Co-fermentation of hemicellulosic hydrolysates and starch from sweet sorghum by Clostridium acetobutylicum: A synergistic effect for butanol production

Abstract

Hemicellulose is a cheap and abundant substrate for biofuel production. However, industrial scale production of biofuels from hemicellulose is relatively inefficient because of expensive pretreatment processes and poor pentoses utilization by most microorganisms. In this study, a cost-effective autohydrolysis process using water as the only reagent to hydrolyze lignocellulose was exploited. Sweet sorghum stalk was also utilized as an economic source of hemicellulose. The autohydrolyzed lignocellulosic compounds from sweet sorghum were used as substrate in acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum which is able to ferment pentoses and hexoses into ABE. Separation of 79 % of hemicellulose and roughly 20 % of cellulose from sweet sorghum stalk was detected as a desirable result for autohydrolysis at 210 °C; however, over-production of inhibitors made it an inappropriate pretreatment for ABE fermentation. No butanol production was detected in autohydrolysates of 210 °C, even after using detoxification methods for inhibitory compounds removal. On the other hand, only 20 % of sweet sorghum’s bagasse hemicellulose was separated at 150 °C; yet, it was detected as the most desirable hydrolysate for ABE fermentation. Nevertheless, inherent inefficiency of C. acetobutylicum to ferment xylo-oligomers (as the sole carbon source) led to less than 1 g/L of ABE production in autohydrolysates at 150 °C. Co-fermentation of these hydrolysates with sorghum grain starch was investigated as a solution and it significantly increased the ABE production up to 8.3 g/L. Furthermore, the synergistic effect of co-fermentation was investigated where 35 % improvement in ABE production was detected. Accordingly, xylose utilization increased from 45 % to 80 %.