Department of Food and Human Health Sciences, Graduate School of Human Life Science, Osaka City University, Sumiyoshi, Osaka 558-8585, Japan
The effects of citric acid on the rheological properties of cornstarch pastes were studied by steady shear and dynamic oscillatory viscoelasticity, intrinsic viscosity measurements and microscopic observation. The pH of cornstarch dispersion was adjusted between 6.0 and 3.0. The viscosity of the pastes was increased by lowering the pH (between 5.5 and 3.6), while the viscosity of samples with pH below 3.5 decreased further than that of the control (pH ) 6.3). Citric acid promoted the collapse of starch granules; however, adding excessive citric acid led to the hydrolysis of glucose chains. No decrease in the viscoelasticity was observed for cornstarch pastes by adding acid at 25 °C after gelatinization. KEYWORDS: Cornstarch; viscoelasticity; citric acid; pH
Starch has been widely used as a thickener, stabilizer, or gelling ingredient in the food industry. The main constituents of starch are amylose and amylopectin, which consist of linked R-D-glucose residues that make large polysaccharide molecules. Starch is present in the form of granules, which are not soluble in water. When starch is heated in water, the starch granules swell and rupture. Amylose and amylopectin can be leached out from the granules, and the starch suspension becomes a viscous paste. This process is known as gelatinization. Starch has been added to many kinds of food using this property. To overcome the disadvantages of native starches such as its gummy or cohesive texture and the formation of rigid and opaque gels, many kinds of modified starches have been designed. Acid-hydrolyzed starch is one of them. Because starch is hydrolyzed by acid, aqueous hydrochloric acid is used to make acid-hydrolyzed starch by compulsorily causing hydrolysis of amylose and amylopectin chains (1-4). However, sour substances (various acids) are included in many foods and are added to many foods as acidulents or preservatives. It is necessary to investigate the effects of acids on the viscoelasticity of starch because it may lead to understanding how to control the viscosity of starch products. In this study, cornstarch and citric acid were selected as the model for actual starch food products. Cornstarch is the most widely used starch in the food industry. Citric acid is contained in citrus fruits and is added to foods as a sour seasoning. It is also important as a sour basic substance for difference tests on the sensory evaluation. There are many reports concerning the effects of acids or pH on gelatinization and retrogradation of starch (5-11) while * To whom correspondence should be addressed. Tel.: +81-6-66052818. Fax: +81-6-6605-3086. E-mail: email@example.com.
there are no report concerning the viscoelasticity of cornstarch to our knowledge. Our objective is to investigate how citric acid affects the rheological properties of starch pastes and contributes to the effect of increasing the viscosity for starch pastes. MATERIALS AND METHODS Materials. A cornstarch sample (cornstarch Y) was kindly provided by Sanwa Starch Co. Ltd. (Nara, Japan). The concentration of cornstarch was fixed at 3.0 wt %; this is the concentration generally used for thickeners in sauces or soups. The moisture (water) content of the cornstarch was determined to be 13.4% by drying it at 130 °C in atmospheric pressure. Citric acid (anhydrous) and dimethyl sulfoxide were of reagent grades. The pH was adjusted between 6.0 and 3.0. Sorbic acid potassium salt (0.05 wt %) was used to prevent microbial growth. Preparation of Cornstarch Pastes. Powders of cornstarch were dispersed in distilled water or in citric acid solutions with pH adjusted beforehand. A control sample of pH 6.3 was made of...