Prefermentation improves ethanol yield in separate hydrolysis and cofermentation of steam-pretreated wheat straw

Agricultural residues, such as wheat straw, are feasible substrates for ethanol fermentation provided that pentoses and hexoses can be converted efficiently. Separate hydrolysis and cofermentation (SHCF) constitute a framework for improvement of conversion efficiency, because it permits independent optimization of the enzymatic hydrolysis and cofermentation steps. A drawback is that the high glucose concentrations present in SHCF repress xylose utilization and constrain ethanol yields. To improve xylose utilization the xylose-rich hydrolyzate liquor was separated from glucose-rich solids and the phases were cofermented sequentially. Prefermentation of the xylose-rich hydrolyzate liquor followed by fed-batch cofermentation of glucose-rich prehydrolyzed solids enabled sequential targeting of xylose and glucose conversion. The aim was to improve the xylose conversion by lowering the glucose repression of the xylose uptake. Various prefermentation configurations and feed patterns for prehydrolyzed solids were examined. Prefermentation increased ethanol yields overall, and fed-batch prefermentation reduced xylitol production. The best results were obtained by balancing promotion of efficient xylose conversion with maintained yeast viability. Fed-batch prefermentation and a single addition of prehydrolyzed solids, elicited an ethanol yield of 0.423 g·g−1 and a xylitol yield of 0.036 g·g−1.