Citric Acid Production by Aspergillus niger in Stirred Tank Bioreactor

The optimization of citric acid production by submerged fermentation using Aspergillus niger TISTR 2365 was studied on upstream and downstream processing using a modified medium. The Box-Behnken Design was used for optimization, varying carbon to nitrogen ratios (70, 85, 100 g/L), shaking speed (200, 250, 300 rpm), and CaCl2 supplementation (0.01, 0.055, 0.1 g/L) across 17 experimental runs. Batch fermentations were conducted in a 250 mL Erlenmeyer flask for 7 days at 30 °C. Results found the optimum citric acid was obtained at 27.73 g/L at a carbon to nitrogen ratio of 97.32 g/g, shaking speed of 300 rpm, and CaCl2 supplementation of 0.10 g/L. Model validation of citric acid production of 25.43 g/L was confirmed in a 5-L stirred tank bioreactor, with an 8.29% deviation from the predicted value. Three operational runs in a bioreactor yielded citric acid concentrations of 26.88 g/L, 21.76 g/L, and 26.88 g/L, respectively. The highest citric acid yield was achieved in the third batch, reaching 0.25 g/g glucose, while the average yield from substrate utilization was approximately 0.20 g/g glucose. The results highlight the effectiveness of fermentation conditions and the role of CaCl₂ in enhancing citric acid production. Downstream processing was performed to purify the citric acid by the precipitation method. Purified citric acid was then quantified and analyzed by HPLC and FTIR.