membrane, due to changes in the membrane components induced by temperature (Reina et al., 1998). Nisin has shown a synergistic effect on microbial inactivation when combined with PEF (Calderón-Miranda et al., 1999; Dutreux et al., 2000; Pol et al., 2000). The site of action for both nisin and PEF is the cell membrane. This could explain why their combined effect on microbial inactivation is synergic (Calderón-Miranda et al., 1999; Dutreux et al., 2000). The degree of microbial inactivation is known to depend on two main processing parameters: the electric ﬁeld strength and the treatment time. Several mathematical models have been used to model microbial inactivation by PEF. Hülsheger et al. (1981) proposed the ﬁrst mathematical model relating the survival fraction with electric ﬁeld strength and treatment time: [ - ( E-Ec )]
Ê tˆ s=Á ˜ Ë tc ¯
where s is the survival fraction, t is the treatment time (msec) and the product between the number of pulses and the pulse width, E is the electric ﬁeld strength (kV/cm), k is a speciﬁc constant for each microorganism, and tc and Ec are the critical treatment time (msec) and critical electric ﬁeld strength (kV/ cm), respectively (two threshold values above which inactivation occurs). Peleg (1995) suggested that a model based on Fermi’s equation could better describe the phenomenon. The model proposed considers a gradual transition from marginal effectiveness (or none at all) at weak electric ﬁelds to effective lethality under strong ﬁelds, instead of lethality occurring during abrupt changes in the destruction kinetic at the Ec threshold value: © 2003 CRC Press LLC
S(V ) =
100 Ê Ê V - Vc (n) ˆ ˆ ˜ Á 1 + expÁ Ë a(n) ¯ ˜ Ë ¯
where S(V) is the percent of surviving microorganisms, V is the ﬁeld strength (kV/cm), Vc(n) is a critical level of V where the survival level is 50%, and a(n) (kV/cm) is a parameter indicating the steepness of the survival curve around Vc. Both Vc and a are functions of the number of pulses (n). Raso et al. (2000) reported that Salmonella senftenberg inactivation did not follow ﬁrst-order or second-order kinetics. They found that the best model to relate microbial inactivation and treatment time was a nonlinear loglogistic model initially described by Cole et al. (1993). The nonlinearity observed could originate from the different sensitivities to PEF within the microbial population (Cole et al., 1993; Raso et al., 2000). Current Limitations and Status Pulsed electric ﬁeld technology has not yet been industrially implemented. The slow commercialization of new preservation technologies has been partially attributed to the difﬁculty in demonstrating the equivalency of nonthermal methods to existing thermal methods. It is foreseen that the ﬁrst commercial applications of PEF technology could include acid foods, such as fruit juices, where Food and Drug Administration regulation requires a 5-log cycle reduction of speciﬁed pathogenic microorganisms (BarbosaCánovas et al., 1999). Some particular requirements for the industrial implementation of PEF technology include the design of PEF systems capable of assuring uniform treatment. The most important aspects of successful design include a treatment chamber capable of producing a homogeneous electric ﬁeld (Qin et al., 1998), along with a good system of measurement and control. Other aspects of a successful design should account for minimizing the occurrence of dielectric breakdown produced by electric tracking along the surfaces of particulates (Barbosa-Cánovas et al., 1999).
High Hydrostatic Pressure Processing
Description of the Technology High hydrostatic pressure (HHP) refers to the exposure of foods within vessels to high pressures (300 to 700 MPa) for a short period, typically ranging from a few seconds to several minutes. Food is pressurized by direct and indirect methods utilizing water as a pressure-transmitting medium (Farr, 1990; Mertens and Deplace, 1993). HHP is a nonthermal process, as it only involves minor increases in temperature during pressurization. For a © 2003 CRC Press LLC
working pressure of 600 MPa, the temperature increment of pure water is only approximately 15ºC (Denys et al., 2000).
Equipment and Engineering Principles High hydrostatic pressure technology is based on the use of pressure to compress food located inside a pressure vessel. The pressure vessel is the most important component of HHP equipment, consisting of a forged monolithic cylindrical piece built of alloy steel with high tensile strength. Multilayer or wire-wound prestressed vessels are used for pressures higher than 600 MPa. Prestressed vessels are purposely designed with residual compressive stress in order to lower the maximum stress level in the vessel wall during pressurization, hence reducing the cost of producing this important piece of equipment (Mertens and Deplace, 1993). In HHP equipment utilized in food applications, pressure is transmitted by two methods: direct or indirect. In the direct method, a piston is pushed at its larger diameter end by a low-pressure pump, directly pressurizing the pressure medium at its smaller diameter end (Figure 14.4). This method allows very fast compression but requires a pressure-resistant dynamic seal between the piston and the internal vessel surface to avoid leaks and contamination of the food. In the indirect method, high-pressure intensiﬁers are used to pump the pressure medium from the reservoir into the closed vessel until the desired pressure is achieved (Mertens and Deplace, 1993). The applied pressure is isostatically transmitted by a ﬂuid (Pascal’s law) (Earnshaw, 1996). In this way, uniform pressure from all directions compresses the food, which then returns to its original shape when the pressure is released (Olson, 1995). High hydrostatic pressure is essentially a batch-wise process for prepackaged foods, whereas for pumpable liquids semicontinuous processes have been developed. In both cases, the pressure treatment is accomplished in cycles. An initial time is required to reach the desired working pressure (come-up time), after which the pressure is maintained for the required processing time (holding time). Finally, the pressure is released, taking only a few seconds (release time) to complete the process (Horie et al., 1992). Pressure transmission is instantaneous and independent of product size and geometry. Contrary to heat, pressure transmission is not time or mass dependent; thus, the time required for pressure to reach the internal points of the food being processed is minimized (Mertens and Deplace, 1993). However, the pressure action on microbial inactivation is time dependent (Hoover et al., 1989).
Effects of High Hydrostatic Pressure on Microbial Inactivation Pressures between 300 and 600 MPa can inactivate food spoilage and pathogenic microorganisms (Isaacs et al., 1995; Palou et al., 1999). Pressure induces © 2003 CRC Press LLC
a) Direct Pressure System
Wire-wound press frame Upper closure Wire-wound pressure vessel Processed product Pressure media, water High pressure piston Pressure media on the low-pressure side Lower closure
b) Indirect Pressure System
Wire-wound press frame Upper closure Wire-wound pressure vessel Processed product
Pressure media, water
Pressure intensifier (PL < 200 MPa) PL
FIGURE 14.4 High-pressure machines with wire-wound vessels and two-pressure transmission system.
