Statistical approach to the optimization of citric acid production using filamentous fungus Aspergillus niger grown on sweet potato starch hydrolyzate

Optimization of citric acid production from Sweet Potato Starch Hydrolyzate (SPSH) using Aspergillus niger was investigated. Effect of five independent variables viz. concentrations of SPSH, (NH4)2HPO4, KH2PO4, fermentation time and pH on the response (citric acid concentration) and their reciprocal interactions were established using Response Surface Methodology (RSM). The central composite design was used to generate a total of 50 fermentation runs, which were subsequently conducted. A second-order mathematical model was obtained to predict the citric acid concentration as a function of the five variables. A statistical model predicted the highest citric acid concentration as 83.01 g L−1, at the optimal condition of SPSH of 153.77 g L−1, (NH4)2HPO4 of 3.55 g L−1, KH2PO4 of 2.58 g L−1, time of 8 days and pH of 6. The optimized condition was validated with the actual citric acid concentration of 83.00 g L−1. This work showed that SPSH could serve as alternative carbon source for citric acid production and the results could be scaled up to pilot production.