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Read this article to learn about the media used for the growth of microorganisms in industrial fermentation.
The media used for the growth of microorganisms in industrial fermentation must contain all the elements in a suitable form for the synthesis of cellular substances as well as the metabolic products. While designing a medium, several factors must be taken into consideration. The most important among them is the ultimate product desired in the fermentation.
For growth-linked products (primary metabolites e.g. ethanol, citric acid), the product formations is directly dependent on the growth of the organisms, hence the medium should be such that it supports good growth. On the other hand, for products which are not directly linked to the growth (secondary metabolites e.g. antibiotics, alkaloids, gibberellins), the substrate requirements for product formation must also be considered.
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In the laboratory, pure defined chemicals may be used for culturing microorganisms. However, for industrial fermentations, undefined and complex substrates are frequently used for economic reasons. Cheaper substrates are advantageous since they minimize the production cost of the fermented products.
Wastes from agriculture, and byproducts of other industries are generally preferred, although they are highly variable in composition. Raw materials used in fermentation largely depend on their cost at a particular time, since there are seasonal variations.
The choice of the medium is very critical for successful product formation. For industrial fermentation, the microorganisms, in general, utilize a luxury metabolism. Therefore, good production yields are expected with an abundant supply of carbon and nitrogen sources, besides requisite growth factors. The media used in fermentation processes may be synthetic or crude.
Synthetic media:
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Media with all the requisite constituents in a pure form in the desired proportion represents synthetic media. Use of this type of media in fermentations is not practicable.
Crude media:
The non-synthetic media with naturally available sources are better suited for fermentation.
In practice, crude media with an addition of requisite synthetic constituents is ideal for good product yield in fermentation.
The most frequently used substrates for industrial fermentation with special reference to the supply of carbon and nitrogen sources and growth factors are briefly described below.
Substrates Used As Carbon Sources:
Carbohydrates constitute the most predominant source of energy in fermentation industry. Refined and pure carbohydrates such as glucose or sucrose are rarely used for economic reasons.
Molasses:
Molasses is a byproduct of sugar industry and is one of the cheapest sources of carbohydrates. Sugar cane molasses (sucrose around 48%) and sugar beet molasses (sucrose around 33%) are commonly used. Besides being rich in sugar, molasses also contain nitrogenous substances, vitamins and trace elements.
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There occurs variation in the composition of the molasses which mostly depends on the climatic conditions and production process. Hydrol molasses, a byproduct in glucose production from corn, is also used as a fermentation substrate.
Malt extract:
Malt extract, an aqueous extract of malted barley, contains about 80% carbohydrates (glucose, fructose, sucrose, and maltose). Nitrogen compounds constitute around 4.5% (proteins, peptides, amino acids, purines, pyrimidine’s).
Starch, dextrin and cellulose:
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The polysaccharides-starch, dextrin and cellulose can be metabolised by microorganisms. They are frequently used for the industrial production of alcohol. Due to its wide availability and low cost, the use of cellulose for alcohol production is extensively studied.
Whey:
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Whey is a byproduct of dairy industry and is produced worldwide. Most of it is consumed by- humans and animals. Whey is a reasonably good source of carbon for the production of alcohol, single-cell protein, vitamin B12, lactic acid and gibberellic acid. Storage of whey is a limiting factor for its widespread use in fermentation industry.
Methanol and ethanol:
Some of the microorganisms are capable of utilizing methanol and/or ethanol as carbon source. Methanol is the cheapest substrate for fermentation. However, it can be utilized by only a few bacteria and yeasts. Methanol is commonly used for the production of single-cell protein. Ethanol is rather expensive. However, at present it is used for the production of acetic acid.
Substrates Used As Nitrogen Sources:
The nitrogen supply to the fermentation microorganisms may come from inorganic or organic sources.
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Inorganic nitrogen sources:
Ammonium salts and free ammonia are cheap inorganic nitrogen sources, particularly in industrialised countries. However, not all the microorganisms are capable of utilizing them, hence their use is limited.
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Organic nitrogen sources:
Urea is fairly a good source of nitrogen. However, other cheaper organic forms of nitrogen sources are preferred.
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Corn steep liquor:
This is formed during starch production from corn. Corn steep liquor is rich in nitrogen (about 4%) and is very efficiently utilized by microorganisms. It is rich in several amino acids (alanine, valine, methionine, arginine, threonine, glutamate).
Yeast extracts:
They contain about 8% nitrogen and are rich in amino acids, peptides and vitamins. Glucose formed from glycogen and trehalose during yeast extraction is a good carbon source. Yeast extracts are produced from baker’s yeast through autolysis (at 50-55°C) or through plasmolysis (high concentration of NaCI). Yeast extracts are very good sources for many industrially important microorganisms.
Soy meal:
After extracting the soy bean oil from the soy bean seeds, the left out residue is soy meal. It is rich in proteins (about 50%) as well as carbohydrates (about 30%) contents. Soy meal is often used in antibiotic production.
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Peptones:
The protein hydro-lysates are collectively referred to as peptones, and they are good sources for many microorganisms. The sources of peptones include meat, soy meal, peanut seeds, cotton seeds and sunflower seeds.
The proteins namely casein, gelatin and keratin can also be hydrolysed to yield peptones. In general, peptones derived from animal sources have more nitrogen content while those from plant sources have more carbohydrate content. Peptones are relatively more expensive, hence not widely used in industries.
Sources of Growth Factors:
Some of the microorganisms are not capable of synthesizing one or more growth factors such as vitamins. These growth factors are very expensive in pure form, hence crude sources are preferred. Yeast extract is a rich source of almost all growth factors. Generally, the substrates derived from plant or animal sources in a crude form are reasonably rich in mineral content. Sometimes, however mineral (phosphate, sulfate) supplementation may be required.