Optimization of Sweet Potato Starch Hydrolyzate Production and Its Potential Utilization as Substrate for Citric Acid Production

Aims: The aims of this work was optimization of two-step enzymatic hydrolysis of sweet potato starch using statistical approach and subsequent utilization of the hydrolyzate obtained for citric acid production.

Methodology: Box Behnken design was used in this study to generate a total of 17 individual experiments for each step of the hydrolysis (liquefaction and saccharification steps). These were designed to study the effect of temperature, time and pH on the sweet potato starch hydrolyzate (SPSH) concentration. The optimization was carried out using response surface methodology (RSM). The SPSH obtained was used to culture Aspergillus niger for citric acid production.

Results: A statistically significant quadratic regression model (P<0.05) was obtained for the liquefaction step. Statistical model predicted the highest sweet potato starch hydrolyzate (SPSH) concentration to be 172.23 g/L at optimal condition of temperature 61.05ºC, time 55.02 min and pH 6.5. A statistically significant quadratic regression model was also obtained for the saccharification step. Statistical model predicted the highest SPSH concentration to be 241.92 g/L, established at the optimal condition of temperature 52ºC, time 44 min and pH 4.5. The optimal liquefaction and saccharification conditions were validated with the actual SPSH concentration of 172.00 and 241.01g/L, respectively. The maximum citric acid production of 86g/L was achieved on the 8th day of cultivation when the SPSH was used for the cultivation of A. niger.

Conclusion: RSM was successfully applied the two-step enzymatic hydrolysis of sweet potato starch. This work showed that the sweet potato starch hydrolyzate could serve as sole carbon source for citric acid production.