Nutritional Requirements of an Aspergillus niger Mutant for Citric Acid Production

The nutritional requirements for high conversion of sugar to citric acid in shaken flask fermentations were investigated for the mutant strain 72--44 of Aspergillus niger. Single variable and factorial experiments were used to determine the effects of nitrogen, phosphorus, magnesium, manganese, copper, zinc and iron on the yield of citric acid. When 2% (v/v) peanut oil was added to the fermentation media it markedly improved the yield of citric acid. Two media were devised which yielded on average 10--10°5 g. anhydrous citric acid from 14 g. sugar, with occasional yields as high as 13 g. Both media contained 2% (v/v) peanut oil and the following trace metals (mg./l.): Fe2+ 2°0; Cu2+ 0°3; Zn2+ 0°1. In medium A the major nutrients were (g./l.): ion-exchange purified cane sugar, 140; KH2PO4, 1°0; MgSO4.7H2O, 0°25; NH4NO3, 1°87; and in medium B (g./l.): ion-exchange purified cane sugar, 140; KH2PO4, 0°3; MgSO4.7H2O, 0°15; NH4NO3, 1°4. Interactions were found between: zinc and iron, nitrogen and iron, nitrogen and phosphorus. The implications of these findings for the fermentation of crude sugar substrates in citric acid production are discussed.