Studies on the Production of Citric Acid by Rhizopus stolonifer

A fermentation process was developed for citric acid production from orange peel waste by Rhizopus stolonifer. In a screening programme to select the best citric acid producing fungus, it was showed that after 3 days of incubation, the amount of citric acid in the screening medium varied depending on the isolate used. Rhizopus stolonifer produced the highest citric acid titre (5.8 g/L) and was therefore selected for fermentation studies. The addition of 3% and 4% (v/v) methanol into the isolation medium resulted in a citric acid yield of 7.8 g/L and 7.1 g/L respectively. This level corresponded to 135 and 122% respective increases in citric acid yield in comparison to the control medium that produced only 5.8 g/L. The basal medium was supplemented with different concentrations of lactose, maltose and sucrose. It was found that sucrose at 15% (w/v) concentration caused the best citric acid yield (33.7 g/L). Yields of citric acid generally increased with sugar concentrations and maximum production rates were achieved at 10–15% of sugar.