a number of changes in the microbial cell membrane, cell morphology, and biochemical reactions that ultimately can cause microbial inactivation (Hoover et al., 1989). Cell membranes are the primary site of pressure damage done to microbial cells (Farr, 1990; Mackey et al., 1994; Burl et al., 2000; Ritz et al., 2000). The microbial membranes play an important role in the transport and respiration functions; thus, a great change in membrane permeability can cause the death of cells (Lechowich, 1993). Changes in cell morphology involve the collapse of intercellular gas vacuoles, anomalous cell elongation, and cessation of movement in the case of motile microorganisms (ZoBell and Cobet, 1964; Mackey et al., 1994). Minor changes in the biochemistry of living cells also play an important role in microbial inactivation. Products that differ in the number of ionizable groups are strongly inﬂuenced by pressure; therefore, water and acid molecules increase their ionization under pressure (Earnshaw, 1996). The pH of water and buffered solutions decreases during pressurization by 0.2 to 0.3 units per 100 MPa of applied pressure (Funtenberger et al., 1995), which in turn may enhance the effect of pressure on microbial inactivation (Palou et al., 1999). Pressure may
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inhibit the availability of energy in cells by affecting energy-producing enzymatic reactions, promoting microbial inactivation. Pressure above 150 MPa induces partial unfolding and dissociation of protein structures, due to modiﬁcations of hydrogen and electrostatic bonds, including hydrophobic interactions (Jaenike, 1987; Cheftel, 1991). The extent of microbial inactivation achieved depends on the type and number of microorganisms, magnitude and duration of HHP treatment, temperature and composition of the suspension media, or food (Hoover et al., 1989; Palou et al., 1999). In general, yeasts and molds are more easily inactivated by pressure than bacteria. Among bacteria, vegetative forms are more susceptible than spores. Spores of bacteria are extremely resistant to pressure action (Nakayama et al., 1996), whereas ascospores of heat-resistant molds are inactivated at pressures of about 300 MPa (Butz et al., 1996). Inactivation of spores increases when combining pressure with other preservation factors, such as heat or antimicrobials (Roberts and Hoover, 1996). Results from combining pressure with low pH are contradictory. Although some authors claim that spore inactivation increases with low pH (Roberts and Hoover, 1996), others have found that lowering pH does not affect spore inactivation, even at extremely low pH values (Sale et al., 1970). Moderate pressures can be utilized to induce spores to germinate and then inactivate them by means of higher pressures or other preservation techniques (Gould, 1973; Wuytack and Michiels, 2001). Because of their relative pressure sensibility, vegetative cells are prime targets for preservation by high-pressure technology, particularly for products with food composition factors such as high acidity (e.g., fruit juices), ensuring that pressure-resistant spores are unable to grow (Gould, 1995). Gram-positive bacteria are more resistant than Gram-negative types (Mackey et al., 1994), and bacteria in the stationary growing phase are more resistant than bacteria in the logarithmic growing phase (ZoBell, 1970; Mackey et al., 1995). When combined with other preservative factors such as water activity, pH, antimicrobials, or temperature, pressure action may have an antagonistic, additive, or synergistic effect. Foods with low water activity (aw) due to high sugar concentrations decrease the sensibility of Rodotorula rubra and Zygosaccharomices bailii to pressure (antagonistic effect) (Oxen and Knorr, 1993; Palou et al., 1997b). Combinations of EDTA with nisin and lysozyme increased the lethality of Escherichia coli in an additive manner (Hauben et al., 1996). Low pH, antimicrobial peptides, and the use of combined moderate temperatures promote pressure efﬁcacy (synergistic effect) for microbial inactivation (Mackey et al., 1995; Pandya et al., 1995; Masschalack et al., 2001). Generally, an increase in pressure increases microbial inactivation; however, increasing the treatment time does not necessarily increase the microbial death rate. The types of HHP inactivation kinetics observed with different microorganisms are quite variable (Palou et al., 1999). Most researchers have observed a ﬁrst-order kinetics (Hashizume et al., 1995; Palou et al., 1997a; Mussa et al., 1999; Erkmen and Karaman, 2001). Other authors report a biphasic inactivation with the ﬁrst phase as described by a © 2003 CRC Press LLC
ﬁrst-order kinetics, followed by a tailing effect. The tailing effect occurs as a result of different pressure sensitivities among microorganisms from the same population (Earnshaw et al., 1995; Reyns et al., 2000). An empirical model to describe the microbial inactivation was originally proposed by Peleg (1995) for pulsed electric ﬁelds and was adapted by Palou et al. (1998) to interpret Zygosaccharomices bailii inactivation by HHP. Because this model is empirical, it can be used only to compare inactivation patterns and pressure sensitivities of microorganisms under the same processing conditions (Palou et al., 1998).
Current Status and Limitations High hydrostatic pressure is not a novel technology in the food industry, although interest has been renewed in the last decade. Hite conducted the ﬁrst studies on HHP in 1899 (Hite, 1899), but it was not until the 1980s that Japan, the U.S., and Europe began working again on this promising technology. Japan launched the ﬁrst commercial HHP processed products (Earnshaw, 1996). In 1990, Medi-ya Food Co. (Japan) launched the ﬁrst highpressure-treated products worldwide, including jams of various ﬂavors. Other HHP products marketed worldwide are fruit sauces and desserts (Medi-ya Food Co.), mandarin juice (Wakayama Co.), grapefruit juice (Pokka Co.), and avocado paste (guacamole) produced in Mexico (Avomex) (Palou et al., 1999). In the U.S., it is expected that the Food and Drug Administration (FDA) will soon approve the use of HHP for processing high-acid foods. It is unlikely that pressure processing of food will replace canning or freezing technology because it is relatively expensive; however, applications could be found for pressure processing for high-quality products where thermal processing is not suitable and in cases where this technology could confer added value in terms of nutritional or sensorial characteristics (Mertens, 1995; Earnshaw, 1996).
Description of the Technology Food irradiation is a process by which food is exposed to ionizing radiation for the purpose of preservation. In 1896, one year after Roentgen’s discovery of x-rays, it was suggested that ionizing radiation was lethal to bacteria (WHO, 1988; Hackwood, 1991). Food irradiation was ﬁrst patented in England in 1905 and ﬁrst used in the U.S. in 1921 to inactivate Trichiniella spiralis in pork muscle. Since the 1940s, irradiation has been used as a method to supply safe food to U.S. army combat troops (Barbosa-Cánovas et al., 1998). Despite scientiﬁc evidence regarding its effectiveness and safety, © 2003 CRC Press LLC
including regulatory approval for speciﬁc uses, consumer acceptance concerns have postponed applications of irradiation in the food industry (Buzby and Morrison, 1999).
Equipment and Engineering Principles The energy employed in food irradiation technology is referred to as ionizing irradiation. The term ionizing comes from the fact that certain rays produce electrically charged particles, called ions, when these rays strike a material (Urbain, 1986). The propagation of energy through space is known as radiation or radiant energy. Radiation has a dual nature, of wave and particle (corpuscle). Electromagnetic energy consists of self-propagating electric and magnetic disturbances that travel as waves, involving electric and magnetic vectors characterized by frequency and wavelength. Corpuscular energy can be visualized as a “particle” traveling through space, in which the energy is concentrated into bundles called photons (Urbain, 1986). According to quantum theory, the energy content in one photon can be expressed as: E = hw = hc l
where h is Planck’s constant, equal to 6.63¥10–34 J s; w is the frequency in (Hz); c is a constant representing the velocity of the propagation of the electromagnetic wave (m/sec), closely equal to 3¥108 m/sec; and l is the wavelength (m). Ionizing radiation occurs when one or more electrons are removed from an atom. Electrons orbiting at minimum energy level or ground state can be raised to higher levels, becoming electronically excited (excitation). If enough energy is transferred to an orbital electron, the excited electron may be ejected from the atom (ionization). A minimum amount of absorbed energy, called ionization potential, exists for each electron energy level necessary to exit the atom domain. If the energy absorbed by the electron is greater than its ionization potential, the excess energy enters a kinetic state, enabling the electron to leave the atom domain (Urbain, 1986). Although each electron behaves individually, electrons can be used in large numbers, called electron beams, to irradiate food (Barbosa-Cánovas et al., 1997). Electron beams are produced from commercial electron accelerators (WHO, 1988). One advantage of electron beam radiation is that the electron accelerators can be switched off when not in use, leaving no radiation hazard; however, the penetration of electron beams into foods is limited (Kilcast, 1995). Electrostatic forces tend to attract charged particles such as electrons (negatively charged), limiting the electron beam penetration into foods. Incident electrons also strike the orbital electrons of other atoms, limiting the penetration depth of incident electrons. Some of the kinetic energy from the incident electrons can be transferred to an orbital © 2003 CRC Press LLC
electron, which becomes excited, or leaves the atom, creating a chain of reactions that end when the energy is completely dissipated. The random pathway of the incident electrons also hinders electron penetration (Urbain, 1986). Therefore, electron beam radiation is limited to small food items, such as grains, or to the removal of surface contamination of prepared meals (Kilcast, 1995). Photons of electromagnetic radiation such as those created by x-rays and gamma rays travel without charge; thus, they do not interact with electrostatic forces while traveling through the food, and can penetrate deeper than electron beams. One mode of producing x-rays is through the use of electron beams. If the highly accelerated electrons penetrate a thin foil of certain metals, such as tungsten, tantalum, or any other material capable of withstanding high heat, x-rays are produced (Figure 14.5). The fact that electron production can be switched off makes this method of x-ray production an ionization source of interest (Radomyski et al., 1994). Radioactive isotopes such as cobalt 60 or cesium 137 are used to produce gamma rays. Although radiation isotopes cannot be switched off, they have the advantage of producing gamma rays, which offer the largest penetration among all ionization sources (Kilcast, 1995). Dose is the most important parameter in food irradiation. The quantity of energy absorbed by the food is measured in grays (Gy). One gray equals one joule per kilogram of matter. A gray (equal to 100 rad) is a very small quantity; therefore, the dose is expressed in kilograys (kGy). Up to 10 kGy is still a very small amount of energy, equal to the amount of heat required to raise the temperature of water 2.4∞C (WHO, 1988). For this reason irradiation is considered a “cold” or nonthermal technology (Farkas, 1988; Barbosa-Cánovas et al., 1998; Foley et al., 2001).
