Nutritional Requirements of an Aspergillus niger Mutant for Citric Acid Production

SUMMARY: 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.