Review on Microbial Production of Organic Acid

The first individual process for citric acid production was the liquid surface culture (LSC), which was introduced in 1919 by Société des Produits Organiques in Belgium, and in 1923 by Chas Pfizer & Co. in US. After that, other methods of fermentation, such as submerged fermentation were developed. Although this technique is more sophisticated, surface method required less effort in operation and installation and energy cost. Organic acids such as citric, malic and pyruvic are used extensively as acidulants in the food industry. Inulin, a fructose polymer found as a carbohydrate reserve in plants, was investigated as a possible substrate for organic acid production. Aspergillus niger produced citric acid on sucrose and inulin during surface fermentation. Aeration was found to have a significant effect on acid production. Submerged fermentation of inulin produced a higher yield of citric acid than surface fermentation with optimum acid production occurring at a concentration of 14%. An investigation into the enzymes produced during citric acid fermentation by Aspergillus niger revealed that both extracellular and cell lysate invertase and inulinase were produced during fermentation. The yeast-like fungus Aureobasidium pullulans was shown to produce poly- sc L-malic during submerged fermentation of sucrose, glucose, galactose, maltose, inulin, xylose, fructose, and an equimolar mixture of glucose and galactose. Citric acid cannot be recovered directly from the fermented liquor by crystallization because of excess of impurities. In the classic protocol, citric acid is precipitated as calcium citrate by the addition of lime. The washed precipitate is treated in aqueous suspension with H2SO4, yielding gypsum as a by-product. The citric acid solution is concentrated by vacuum evaporation and crystallized at low temperatures. Citric acid (C6H8O7, 2 – hydroxy – 1,2,3 – propane tricarboxylic acid), a natural constituent and common metabolite of plants and animals, is the most versatile and widely used organic acid in the field of food (60%) and pharmaceuticals (10%). It has got several other applications in various other fields. Currently, the global production of citric acid is estimated to be around 736000 tones/year, and the entire production is carried out by fermentation.