Electron injector Beam
Scanning magnet e e e e
FIGURE 14.5 Schematic representation of an x-ray generator utilizing a linear induction electron accelerator.
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Effects of Irradiation on Microbial Inactivation Irradiation is currently used as a tool to control naturally occurring processes such as ripening or senescence of raw fruits and vegetables and is an effective way to inactivate spoilage and pathogenic microorganisms. Irradiation causes microbial death by inhibiting DNA synthesis (Farkas, 1988). Other mechanisms involved in irradiation microbial inactivation are cell membrane alteration, denaturation of enzymes, alterations in ribonucleic acid (RNA) synthesis, effects on phosphorylation, and DNA compositional changes (Urbain, 1986). According to the dose used and the goal of the treatment, food irradiation can be classiﬁed into three categories (Wilkinson and Gould, 1996): 1. Radurization is a process comparable with thermal pasteurization. The goal of radurization is to reduce the number of spoilage microorganisms, using doses generally below 10 kGy. 2. Radicidation is a process in which the irradiation dose is enough to reduce speciﬁc non-spore-forming microbial pathogens. Doses generally range from 2.5 to 10 kGy, depending on the food being treated. 3. Radappertization is a process designed to inactivate spore-forming pathogenic bacteria, similar to thermal sterilization. Irradiation doses must be between 10 and 50 kGy. Irradiation microbial inactivation kinetics has been reported to be log linear (Urbain, 1986; Farkas, 1988; Radomysky et al., 1994; Monk et al., 1995; Tarte et al., 1996; Serrano et al., 1997; Buchanan et al., 1998; Thayer et al., 1998; Gerwen et al., 1999). The irradiation D value is a good parameter to predict the effectiveness of irradiation, as it represents the dose required to inactivate target cells by 90% (Wilkinson and Gould, 1996; Gerwen et al., 1999). The D value of a particular microorganism depends on both species and strain, as well as on certain extracellular environmental conditions such as pH, temperature, and chemical composition of the irradiated food (Urbain, 1986; Monk et al., 1995; Barbosa-Cánovas et al., 1998). In comparing the irradiation D value of bacterial spores with the D value of vegetative bacterial cells, viruses are generally found to be the most radiation-resistant microorganisms, followed by spores and yeasts (Wilkinson and Gould, 1996). Gerwen et al. (1999) compared the D values of several bacterial spores with the D values of several bacteria in the vegetative stage. The average D value of the bacterial spores studied (2.48 kGy) was signiﬁcantly higher than the average D value of vegetative bacterial cells tested (0.762 kGy), which demonstrates that spores are usually more resistant to irradiation than vegetative bacterial cells. Molds and Gram-positive vegetative bacteria are more tolerant than Gram-negative bacteria (Wilkinson and Gould, 1996). The irradiation D value of Escherichia coli O157:H7 increased in a range from 0.12 to 0.21 kGy for nonacid-adapted cells to 0.22 to 0.31 kGy for acid-adapted cells when © 2003 CRC Press LLC
tested in ﬁve apple juice brands containing different suspended solids (Buchanan et al., 1998). Resistance of Salmonella typhimurium increased at reduced irradiation temperatures (Thayer and Boyd, 1991). Water content also affects microbial sensitivity to irradiation. The lower the water activity of the food product, the more resistant the microorganisms become (Radomyski et al., 1994). The presence of oxygen enhances the indirect action of radiation, increasing its effectiveness and causing a reduction in the D value (Urbain, 1986). Chemical compounds with nutritional or ﬂavor functions can also be affected by ionizing irradiation; however, the chemical changes produced by irradiation in food components, at recommended doses, are small when compared with other preservation technologies such as heat (Luchsinger, et al., 1996; Kilkast, 1995). Irradiation doses below 1 kGy cause an insignificant loss of nutrients. At higher irradiation doses, such as those required for food sterilization, some vitamins such as A, B1, C, E, and K can degrade to some extent (Urbain, 1986). The loss of vitamins (thiamin, riboﬂavin, and a-tocopherol) in various meats due to gamma irradiation is negligible insofar as the diet of U.S. consumers is concerned (Lakritz et al., 1998). Vitamin losses seem to be more than compensated for by the advantages of irradiation processing in controlling bacteriological contamination (Fox et al., 1995). Thiamin is the most radiation sensitive of the water-soluble vitamins and is therefore a good indicator of the effect of irradiation treatment (Graham et al., 1998). Irradiation may cause some changes in the sensory characteristics of food and the functional properties of food components. Irradiation initiates the autoxidation of fats, which gives rise to rancid off ﬂavors (Kilcast, 1995; Byun et al., 1999). The extent of irradiation-induced lipid oxidation depends on factors associated with oxidation such as temperature, oxygen availability, fat composition, and pro-oxidants (Barbosa-Cánovas et al., 1998). Irradiationinduced lipid oxidation was retarded when antioxidants were added to groundbeef patties prior to irradiation (Lee et al., 1999). Lipid oxidation was also diminished when oxygen availability was lowered (Jo et al., 1999; Foley et al., 2001). Proteins with sulfur-containing amino acids can break down after irradiation treatment, yielding unpleasant off ﬂavors (Kilcast, 1995; Ahn et al., 2000). Dairy products such as milk and other commodities are especially prone to develop off ﬂavors even at low radiation doses (Josephson and Peterson, 1982; Kilcast, 1995). Irradiation can break down high-molecular-weight carbohydrates such as pectins and other cell wall materials into smaller units, causing softening of fruits and vegetables (Kader, 1986).
Current Status and Limitations Irradiation is the process of choice for spice treatment in many countries, including Canada and the U.S. (Farkas, 1988). Although irradiation is an effective method for food preservation, public apprehension about irradiation
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has delayed its many potential commercial applications in the food industry. Many consumers naturally tend to relate irradiation with nuclear energy and develop a negative attitude toward irradiated foods (Bord, 1991). Concerns about irradiation include safety, nutritional quality, potential harm to employees, and potential danger to people living near an irradiation facility (Bruhn, 1995). In the past, lack of regulation was the main reason for limiting potential irradiation applications in the food industry. At the international level, the Food and Agricultural Organization (FAO), International Atomic Energy Agency (IAEA), and World Health Organization (WHO) met in 1980 and concluded that irradiated foods are safe and healthful at levels up to 10 kGy (Radomyski et al., 1994). In the U.S., the FDA has approved the unconditional use of radiation on wheat and wheat ﬂour (1963), potatoes (1964), herbs (1983), spices (1986), pork for parasite control (1985), poultry meat (1992) (Wilkinson and Gould, 1996), and more recently on ground beef (1997). Ofﬁcial regulations are now available for a wide range of foodstuffs, but the availability of ofﬁcial regulations has not been reﬂected in an increase of commercially irradiated products (Bord, 1991).
