Effect of acetate and format inhibitors on the production of <i>Aspergillus niger</i> inulinase and fermentation modeling, sensitivity, and economic analysis

This study examines the effect of acetic acid (AA) and formic acid (FA) in different concentrations, on producing inulinase by Aspergillus niger A42 (ATCC 204447) as well as determining the best model for fermentations. The highest inulinase and invertase-type activities were 340.32 and 546.25 U/mL and 398.24 and 608.50 U/mL in the fermentation medias containing 2.5 g/L AA and 12.5 g/L FA, respectively. It was observed that 7 g/L or more AA addition inhibited the inulinase production by A. niger. Indeed, a negative affect was seen in the A. niger inulinase production. When the FA concentrations were less than 3 g/L, the inulinase activities were negatively affected. Conversely, the inulinase activities were surprisingly increased with the concentrations more than 4 g/L, showing an inducing effect. The fermentations that yielded maximum inulinase activity were modeled using ten mathematical models. Inulinase productions were successfully represented by Huang (RMSE = 8.28 U/mL, MAE = 5.03 U/mL, R2 = 0.998, BF = 1.08, and AF = 1.11) and Weibull (RMSE = 47.98 U/mL, MAE = 26.83 U/mL, R2 = 0.901, BF = 1.08, and AF = 1.31) models. Also as a result of fermentations, the enzyme activity was determined as inulinase type activity since the I/S ratio was greater than 10-2. In conclusion, optimal inulinase production by A. niger was achieved with specific concentrations of AA and FA, showing distinct inhibitory and inducing effects, and the fermentations were accurately modeled by the Huang and Weibull models.