Continuous fermentation of clarified corn stover hydrolysate for the production of lactic acid at high yield and productivity

Significant amount of research has been done on the production of lactic acid from lignocellulosic biomass. However, most of these studies have been restricted to production using pure cultures that require expensive reactor systems operating with high sterility and clean sugar feedstocks translating into high overall processing costs. We produced a stable lactic acid producing culture through extended non-sterile cultivation of a strain of Bacillus coagulans (strain AD) isolated in our laboratory from a thermophilic biogas reactor. The culture operated with complete and consistent production of lactic acid as the only fermentation product in a similar manner to that of the pure culture. In the present study, we assessed the effect of pH and residence times on the continuous fermentation of clarified corn stover hydrolysate for lactic acid production using our stable culture. A continuous fermenter was set up at 50 degrees C with pH conditions ranging from 5.5 to 6.0 and residence times ranging from 6 to 96 h. Maximum lactic acid productivity of 3.69 g/L/h was found at pH 6.0 and residence time of 6 h with a lactic acid yield of 0.95 g/g biomass sugars. It was found that the productivity increased with decrease in residence time but there was no significant effect of pH or residence time on the overall lactic acid yield. Maximum lactic acid yield was found to be 1.086 g/g biomass sugars at pH 6.0 and a residence time of 24 h showing that the culture consumed sugars other than the conventional C5 (xylose) and C6 (glucose) that was used for the yield calculation. Our study shows the potential for high-rate production of lactic acid with low operational costs. (C) 2016 Elsevier B.V. All rights reserved.