Optimization of citric acid production by Aspergillus niger

Among the various fungal strains screened for citric acid production, Aspergillus niger is known to produce considerable amounts of citric acid and other organic acids when cultivated in submerge fermentation. In this study, optimization of the medium components was carried out using "one-factor-at-a-time" and response surface methods (RSM). One-factor-at-a-time indicated that the amount of citric acid production was increased along with the increasing of agitation speed (from 150 to 200 rpm), raising the incubation temperature (from 17 to 32) and decreasing pH value (to 2). Moreover, in comparison with the other groups, citric acid production in the treatment containing soy bean powder and minerals revealed a significant increase (up to 25 g/l). The results showed that application of ultrasonic wave during the growth phase could remarkably enhance the production of citric acid. Based on the results of one-factor-at-a-time, sucrose and soy bean powder were the selected additives to test their effect on citric acid production using RSM. The two variables were identified to have significant effects on citric acid production and the maximum citric acid production of 58 g/l was resulted from the combination of 230.87 g/l sucrose and 200.81 g/l soy bean powder. In this study, the significant increase in the production of citric acid in the optimal conditions indicated the using of appropriate statistical methods and also the correct levels of selected variables.