Description of the Technology Ultrasound technology entails the transmission of mechanical waves through materials at frequencies above 18 MHz (Blitz, 1971). Most ultrasound applications are not associated with food preservation; however, when applied with enough intensity, ultrasound has a lethal effect on microorganisms. Therefore, ultrasound can potentially be used as a food preservation technique (Lillard, 1994; Earnshaw, 1998; Betts et al., 1999).
Equipment and Engineering Principles Ultrasound is generated in a material because of mechanical disturbances caused by continuous vibrations that are characterized by amplitude and frequency (Blitz, 1971; Povey and McClements, 1988). Transducers transform electric energy into mechanical energy, which in turn is ampliﬁed and transmitted through a liquid or solid material to generate traveling sonic energy waves (Povey and McClements, 1988; Sala et al., 1995). The energy of vibrations traveling through a material causes alternating compressive and tensile strains due to minute displacements of particles, described by a sinusoidal equation as follows:
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Ê xˆ y = y0 sin w t Ë c¯ where y is the particle displacement, y0 is the particle displacement amplitude, w is the frequency, t is the time, x is the distance of the particle from the source, and c is the velocity of the light through the material (Blitz, 1971). When ultrasound waves travel through liquids, they create periodic cycles of expansion and compression (Figure 14.6). If the negative pressure created in the liquid during the expansion cycle is low enough to overcome the tensile strength, it can cause liquid failure and, as a consequence, the formation of bubbles. These bubbles expand and contract and may ﬁnish collapsing during a dynamic process known as cavitation (Suslick, 1988; Leighton, 1998). Bubbles enlarge and collapse with different intensities, instantaneously increasing the local temperature and pressure up to 5000 K and 100 MPa, respectively. Frequency and amplitude, medium viscosity, temperature, and pressure determine the extent of cavitation (Betts et al., 1999). Another phenomenon resulting from variation in bubble size and collapse is the development of strong micro-streaming currents. The phenomenon of acoustic micro-streaming is associated with high ﬂow-velocity gradients and shear stresses that alter media characteristics (Suslick, 1988). Ultrasound applications are typically divided into two categories according to the power and frequency utilized. Low-power/high-frequency ultrasound operates at frequencies in the megahertz range and with acoustic power ranging from a few W to several tens of mW (Suslick, 1988). Such acoustic waves are capable of traveling through a medium
Distance from the source (x)
FIGURE 14.6 Schematic representation of layers of a material in which longitudinal sonic energy is propagated. © 2003 CRC Press LLC
without altering the material, allowing nondestructive measurements of food processes (Povey and McClements, 1989). High-power/low-frequency ultrasound is operated at frequencies in the kilohertz range, at which the acoustic power may extend from a few milliwatts to kilowatts (Suslick, 1988). Examples of use of this type of ultrasound include surface sanitation, microbial inactivation, and enzyme activity alteration (McClements, 1995; Povey, 1989).
Effects of Ultrasound on Microbial Inactivation Ultrasound microbial inactivation is mainly attributed to cavitation. Cavitation is associated with shear disruption, localized heating, and free radical formation (Earnshaw, 1998). Sudden changes in temperature and pressure that occur during cavitation are considered the main reasons for cell membrane damage and therefore microbial inactivation. The hightemperature spots created during bubble implosion alone cannot explain microbial inactivation, as their effect is very local (Earnshaw, 1998). It is likely that the lethal effect of microbial inactivation mainly relies on the inability of a microbe to withstand the extreme pressure variations taking place during the cavitation process (Scherba et al., 1991). Microstreaming may also contribute to inactivation, as microcurrents created around the bubbles are of such intensity that they can catalyze chemical reactions and disrupt microorganisms (Suslick, 1988). The third mechanism involved in microbial inactivation with ultrasound is the formation of free radicals and hydrogen peroxide, both of which have bactericidal properties (Riesz and Kondo, 1992). Efﬁciency of ultrasound microbial inactivation depends on the microorganism itself as well as on the medium properties and operating conditions. Different microorganisms show different resistances to ultrasound inactivation. Bacterial spores are more resistant to ultrasound than vegetative cells (Raso et al., 1998b). Gram-positive bacteria are more resistant than Gramnegative types, probably due to a thicker cell wall and the ﬁrmer adherent layer of peptidoglycans (Earnshaw, 1998). The combination of ultrasound with temperature increases the effectiveness of ultrasound against vegetative cells of Staphylococcus aureus (Ordoñez et al., 1984) and Saccharomyces cerevisiae (Guerrero et al., 2001) and increases effectiveness against spores of Bacillus cereus (Joyce et al., 1960) and Bacillus subtilis (Raso et al., 1998b). Combining ultrasound and pressure simultaneously is a very efﬁcient means to increase the lethality of ultrasound waves. Increased antimicrobial efﬁcacy due to pressure is greater at larger ultrasound operating amplitudes (Raso et al., 1998b,c; Pagán et al., 1999). Microbial inactivation by ultrasound has been reported to follow a ﬁrstorder kinetics (Raso et al., 1998c; Guerrero et al., 2001); however, shoulders and tails have been reported, suggesting that inactivation kinetics might be nonlinear (Raso et al., 1998c). © 2003 CRC Press LLC
Current Status and Limitations Most likely, ultrasound will become an important tool for measurement in food industry applications such as ﬂow rate, undisolved air, bubble size, solids content, particle size, emulsion stability, and degree of crystallization (Povey and McClements, 1988; McClements, 1995; Mason et al., 1996). Unfortunately, very high intensities are required to sterilize foods using ultrasound alone (Mason et al., 1996). However, in combination with other preservation factors such as heat, pH, pressure, or natural antimicrobials, ultrasound could be used to develop minimally processed foods (Ordoñez et al., 1987; Mason et al., 1996; Guerrero et al., 2001).
