There are several organic acids which are manufactured on a large scale. They are marketed in the form of pure chemicals or salts. In 1880, manufacture of calcium lactate in pilot fermentor was started first in Massachusetts through fermentation. In France, a species of Penicillium was used to produce citric acid in a large fermentor.
There are different uses of organic acids, for example citric acid is used in preparation of soft drinks, jam jellies, candy, wines an frozen eggs. It is also used in blood transfusion, cosmetics, food preservatives, etc. Gluconic acid and its derivatives are used in pharmaceuticals as medicine, removal of metal carbon, bottle washing, etc. Following is a list of organic acids and their producers.
Production of Citric Acid :
Today citric acid is produced throughout the world by fermentation process. There are three processes that are used for citric acid production: Japanese Koji process (strains of Aspergillus niger are used), the liquid surface culture fermentation (A. niger floats on surface of a solution) and submerged fermentation process (mycelium grows throughout the solution in deep tank).
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A. niger is a black coloure fungus that produces conidiophores bearing a vesicle. Vesicles give rise to phyalides that originate conidia borne in chain. A. niger culture accumulates citric acid in the condition of nutrient deficiency.
(a) Submerged Culture Process:
Spores of A. niger are used as inocula for citric acid production. Solutions of high test cane syrup, glucose or sucrose are suitable carbohydrate substrates for citric acid production. Syrup of sugarcane molasses is transferred into a large size fermentor and inoculated after sterilization of medium.
Addition of iron (10 mg/ml) and copper (50 mg/ml) into the medium (pH 4.0), devoid of any source of iron and copper, resulted in 77.8% of yield of citric acid. Air is bubbled in the medium to facilitate the growth of fungus.
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The combination of aeration, low pH and high sensitivity to copper and iron make the use of ordinary iron fermentors impossible. Therefore, glass or plastic fermentors have been used. Antifoaming agent is used to prevent the loss of material. The young culture grows and utilizes medium and produces citric acid before the pH 2.0 is achieved.
(b) Chemistry of Citric Acid Production:
A. niger consumes sugar and passes 78% of sugar into Embden-Meyerhof-Parnas (EMP) pathway that is also called glycolysis and results in 2 moles of pyruvic acid. Here aconitase and isocitric dehydrogenase are inhibited by copper.
Iron acts as co- factor of these enzymes in the conversion process. Consequently, acetyl-CoA (derived from EMP pathway) condenses with oxaloacetate to form citric acid. Hence the Krebs cycle is aborted and citric acid accumulates in fermentation broth.