Sequential fermentation for enhanced volumetric productivity of bioethanol from mixed sugars

Abstract

Most native strains efficiently ferment individual preferential sugars but not the mixed sugars to bioethanol. Therefore, a two-stage sequential fermentation was carried out in a single bioreactor vessel to convert mixed sugars, a mixture of synthetic glucose and xylose, into bioethanol using Z. mobilis and P. stipitis at stage-1 (glucose fermentation) and stage-2 (xylose fermentation), respectively. A total of 160 g/L (100 g/L glucose and 60 g/L of xylose) was found to be the optimum for mixed sugar fermentation. In the sequential fermentation, at stage-1, an ethanol titer of 50.25 g/L with a yield of 0.50 g/g was achieved with no residual glucose. In stage-2, no significant conversion of xylose was observed due to the presence of live Z. mobilis cells; however, removing Z. mobilis cells in stage-2, ethanol productivity resumed, and overall ethanol titer and yield were found to be 56.77 g/L and 0.36 g/g, respectively. Further, it was observed when residual ethanol at stage-2 was removed along with complete removal of Z. mobilis cells, overall ethanol titer and productivity improves. On 80% removal of residual ethanol, overall ethanol titer, ethanol yield, and volumetric productivity were found to be 79.03 g/L, 0.50 g/g, and 1.09 g/L/h, respectively, and the same upon complete removal of residual ethanol was found to be to 79.93 g/L, 0.50 g/g, and 1.82 g/L/h, respectively. These ethanol titer and productivity values of mixed sugar sequential fermentation were identical to separate glucose and xylose batch fermentation.