Conventional thermal food processes are among the most widely employed mechanisms in food preservation, primarily because they are very effective in ensuring the required high food safety standards. However, conventional thermal processes frequently lead to undesirable overcooked ﬂavor and loss of desirable fresh ﬂavor, aroma, vitamins, and essential nutrients. These negative effects on food quality can be attributed to the high amount of energy required to heat the food products and to attain the desired microbial inactivation. The high amount of energy required also promotes undesirable changes in nutritional and sensory food characteristics. Contrary to conventional thermal processes, alternative technologies target the use of energy on the microbial inactivation mechanism itself. Alternative technologies minimize the detrimental effects on food quality and better retain their fresh-like characteristics. In order to be considered successful, however, alternative technologies have to achieve at least the same food safety standards as those of conventional thermal processes. Understanding the mechanism of microbial inactivation involved in each alternative technology would allow utilizing them in the most suitable conditions. Each alternative technology has speciﬁc applications in terms of the type of food that can be processed. Microwave heating, high hydrostatic pressure, and irradiation are useful in processing both liquid and solid foods, whereas pulsed electric ﬁelds and ultrasound are more suitable for liquid foods. Also, each of these technologies could be used either alone or in combination with other technologies to optimize product quality, processing time, and microbial inactivation. The future of these alternative technologies is promising; however, technical and legal barriers must be overcome in order to achieve successful industrial implementation. © 2003 CRC Press LLC
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Vega-Mercado, H., Pothakamury, U.R., Chang, F.J., Barbosa-Cánovas, G.V., and Swanson, B.G. (1996b) Inactivation of Escherichia coli by combining pH, ionic strength and pulsed electric ﬁelds hurdles, Food Res. Int., 29(2), 117–121. Vega-Mercado, H., Góngora-Nieto, M.M., Barbosa-Cánovas, G.V., and Swanson, B.G. (1999) Nonthermal preservation of liquid foods using pulsed electric ﬁelds, in Handbook of Food Preservation, M.S. Rahman, Ed., Marcel Dekker, New York, pp. 487–520. Weaver, J.C. and Powel, K.T. (1989) Theory of electorporation, in Elecroporation and Elecrtrofusion in Cell Biology, E. Neumann, A.E. Sowers, and C.A. Jordan, Eds., Plenum Press, New York, pp. 111–126. Wilkinson, V.M. and Gould, G.W. (1996) Food Irradiation: A Reference Guide, Reed Educational and Professional Publishing, Oxford, England. World Health Organization (WHO) (1988) Food Irradiation: A Technique for Preserving and Improving the Safety of Food, World Health Organization, Geneva, Switzerland. Wuytack, E.Y. and Michiels, C.W. (2001) A study on the effects of high pressure and heat on Bacillus subtilis spores at low pH, Int. J. Food Microbiol., 64, 333–341. Yeom, H.W., Streaker, C.B., Zhang, Q.H., and Min, D.B. (2000) Effects of pulsed electric ﬁelds on the activities of microorganisms and pectin methyl esterase in orange juice, J. Food Sci., 65(8), 1359–1363. Yin, Y., Zhang, Q.H., and Sastry, S.K. (1997) High Voltage Pulsed Electric Field Treatment Chambers for the Preservation of Liquid Food Products, U.S. Patent No. 5,690,978. Yonemoto, Y., Yamashita, T., Muraji, M., Tatbe, W., Ooshima, H., Kato, J., Kimura, A., and Murata, K. (1993) Resistance of yeast and bacterial spores to high voltage electric pulses, J. Fermentation Bioeng., 75, 99–102. Zhang, H. and Datta, A.K. (2001) Electromagnetics of microwave heating: magnitude and uniformity of energy absorption in an oven, in Handbook of Microwave Technology for Food Applications, A.K. Datta and R.C. Anatheswaran, Eds., Marcel Dekker, New York, pp. 33–63. Zhang, Q., Barbosa-Cánovas, G.V., and Swanson, B.G. (1995) Engineering aspects of pulsed electric ﬁeld pasteurization, J. Food Eng., 25, 261–281. Zimmermann, U. (1986) Electrical breakdown, electropermeabilization and electrofusion, Reviews of physiology, Biochem. Pharmacol., 105, 176–250. ZoBell, C.E. (1970) Pressure effects on morphology and life processes of bacteria, in High Pressure Effects on Cellular Processes, A.M. Zimmermann., Ed., Academic Press, New York, pp. 85–130. ZoBell, C.E. and Cobet, A.B. (1964) Filament formation by Escherichia coli at increased hydrostatic pressures, J. Bacteriol., 87(3), 710–719.
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Emerging Processing and Preservation Technologies for Milk and Dairy Products
Valente B. Alvarez and Taehyun Ji
CONTENTS Introduction Processing Technologies Heat Treatment High-Pressure Processing Pulsed Electric Field Irradiation Modiﬁed Atmosphere Packaging Membrane Filtration Natural Components in Milk Lactoferrin Lactoperoxidase Xanthine Oxidase Conclusions References
Pasteurization and, more recently, ultra-high-temperature (UHT) processing are the traditional methods used to eliminate pathogens with minimal detriment to the physical and chemical properties of milk. The main purpose of thermally processing raw milk is to make it safe for human consumption. The demand for better quality dairy products with longer shelf lives has prompted the industry to search for new means to enhance these two important properties. Several recent technologies have shown potential commercial applications for improving quality and extending the shelf life of milk
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and other dairy products. High-pressure processing (HPP) can inactivate vegetative bacteria in milk. Pulsed electric ﬁeld (PEF) technology shows some potential in the pasteurization of ﬂuid milk. Irradiation has been an effective method for destroying pathogens but produces undesirable changes in sensory attributes and quality of dairy products. Modiﬁed atmosphere packaging (MAP) using CO2 has an antimicrobial effect and inhibits the growth of some psychrotrophic bacteria in dairy products such as ﬂuid milk, cheese, yogurt, ice cream mixes, and sour cream. Membrane microﬁltration has been applied successfully to reduce the microbial load of raw milk; the technique requires less thermal treatment and therefore improves milk quality. The antimicrobial effect of natural components of milk, such as lactoperoxidase, lactoferrin, and xanthine oxidase, can also preserve dairy foods, thus increasing shelf life. The basic concepts of these technologies, their potential uses and limitations, and their possible effects on the quality and shelf life of dairy products are discussed in this chapter.
Heat Treatment Milk and dairy products are heat treated to inactivate pathogenic microorganisms and some undesirable enzymes. This practice improves product safety and prolongs shelf life. The most common heat process is pasteurization, which can be applied at different temperature and time conditions ranging from 63°C for 30 min to 100°C for 0.01 sec (Anon., 2001). Pasteurization produces both irreversible and reversible changes in milk components and causes both desirable and undesirable effects. Heat treatment of milk increases the amount of colloidal phosphate, decreases Ca++, hydrolyzes phosphoric esters, isomerizes lactose, decreases pH, increases titratable acidity, denatures serum proteins, inactivates enzymes, and forms free sulfhydryl groups that drop redox potential and degrades some vitamins (Walstra et al., 1999). Changes of milk components by heat treatment largely depend on the combination of heating intensity and time duration. High-temperature/short-time (HTST) pasteurization is the most widely used treatment for preserving the quality and extending the shelf life of dairy products. HTST pasteurization destroys all pathogenic microorganisms but not bacterial spores. HTST processing also deteriorates the physical and chemical properties of the product. Plate heat exchanger systems are most commonly used for milk pasteurization in modern industry (Staal, 1986). During ultra-high-temperature (UHT) treatment, the product is held for a few seconds at a higher temperature (135 to 150°C) than with HTST pasteurization. The two UHT methods are direct heating by steam injection or infusion and indirect heat transfer by heat exchange. The UHT process © 2003 CRC Press LLC
destroys all viable microorganisms, including bacterial spores, and extends product shelf life. Additionally, UHT causes considerable changes in milk properties such as excessive heated ﬂavor, phosphotase reduction, activation of spores or spore-forming bacteria, gelation, reactivation of lipase, increased viscosity, and decreased nutritive value (Burton, 1998). High-Pressure Processing High-pressure processing (HPP) is a novel nonthermal food processing technology that uses extremely high pressure to kill vegetative microorganisms (Farr, 1990; Hjelmqwist, 1998). Inactivation of vegetative microbes through HPP may result from denaturation of deoxyribonucleic acid (DNA) replication and transcription, solidiﬁcation of lipids, breakage of biomembranes, or leakage of cell contents. Applications of HPP to food extend the shelf life without thermal denaturation; preserve nutritional value with retention of natural ﬂavor, color, texture and taste; and increase food safety (Cheftel, 1995; Hjelmqwist, 1998). High-pressure processing can operate within a wide range of conditions. High pressure machines can process volumes from 5 mL to 200 L and operating pressures from 30,000 to 130,000 psi (200 to 900 MPa) for typically 30 sec to a few minutes. The typical temperatures are 20 to 40°C, but in some cases the range can go from –20 to 80°C (Farr, 1990; Rovere, 1995). Under HPP, food retains its original shape with minor cellular damage because pressure is applied uniformly in all directions. The effect of HPP on food characteristics depends on the applied pressure, temperature, and duration. Functional properties of food proteins such as hydration, gelation, and emulsiﬁcation characteristics are altered by disruption of protein–water interactions and protein–protein interactions. HPP breaks only hydrogen protein bonds, disrupts hydrophobic and electrostatic interactions, makes greater ionization of molecules, and inactivates microorganisms. Covalent bonds are not affected (Messens et al., 1999). High-pressure processing has been used on dairy products. The process destroys microorganisms in milk and reduces coagulation and ripening time in cheese. HPP technology increases viscosity and apparent elasticity of gel and thus decreases syneresis in yogurt. The effect of HPP on milk depends greatly on the properties and composition of the milk. UHT milk pressurized at 400 MPa and 50°C for 15 min resulted in a reduction of approximately 5 log (CFU/g) for Escherichia coli, and 6 log for Staphylococcus aureus at 500 MPa (Patterson and Kilpatrick, 1998). Besides destroying microorganisms, HPP may alter the physical characteristics of milk. Pasteurized skim milk was HPP processed up to 700 MPa for 3 min at 20°C and kept constant for 22 min. The treatment increased the dynamic viscosity and surface hydrophobicity of milk. Milk turbidity and lightness decreased (Desobry-Banon et al., 1994). The particle size started to decrease with increased pressure from 230 to 430 MPa and reached minimal size with pressure from 430 MPa and above. Because skim © 2003 CRC Press LLC
