Production of organic acids
Many organic acids such as acetic, lactic, citric, gluconic, fumaric etc. are produced by microbial fermentation of the cells.
The commercial production of vinegar (acetic acid) involves a preliminary fermentation of the fruit juice to produce ethyl alcohol and its secondary fermentation .Into acetic acid, under aerobic conditions. Various species of Acetobacter have the ability to oxidize alcohol to acetic acid.
The rate and the final amount of the acetic acid produced depend to a great extent on oxygen supply, the initial alcohol fermentation and the strain of the bacterium.
In one type of the vinegar generator, wooden shavings packed in a column are saturated with mother vinegar (old vinegar containing Acetic acid bacteria) and freshly fermented alcoholic solution is then circulated. Acetic acid bacteria develop as a thin film on the wooden shavings and carry out the oxidation ethanol to acetic acid.
Solutions containing about 5-6 per cent alcohol are converted into ethanol in less than 5 days. Acetic acid fermentation can be further improved by the use of stirred tank fomenters, which are vigorously aerated. In this way desired acetic acid level can be obtained within hours.
Lactic acid fermentation
Lactic acid is produced from various carbohydrates such as corn starch, potato starch, molasses and whey. When starchy materials are used, they are initially hydrolysed to simple sugars. The medium is supplemented with a nitrogen source and calcium carbonate and the fermentation is carried out by inoculation with homofermentative
Lactobacilli such as Lactobacillus bulgariacus or Lactobacillus delbruckii during the fermentation the temperature is controlled at 40-50° C depending on the organism. The medium is kept in constant agitation.
After completion of the fermentation for 4-6 days, the fermented liquor is heated to 82°C and then, filtration is done.
The filtrate containing calcium lactate is spray dried after treating with sodium sulphide. To obtain lactic acid, the calcium lactate is treated with sulphuric acid and the lactic acid, thus, obtained is further purified.
Citric acid fermentation
Citric acid is a key intermediate of TCA cycle. Many fungi, bacteria and yeasts produce it. A variety of carbohydrate sources such as beet molasses, sucrose, commercial glucose, starch hydrolysis etc. are used for citric acid production. The raw material is diluted to 20-25 per cent sugar concentration and mixed with nitrogen source and other salts.
The pH of the medium is maintained around 5.0, when molasses are used and pH is adjusted at 3.0 when sucrose is used. The fermentation is carried out either under the surface, submerged or under solid state conditions.
In the surface culture method, shallow aluminum or stainless steel pans are filled with the growth medium, inoculated with fungal spores and allowed to ferment. In the submerged culture method, the mold is cultured in the fomenters under vigorous stirring and mixing while in solid state fermentation; the mold is gown over carrier material such as biogases etc. which is impregnated with fermentation medium.
Citric acid fermentation by Aspergillus niger is greatly enhanced in the presence of trace metals such as iron, zinc, copper, manganese etc. in the medium. Trace metals have detrimental effects when used in excess. To optimize the level of trace metals, the raw materials are treated with ferrocyanide, charcoal, chelating agents or cation exchange resins. This fermentation, being aerobic one, needs adequate aeration for citric acid production.
Production of enzymes
Microorganisms are known to excrete enzymes into the growth medium and these enzymes have many uses in pharmaceutical, food and textile industries. Some microbial enzymes and their uses are given in the Table.
The quality and quantity of the enzymes produced by the microbes depend on the strain of the microbe and its cultural conditions. For large-scale production of enzymes solid state fermentation or semi-solid fermentation is used.
Here, wheat bran, after being properly sterilized is inoculated with fungal spores and incubated for definite period of time. The fermented mass is then extracted with water and the enzyme is extracted following the characteristic procedure which is specific for each enzyme.
These enzymes can also be produced in large-scale fomenters under vigorous aeration and agitation.
Production of antibiotics
First antibiotic to be extracted from the species of Penicillium notatum is Penicillin in the year 1928 by Sir Alexander Fleming. Since then a large number of antibiotics have been obtained from microorganisms like fungi, bacteria and actinomycetes.
Using strains of P.notatum, P.chrysogenum etc in fermented containing a carbohydrate source, mineral salts and corn steep liquor under vigorous agitation, there can be maximal production of penicillin.
The second antibiotic to be produced was streptomycin from Streptomyces griseous. It is obtained from a fermentation medium containing glucose, soy bean meal and mineral sals. The pH of the medium was maintained at 7.4-7.5. The fermentation is carried out under submerged conditions at 25-30° C for 5-7 days.