Effect of sugars, hydrogen ion concentration and ammonium nitrate on the formation of citric acid by Aspergillus niger.

Aspergillus niger Mulder strain when grown on a synthetic medium containing urea as the sole source of nitrogen at pH 5.2, formed a mixture of citric and gluconic acids. On growing the organism at pH 2.0 the gluconic acid content was reduced but citric acid yield remained low. Addition of NH4NO3 to the medium lowered the gluconic acid yields to undetectable levels with a simultaneous increase in the citric acid content. Of the sugars used for the production of citric acid, sucrose in an unautoclaved medium was found to be the best carbon source. Sucrose medium if autoclaved at pH 2.0, or a mixture of glucose and fructose instead of sucrose gave lower yields of citric acid. Under optimum conditions only citric acid was produced and the yield was 66-68 per litre after a growth period of about 10 days.