Production of critic acid: A review

Abstract

Citric acid is present in citrus fruits. The citric acid produced from fruits is known as natural citric acid, in contrast to fermentation citric acid. It has been found that citric acid is produced fermentatively by many moulds and yeasts of which Aspergillus niger has been widely used. However, besides A. niger, many other species have been considered as citric acid producers, and many trials have been done to increase the potentialities of strains of A. niger, for instance by exposure of the spores to ultra-violet irradiation and x-rays. Citric acid is used in the preparation of many industrial products, and the commercial uses of citric acid have led to an increasing demand for it. The supply of natural citric acid is limited and the demand can only be satisfied by fermentation processes. These processes can use a number of substrates, especially hydrocarbons, carbohydrates and agricultural wastes. Various theories are presented to show the complexity of the reactions that may occur in this fermentation, rather than to suggest that any one of them shows the true course. In citric acid production it has been found that it is essential to provide the citric-acid-producing organisms with not only major nutrients such as carbon, nitrogen and phosphorus, but also with vitamins and other micronutrients such as iron, copper, magnesium, manganese and lead. The fermentation process of citric acid production by moulds and yeasts is also controlled by aeration, temperature and incubation period. Treatment of agricultural wastes such as black-strap molasses with ferrocyanide improves the yield of citric acid. The process of recovering substantially pure citric acid from the highly contaminated citric acid solutions of the fermentation media consists of precipitating the acid in the form of poorly soluble salts or alkali-metal salts, or in extracting the acid with organic solvents.