Nitrogen Sources
Most industrial microbes
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 used to
adjust pH of the fermentation. Organic
nitrogen sources include amino acids, proteins and urea. Nitrogen is often supplied in crude forms
that are essentially by-products of other industries, such as corn steep
liquor, yeast extracts, peptones and soya meal.
Purified amino acids are used only in special situations, usually as
precursors for specific products.
Corn Steep Liquor (CSL)
Corn steep liquor is a
by-product of starch extraction from maize and its first use in fermentations
was for penicillin production in the 1940s.
The extract composition of the liquor varies depending on the quality of
the maize and the processing conditions. Concentrated extracts generally
contain about 4% (w/v) nitrogen, including a wide range of amino acids, along
with vitamins and minerals. Any residual
sugars are usually converted to lactic acid (9-20%, w/v) by contaminating
bacteria. Corn steep liquor can
sometimes be replaced by similar liquors, such as those derived from potato
starch production.
Yeast Extracts
Yeast extracts may be
produced from waste baker’s and brewer’s yeast, or other strains of S. cerevisiae. Alternate sources are Kluveromyces marxianus (formerly classified as K. fragilis) grown
on whey and Candida utilis cultivated
using ethanol, or wastes from wood and paper processing. Those extracts used in the formulation of
fermentation media are normally salt-free concentrates of soluble components of
hydrolyzed yeast cells. Yeast extracts
with sodium chloride concentrations greater than 0.05% (w/v) cannot be used in
fermentation processes due to potential corrosion problems.
Yeast cell hydrolysis is often achieved by autolysis using the cell’s endogenous enzymes, usually without the need for additional hydrolytic enzymes. Autolysis can be initiated by temperature or osmotic shock, causing cells to die but without inactivating their enzymes. Temperature and pH are controlled throughout to ensure an optimal and standardized autolysis process. Temperature control is particularly important to prevent loss of vitamins.
Autolysis is performed at 50-55˚C for
several hours before the temperature is raised to 75˚C to inactivate the
enzymes. Finally the cells are disrupted
by plasmolysis or mechanical disruption.
Cell wall materials and other debris are removed by filtration or
centrifugation and the resultant extract is rapidly concentrated. Extracts are available as liquids containing
50-65% solids, viscous pastes or dry powders.
They contain amino acids, peptides, water-soluble vitamins and some
glucose, derived from the yeast storage carbohydrates (trehalose and glycogen).
Peptones
Peptones are usually too
expensive for large scale industrial fermentations. They are prepared by acid or enzyme
hydrolysis of high protein materials: meat, casein, gelatine, keratin, peanuts,
soy meal, cotton seeds etc. Their amino
acid compositions may vary depending upon the original protein source. For example, gelatine-derived peptones are
rich in proline and hydroxyproline, but are almost devoid of sulphur-containing
amino acids; whereas keratin peptone is rich in both proline and cystine, but
lacks lysine. Peptones from plant
sources invariably contain relatively large quantities of carbohydrates.
Soya Bean Meal
Residues remaining after
soya beans have been processed to extract the bulk of their oil are composed of
50% protein, 8% non-protein nitrogenous compound, 30% carbohydrates and 1%
oil. This residual soya meal is often
used in antibiotic fermentations because the components are only slowly
metabolized, thereby eliminating the possibility of repression of product
formation.
References
- Industrial Microbiology:
An Introduction. Michael J. Waites, Neil L. Morgan, John S
- Principles of Fermentation Technology- Peter Stanbury,
Allan Whitaker, Stephen Hall
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