milk is a colloidal emulsion of casein particles, the functional properties of skim milk are largely dependent on the size of casein particles. High-pressure processing technology was applied to investigate the accelerated ripening of gouda cheese at 14°C, 50 to 400 MPa, for 20 to 100 min (Messens et al., 1999) and cheddar cheese at 25°C, 50 MPa, for 3 days (O’Reilly et al., 2000). The results of those studies showed that HPP treatment using the conditions mentioned above did not accelerate ripening of the cheeses. However, Kolakowski et al. (1998) reported that camembert cheese treated by HPP at 0 to 500 MPa obtained the highest degree of proteolysis at the pressure of 50 MPa for 4 h. The authors also mentioned that the number of microorganisms in gouda and camembert cheeses decreased signiﬁcantly by HPP at pressures above 400 MPa. Another study concluded that increasing temperature accelerated cheese ripening as well as the risk of microbial spoilage (Fox, 1989). Accelerated cheese ripening was attributed to a combination of HPP and increased ripening temperature (O’Reilly et al., 2000). Processing of yogurt by HPP technology has also been investigated. The application of HPP at approximately 414 MPa at room temperature can extend shelf life or even produce shelf-stable yogurt (Farkas, 1996). The continuous development of acidity after packaging, which can lead to syneresis, can be reduced by HPP. Tanaka and Hatanaka (1992) subjected yogurt to pressures of 200 to 300 MPa at 10 to 20°C for 10 min. The treatment did not modify the yogurt texture and did not reduce the numbers of viable lactic acid bacteria. Pressure above 300 MPa prevented overacidiﬁcation; however, the numbers of viable lactic acid bacteria were reduced. Pulsed Electric Field Pulsed electric ﬁeld (PEF) is a nonthermal processing technology that may have the potential to replace traditional thermal pasteurization (Zhang et al., 1995a; Qiu et al., 1998). The use of PEF technology in foods reduces pathogen levels while increasing shelf life; retaining original ﬂavor, color, and nutritional properties; and improving protein functionalities (Dunn, 1996). PEF processing involves the application of a short burst of high-voltage energy to a ﬂuid as it ﬂows between two inert electrodes. The complete PEF system consists of a ﬂuid-handling section, high-voltage pulse generator, and multiple-stage co-ﬁeld PEF treatment chamber (Yin et al., 1997). The effect of PEF treatment on microorganisms is known as electroporation. PEF causes swelling of the cell membrane, resulting in reversible or irreversible ruptures. The high-voltage pulsed electricity discharges that are induced into microbes in the food product develop pores that allow permeation of small molecules in cellular membranes, thus impairing cellular functions (Benz and Zimmermann, 1980; Tsong, 1990; Knorr et al., 1994). Several factors inﬂuence PEF process efﬁciency. The effects of PEF treatment on inactivation of microorganisms largely depend on process conditions such as electric ﬁeld intensity, pulse width, treatment time and temperature, pulse wave shapes; microbial © 2003 CRC Press LLC
entities such as type, concentration, and growth stage of microorganisms; and treatment media, such as pH, antimicrobials, ionic compounds conductivity, and medium ionic strength (Hülsheger et al., 1983; Zhang et al., 1995b; Dunn, 1996; Vega-Mercado et al., 1997; Barbosa-Cánovas et al., 2001). Studies about the effects of PEF on dairy products have been conducted on skim milk, whole milk, and yogurt. Fluid milks containing Escherichia coli, Salmonella dublin, Listeria innocua, and Listeria monocytogenes were processed at PEF conditions of 28.6 to 50 kV/cm, 1.5 to 100 msec, 23 to 100 pulses, and temperatures from 10 to 63°C. Although the PEF conditions were different, the studies reported 2.0 to 4.0 log (CFU/mL) reductions (Dunn and Pearlman, 1987). A study investigated the application of PEF on raw milk by inoculating ultra-high-temperature (UHT) skim milk with Pseudomonas ﬂuorescens, Lactococcus lactis, and Bacillus cereus. The authors reported that the PEF treatment caused 0.3 to 3.0 log reductions of P. ﬂuorescens, L. lactis, and B. cereus in UHT milk and total microorganisms in raw milk. In all cases, PEF had a partial effect on the inactivation and destruction of microorganisms in milk, and the survivability of the cells differed for various organisms (Michalac et al., 2002). Other researchers reported 2.0 to 4.0 log reductions for Escherichia coli in skim milk (Qin et al., 1995), Listeria innocua in skim milk (Calderon-Miranda et al., 1999), Listeria monocytogenes (scott A) in pasteurized whole milk (3.5% milkfat) (Reina et al., 1998), and Staphylococcus aureus in simulated milk ultraﬁltrate (Pothakamury et al., 1995). The log reduction of Saccharomyces cerevisiae, Lactobacillus bulgaricus, and Streptococcus thermophilus in yogurt treated by PEF at 23 to 38 kV/cm, 100 msec, 20 pulses, and 63°C was investigated; the PEF treatment produced a 2.0 log reduction (Dunn and Pearlman, 1987). These results suggest that the use of PEF has some limitations due to the difﬁculty of inactivating endospores and low conductivity in foods caused by solids. The microbial reduction achieved in the studies is not substantial enough to consider PEF treatments as a substitute for current pasteurization methods of dairy products. However, the efﬁciency of PEF processing in microbial reduction in milk may be improved by employing longer treatment times and higher electric ﬁeld strengths. Combination with other nonthermal techniques may be necessary for more efﬁcient PEF treatment for the processing of ﬂuid dairy products. Irradiation Food irradiation is the exposure of food to a source of ionizing radiation energy. In general terms, radiation refers to exposure to or illumination by rays or waves of all types. Microwaves, infrared or ultraviolet light, and xrays are common sources of energy. Gamma (g) radiation is the most common type of energy used in food irradiation (Satin, 1996). The application of irradiation to foods reduces the pathogenic potential of microorganisms present because nucleic acids and macromolecules of food microorganisms © 2003 CRC Press LLC
are very sensitive to ionization. Irradiation disrupts the genetic material of living cell, destroys foodborne pathogens, and reduces the number of spoilage microorganisms. Ionization in food caused by irradiation modiﬁes the structure and composition of large and complicated molecules (Thakur and Singh, 1995). Food irradiation also affects food components such as water, carbohydrate, lipid, protein, vitamins, minerals, and other trace elements through reactive ions or free radicals, which combine with other ions to achieve a more stable state (Satin, 1996). Irradiation of milk and dairy products has been investigated at a wide range of radiation energy intensities (0.07 to 2.5 kGy). Irradiation of milk started to be used during the 1930s in order to increase the vitamin D content in milk. The difference with today’s irradiation methods was the use of ultraviolet (UV) light as the source of ionization rather that g-rays. Fluid milk and evaporated milk were irradiated with UV light (Sadoun et al., 1991; Satin, 1996). In most studies, dairy products have been exposed to high radiation doses suitable for sterilization, causing the development of off ﬂavors, aftertaste, and loss of vitamins A, B1, and B2. Irradiation treatment of milk and dairy products has created ﬂavor problems due to sulfur compounds produced from milk protein fraction and oxidative rancidity from lipid fraction. The off-ﬂavor production level in milk and cheese depended on their composition and the conditions of radiation and storage (Wilkinson and Gould, 1996). Irradiation with a dose of 0.25 kGy at room temperature extended the shelf life of pasteurized milk stored at 4°C; however, the milk lost a certain amount of vitamin A, B1, and B2 (Sadoun et al., 1991). Irradiation of other dairy products includes cheese, frozen desserts, and caseinate ﬁlms. Irradiation at an average dose of 2.5 kGy was an effective method for destroying pathogenic bacteria Listeria monocytogenes and Salmonella in camembert cheese without affecting enzyme activity and ﬂavor (Boisseau, 1994). Gamma irradiation at a dose of 40 kGy at –78°C was sufﬁcient to sterilize ice cream and frozen yogurt, but not for mozzarella or cheddar cheeses (Hashisaka et al., 1990a). The irradiation caused a decrease in overall acceptability of the product due to off ﬂavor and aftertaste, but it had little effect on product color or texture. Irradiation combined with modiﬁed atmosphere packaging or antioxidant was effective at preserving sensory properties of peppermint ice cream packed with helium and strawberry yogurt bars treated with ascorbyl palmitate (Hashisaka et al., 1990b). Water solubility and microbial degradation of caseinate ﬁlms were reduced by gamma irradiation at 64 kGy dosage. The improved changes were associated with the higher number of cross-links on the ﬁlm caused by irradiation. (Mezgheni et al., 2000).
