Medium formulation is an essential stage in the design of fermentation process. Most fermentation media require liquid media, although some solid-substrate fermentations are also operated. Fermentation media must satisfy all the nutritional requirements of the microorganisms and fulfill the technical objectives of the process (1).
There are several stages where media are required in a fermentation process; inoculum (starter culture), propagation steps, pilot-scale fermentations and the main production fermentations (2). On a large scale, the sources of nutrients should be selected to create a medium which should meet as many as many possible of the following criteria: i.
It should produce the maximum yield of product or biomass per gm of substrate used. ii.
It should produce maximum concentration of product or biomass. iii.
It should permit the maximum rate of product formation. iv.
There should be the minimum yield of undesired products. v.
It should be of a consistent quality and be readily available throughout the year. vi.
It should cause minimal problems during media preparation and sterilization. vii.
It should cause minimal problems in other aspects of the production process particularly aeration and agitation, extraction, purification and waste treatment. The initial step in media for media formulation is the examination of the overall process on the stoichiometery for growth and product formation (3). An aerobic fermentation process may be represented as:
Carbon and energy source + Nitrogen source + O2 + other requirements Biomass + products + CO2 + H2O + heat
This primarily involves consideration of the input of the carbon and nitrogen sources, minerals and oxygen and their conversion to cell biomass, metabolic products. Based on this information, it should be possible to calculate the minimum quantities of each element required to produce a certain quantity of biomass and metabolite. Once the nutritional requirements of a microorganism have been determined, then suitable nutrient sources can be incorporated into the media (3).
The medium adopted also depends on the scale of the fermentation. For small scale laboratory fermentations pure chemicals are often used in well defined media. Industrial scale fermentation processes use cost effective complex substrates, where many carbon and nitrogen sources are almost indefinable (1). For example, for preparing a medium for culturing yeast for animal consumption vegetable waste matter can be used as a starting material. Although, such a waste matter is not suitable for direct feeding of animals. Corn-cob and/or stalks, reeds, sunflower stalks, fallen autumn leaves are preferred representatives (4). Carbon sources
A carbon source is required for all biosynthesis leading to reproduction, product formation and cell maintenance. It also serves as the energy source. Carbon requirements may be determined from the biomass yield coefficient (Y), an index of the efficiency of conversion of a substrate into the cellular material: Ycarbon (g/g) = biomass produced (g)
Carbon substrate utilized (g)
Carbohydrates are traditional carbon and energy sources for microbial fermentation, although other sources may be used, such as alcohols, alkanes and organic acids. In addition to main carbon source, animal fats and plant oils may be incorporated into some media as supplements (1,3). Nitrogen sources
Industrially important microorganisms can utilize both inorganic and organic nitrogen sources. Inorganic nitrogen may be supplied as ammonium salts, often ammonium sulphate and diammonium hydrogen phosphate, or ammonia. Ammonia can also be use to adjust the pH of the fermentation. Organic nitrogen sources include amino acids, proteins and urea. Growth is fastened with a supply of organic nitrogen, and a few microorganisms have an absolute requirement of amino acids. Nitrogen is often supplied in crude forms that are essentially...
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