Studies on citric acid production by Aspergillus niger using undersized semolina

In this research, the effects of initial phi, methanol, ammonium nitrate and different substrate concentrations on citric acid production from undersized semolina by Aspergillus niger were investigated. During fermentation period; nH, citric acid concentration, biomass concentration and the amount of total carbohydrates (as glucose) were determined. Growth and citric acid production of A. niger were modeled and maximum specific growth rate, maximum specific citric acid production rate, lag time and asymptote value were calculated. Optimum initial pH value for citric acid production from undersized semolina was found as 6.5. About 0.0506, 0.3904, and 1.1904 g/L of citric acid was produced in 0.01, 0.1, 0.5% undersized semolina medium with initial pH 6.5, respectively. As undersized semolina concentration increased, citric acid concentration increased. Citric acid and biomass formation rates were increased with the addition of 3.0% methanol and 0.03% ammonium nitrate. Maximum citric acid yield (70.4%) was obtained in 0.01% undersized semolina medium containing 3.0% methanol and 0.03% ammonium nitrate with initial pH 6.5. Maximum specific citric acid production rates of A. niger were calculated by modified Gompertz model. In the fermentation medium containing 0.5% undersized semolina, 0.5% undersized semolina+3%methanol, 0.5% undersized semolina+0.03% ammonium nitrate, 0.5% undersized semolina+3% methanol+0.03% ammonium nitrate, maximum specific citric acid production rates of A. niger were calculated as 0.007, 0.010, 0.012, 0.010 h"1, respectively. Undersized semolina can be used as an economic raw material in the production of citric acid by A. niger. Key words: Aspergillus niger, citric acid production, undersized semolina, methanol, ammonium nitrate, modeling, yield.