Modiﬁed Atmosphere Packaging Modiﬁed atmosphere packaging (MAP) is a technology used to extend the shelf life of fresh and processed food products. The MAP process consists © 2003 CRC Press LLC
of ﬂushing the food packaging with antimicrobial gases just before sealing (Yam and Lee, 1995). Modiﬁed atmosphere packaging has been used successfully to extend the shelf life of solid dairy foods such as shredded cheese. In MAP, gas is added directly into liquids or semiliquid foods such as milk, yogurt, cottage cheese, sour cream, and ice cream (Hotchkiss and Chen, 1996). The gas concentration used in MAP can vary from 40 to 1100 ppm, depending on the type of product and packaging characteristics. Antibacterial agent carbon dioxide and inert nitrogen are the gases used in these technologies. Carbon dioxide is widely used to inhibit the growth of some psychrotrophic bacteria that deteriorate refrigerated food and to restrict the growth of typical aerobic Gram-negative spoilage bacteria. The gas enters microbial cells and lowers the pH, thus retarding the growth of microbes (Hintalian and Hotchkiss, 1987; Buick and Damoglou, 1989). The effectiveness of MAP in foods is improved with exclusion of the oxygen necessary for the growth of aerobic spoilage bacteria. Oxygen also causes oxidative rancidity and color changes (Church, 1994). Several researchers have investigated MAP in cottage cheese. Flushing CO2 gas into the headspace of plastic containers ﬁlled with cottage cheese inhibited the growth of yeasts, molds, and psychrotrophic bacteria for 112 days; however, ﬂavor and texture were retained for only 45 days (Kosikowski and Brown, 1973). The application of CO2 gas above 750 mL/L in cottage cheese kept the quality and maintained the color and ﬂavor for 28 days at 4°C (Manier et al., 1994). MAP using 500 mL/L CO2 gas in cottage cheese inhibited the growth of yeasts and fungi (Fedio et al, 1994). Pseudomonas spp. and Listeria monocytogenes were inhibited in cottage cheese with 400 mL/L CO2 (Moir et al., 1993). CO2 extended the shelf life of cottage cheese cream but caused a slight ﬂavor alteration due to the CO2 dissolved in the cream (Chen and Hotchkiss, 1993). Modiﬁed atmosphere packaging has also been applied to other cheese products to maintain quality, inhibit microorganism growth, and extend shelf life. MAP with plain CO2 was applied to whey cheeses stored at 4°C. Chemical composition was retained and lipolysis was inhibited completely (Mannheim and Soffer, 1996). MAP with several ratio combinations was applied to cameros cheese to investigate its effect on shelf life and microbial quality. Gas ratios of 50:50 and 40:60 (CO2:N2) were the most effective in reducing proteolysis and lipolysis, which helped in retaining good sensory properties. These gases were also effective in inhibiting the growth of mesophiles, psychrotrophs, enterobacteriaceae, and coliforms, thus extending shelf life (Gonzalez-Fandos et al., 2000). Effective MAP technology is associated with proper storage temperature, proper amount of gas dissolved, and high-barrier packaging. A disadvantage of MAP is that the gas applied dissipates quickly during modern processing.
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Membrane Filtration Membrane ﬁltration occurs when ﬂuids and solvents are selectively transported and passed through a barrier by applying a driving force across the barrier. The membrane materials that act as the barrier can be organic polymers, metals, ceramics, layers of chemicals, liquids, or gases (Mohr et al., 1989). Membrane ﬁltration has been used for the separation, concentration, demineralization, fractionation, or clariﬁcation of liquids. Membrane ﬁltrations are classiﬁed by membrane types, pore sizes, and process conditions into microﬁltration (MF), nanoﬁltration (NF), ultraﬁltration (UF), and reverse osmosis (RO). These membrane ﬁltrations have been applied in the dairy industry for the removal of bacteria in milk, standardization of milk, concentration of milk protein, fractionation of caseins, cheesemaking, and whey processing for many years (Rosenberg, 1995). Membrane technologies have also been used in the dairy industry for new product development, improving product quality, and enhancing process proﬁtability. The effectiveness of a ﬁltration system depends on the types and characteristics of membrane; therefore, the selection of membrane is important with respect to operating costs, energy requirements, reducing operating time, and increasing product quality. Microﬁltration is applied for the removal of bacteria from milk and cheese milk. It is also used in the preparation of casein-enriched cheese milk and to modify the a- and b-casein ratio of milk (Mistry and Maubois, 1993). MF of skim milk reduced bacteria by about 99.5%, extended shelf life, and retained the milk properties intact (Papachristou and Lafazanis, 1997). Rodriguez et al. (1999) studied the effect of UF and MF technologies on the texture of semihard, low-fat cheese. The use of MF milk improved cheese texture and produced lower retention of whey protein as compared with UF membrane; however, the UF process produced better cheese yields than the MF process. Nanoﬁltration membranes have high permeability of salts such as sodium chlorides and potassium chlorides and very low permeability for organic compounds such as lactose, protein, and urea. These characteristics make NF suitable for dairy application. NF concentrates and demineralizes whey at the same time, which traditionally was processed using evaporation or reverse osmosis followed by electrodialysis. With these properties, the use of NF reduces the cost of energy consumption and wastewater disposal considerably (Van der Horst et al., 1995). Demineralization of whey has been accomplished by NF (Van der Horst et al., 1995). Ultraﬁltration and RO are widely used in cheese manufacturing and whey treatment. UF and RO have been used in whey concentration and the development of whey protein concentrates (Rodriguez et al., 1999), and the UF process has been applied to making fresh cheeses (Mahaut and Korolczuk, 1992; Schkoda and Kessler, 1996) and semihard cheeses (De Boer and Nooy, 1980; De Koning et al., 1981; Delbeke, 1987). The use of UF processing in making low-fat cheddar cheese did not signiﬁcantly improve ﬂavor, body,
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or texture characteristics (McGregor and White, 1990). Aroma and ﬂavor degradation were reported in cheeses made from UF concentrated milk (Bech, 1993). The advantages of UF milk in cheesemaking are cheese yield increases of about 16 to 20%, reduction of the quantity of coagulant used by 80%, and decrease of biological oxygen demand (BOD) of the whey produced (El-Gazzar and Marth, 1991). RO concentrated 35,000 kg of whey fourfold, reducing the BOD of permeates (Bissett and Schmidtke, 1984).
