High-performance co-immobilized microbial culture for sweet potato-based ethanol production using modified VMFB bioreactor.

The purpose of this study was to investigate the optimal cultivation strategy for long-term stable ethanol production from sweet potato starch in vertical mass flow bioreactor (VMFB) by co-immobilizing Aspergillus awamori, Rhizopus japonicus and Zymomonas mobilis. In exploring the optimum initial substrate concentration, the maximum ethanol concentration of 45.11 g/L was achieved at the 54th hour at a feed concentration of 12 % (w/v), which resulted in a conversion rate of 66.26 % compared to the theoretical value. In the repeated batch process (multiple cycles), stable ethanol production was achieved from 6 % (w/v, 3cycles) to 12 % (w/v, 5cycles) of sweet potato starch producing total ethanol concentrations of 71.03 g/L (3cycles, 72 h) and 241.54 g/L (5 cycles, 348 h), respectively. In fed-batch fermentation with a total addition of 24 % (w/v) sweet potato starch (four separate additions), the maximum ethanol concentration and yield after 114 h were 102.6 g/L and 0.43, respectively. To evaluate the potential for future industrialization, dried raw starch was used instead of gelatinized starch as a substrate. The results demonstrated that the fed-batch fermentation process had higher ethanol yield and starch conversion rates, with values of 0.32 and 55.96 % at 144 h, respectively.