1. Acetic Acid:
It is prepared from fermented alcohols with the help of acetic acid bacteria, Acetobacter aceti. Alcoholic fermentation is anaerobic process, but the conversion of alcohol to acetic acid is aerobic one.
As soon as 10-13% acetic acid is formed, the liquid is filtered. It is used after ripening as vinegar. The type and quality of vinegar depends upon substrate used for alcoholic fermentation and ripening. For other purposes, acetic acid is purified. The organic acid is employed in pharmaceuticals, colouring agents, insecticides, plastics, etc.
2. Citric Acid:
It is obtained through the fermentation carried out by Aspergillus niger and Mucor species on sugary syrups. Yeast Candida lipolytica can also be employed, provided its nutrient medium is made deficient of iron and manganese. Citric acid is employed in dyeing, engraving, medicines, inks, flavouring and preservation of food and candies.
3. Lactic Acid:
It was the first organic acid to be produced from the microbial fermentation in starchy and sugary medium. Lactic acid fermentation is carried out by both bacteria (e.g., Streptococcus lactis, Lactobacillus species) and fungi (e.g., Rhizopus). The acid derived from fungal sources is costlier but is of high purity. Any starchy or sugary medium is used.
Lactic acid is used in confectionery, fruit juices, essences, pickles, curing of meat, lemonades, canned vegetables and fish products. It is also employed as mordant in tanning, printing of wool in the preparation of plastics and pharmaceuticals.
4. Gluconic Acid:
The acid is prepared by the activity of Aspergillus niger and Penicillium species. Calcium gluconate is used widely as a source of calcium for infant, cows and lactating mothers. It is also used in preparation of pharmaceuticals.
5. Butyric Acid:
The acid is produced during fermentation activity of bacterium Clostridium acetobutylicum. Rincidity of butter is also due to its formation.
6. Alcohols:
Ethanol, methanol, propanol and butanol are alcohols that can be produced commercially by fermentation activity of some fungi (e.g., Yeast, Mucor, Rhizopus) and bacteria (e.g., Clostridium acetobutylicum, C. saccharotobutylicum). The alcohols are important industrial solvents.
Enzymes:
Enzymes are proteinaceous substances of biological origin which are capable of catalysing biochemical reactions without themselves undergoing any change. The word enzyme was coined by William Kuhne (1867) after the fact the yeast provided the most well studied bio-catalytically controlled reactions of alcoholic fermentation (Gk. en- in, zyme- yeast). Buchner (1901) found yeast extract to have enzymatic activity. The number of enzymes now runs into several thousands.
All of them are macromolecules (large sized molecules) with a specific three-dimensional shape. Enzymes are substrate specific and carry out a specific catalytic action. They work best at room temperature and near-neutral pH with the exception of several digestive enzymes. Use of enzymes in biotechnology had a number of problems which have been largely overcome by the technique of immobilisation of enzymes inside artificial cells or gels. About 300 enzymes are being used in industry and medicines. Most of them are obtained from microbes.