Natural Components in Milk
Lactoferrin, lactoperoxidase, and xanthine oxidase are naturally present in milk and have some speciﬁc properties that have been found to be beneﬁcial for the shelf life and quality of dairy products. These compounds are nonimmune antimicrobial proteins that have been investigated by several researchers (Bellamy et al., 1992; Grappin and Beuvier, 1997; Pakkanen and Aalto, 1997; Schanbacher et al., 1998).
Lactoferrin Lactoferrin is an iron-binding glycoprotein in milk that inhibits the growth of pathogenic bacteria by its high afﬁnity with iron. The antimicrobial mechanism of lactoferrin is more complex than simple binding of iron (Bellamy et al., 1992). Lactoferrin also disrupts bacterial cell membranes by binding bacterial lipopolysaccharide (Nuijens et al., 1996) and modiﬁes membrane permeability by binding porin molecules in the outer membrane (Erdei et al., 1994; Naidu and Arnold, 1994). This interaction with the cell membranes facilitates the bactericidal properties of lactoferrin (Bellamy et al., 1993). When the antimicrobial effects of lactoferrin on Escherichia coli (Law and Reiter, 1977), Salmonella typhimurium, Shigella dysenteriae (Batish et al., 1988), and Listera monocytogenes (Payne et al., 1990) were investigated, lactoferrin was shown to inhibit the growth of these microbes.
Lactoperoxidase Lactoperoxidase is an antibacterial enzyme naturally present in colostrum and milk. This enzyme catalyzes the oxidation of thiocyanate (SCN–) in the presence of hydrogen peroxide (H2O2), producing hypothiocyanite (OSCN–), which is a toxic intermediary oxidation product. This product inhibits bacterial metabolism by oxidation of essential sulfhydryl groups in proteins. This reaction produces a severe change in the cytoplasmic membrane of spoilage bacteria (Reiter, 1978; Pruitt and Reiter, 1985). The use of lactoperoxidase is not approved in the United States because its activation requires © 2003 CRC Press LLC
a thiocyanide compound, which is unsafe for children (Anon., 1998). The activation of the lactoperoxidase system in refrigerated raw milk retarded the growth of psychrotrophic bacteria for several days (Bjorck, 1978; Haddadin et al., 1996). The lactoperoxidase system has been used to extend the shelf life of raw, pasteurized (Martinez et al., 1988), and UHT-treated milk (Denis and Ramet, 1989) and to preserve cream (Toledo Lopez and Garcia Galindo, 1987), cottage cheese (Earnshaw et al., 1989), mozzarella cheese, and yogurt (Kumar and Mathur, 1989).
Xanthine Oxidase Xanthine oxidase is a complex metallo-ﬂavo enzyme present in the fat globule membrane (Mondal et al., 2000). The enzyme catalyzes the reaction to produce bactericide superoxide radicals and hydrogen peroxide in the presence of oxygen. Hydrogen peroxide can also be used to activate the lactoperoxidase system (Reiter, 1978). Xanthine oxidase was studied for its activity in dairy products such as raw, evaporated, and powdered milks; ice cream; yogurt; cheese (Cerbulis and Farrell, 1977; Zikakis and Wooters, 1980); and butter and creams (Stannard, 1975; Cerbulis and Farrell, 1977). Nielsen (1999) reported that xanthine oxidase, an oxido-reductase enzyme, catalyzes the oxidation of purine bases and reduces nitrate to nitrite. Nitrate inhibits the germination of spore butyric acid bacteria in cheese.
The development and use of emerging technologies and new processing procedures have been shown to be promising. Most of them provide speciﬁc advantages to improve the shelf life and quality of dairy products; however, they also present some disadvantages and limitations. Several of these new technologies are still in experimental stages and may not fall under current Food and Drug Administration (FDA) regulations associated with low-acid canned foods which relate only to processing procedures involving a thermal treatment to render the commercial sterility of the product. Although some of these nonthermal processes may have capabilities to produce shelf-stable products, public health concerns are signiﬁcant. The FDA speciﬁes that, “Any nonthermal process used to create a commercially sterile food product must result in a food product that has not been prepared, packed, or held under unsanitary conditions whereby it may have become contaminated with ﬁlth or whereby it may have been rendered injurious to health.” Consequently, the future applications and utilization of nonthermal technologies and new processing procedures will depend greatly on more research results, economics, and legal approval.
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Anon. (2001) Extending shelf life in dairy foods, in Innovations in Dairy: Dairy Industry Technology Review, DMI Bull., Dairy Management, Inc., Rosemont, IL, pp. 1–6. Anon. (2001) Grade A Pasteurized Milk Ordinance, Publication No. 229, Public Health Service/Food and Drug Administration, Washington, D.C., p. 4. Barbosa-Cánovas, G.V., Pierson, M.D., Zhang, Q.H., and Schaffner, D.W. (2001) Pulsed electric ﬁelds, J. Food Sci. (special suppl.), 65–79. Batish, V.K., Harish, C., Zumdegni, K.C., Bhatia, K.L., and Singh, R.S. (1988) Antibacterial activity of lactoferrin against some common food-borne pathogenic organisms, Aust. J. Dairy Technol., 43, 16–18. Bech, A.M. (1993) Characterizing ripening in UF-cheese, Int. Dairy J., 3, 329–342. Bellamy, W.R., Takase, M., Yamauchi, K., Wakabayashi, H., Kawase, K., and Tomita, M. (1992) Identiﬁcation of the bactericidal domain of lactoferrin, Biochim. Biophys. Acta, 121, 130–136. Bellamy, W.R., Wakabayashi, H., Takase, M., Kawase, K., Shimamura, S., and Tomita, M. (1993) Roll of cell-binding in the antibacterial mechanism of lactoferrin B, J. Appl. Bacteriol., 75, 478–484. Benz, Z. and Zimmermann, U. (1980) Pulse-length dependence of the electrical breakdown in lipid bilayer membranes, Biochim. Biophys. Acta, 597, 637–642. Bissett, D.W. and Schmidtke, N.W. (1984) Concentrating whey by hyperﬁltration at a small Canadian cheese plant, IDF Bull., 184, 96. Bjorck, L. (1978) Antimicrobial effect of the lactoperoxidase system on psychrotrophic bacteria in milk, J. Dairy Res., 45, 109–118. Boisseau, P. (1994) Irradiation and the food industry in France, Food Technol., 48(5), 138–140. Buick, R.K. and Damoglou, A.P. (1989) Effect of modiﬁed atmosphere packaging on the microbial development and visible shelf life of mayonnaise-based vegetable salad, J. Sci. Food Agr., 46, 339–347. Burton, H. (1998) Properties of UHT-processed milk, in Ultra-High Temperature Processing of Milk and Milk Products, Burton, H., Ed., Elsevier, New York, pp. 254–291. Calderon-Miranda, M.L., Barbosa-Canovas, G.V., and Swanson, B.G. (1999) Inactivation of Listeria innocua in skim milk by pulsed electric ﬁelds and nisin, Int. J. Food Microbiol., 51(1), 19–30. Cerbulis, J. and Farrell, H.M. (1977) Xanthine oxidase activity in dairy products, J. Dairy Sci., 60(2), 170–176. Cheftel, J.C. (1995) High-pressure microbial inactivation and food preservation, Food Sci. Technol. Int., 1, 75–80. Chen, J.H. and Hotchkiss, J.H. (1993) Growth of Listeria monocytogenes and Clostridium sporogenes in cottage cheese in modiﬁed atmosphere packaging, J. Dairy Sci., 76, 972–977. Church, N. (1994) Developments in modiﬁed-atmosphere packaging and related technologies, Trends Food Sci. Technol., 5, 345–352. De Boer, R. and Nooy, P.F.C. (1980) Low-fat semi-hard cheese from ultraﬁltered milk, North Eur. Dairy J., 46, 52–61.
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