Abstract
Efficient conversion of both glucose and xylose in lignocellulosic biomass is necessary to make second-generation bioethanol from agricultural residues competitive with first-generation bioethanol and gasoline. Simultaneous saccharification and co-fermentation (SSCF) is a promising strategy for obtaining high ethanol yields. However, with this method, the xylose-fermenting capacity and viability of yeast tend to decline over time and restrict the xylose utilization. In this study, we examined the ethanol production from steam-pretreated wheat straw using an established SSCF strategy with substrate and enzyme feeding that was previously applied to steam-pretreated corn cobs. Based on our findings, we propose an alternative SSCF strategy to sustain the xylose-fermenting capacity and improve the ethanol yield. The xylose-rich hydrolyzate liquor was separated from the glucose-rich solids, and phases were co-fermented sequentially. By prefermentation of the hydrolyzate liquor followed fed-batch SSCF, xylose, and glucose conversion could be targeted in succession. Because the xylose-fermenting capacity declines over time, while glucose is still converted, it was advantageous to target xylose conversion upfront. With our strategy, an overall ethanol yield of 84% of the theoretical maximum based on both xylose and glucose was reached for a slurry with higher inhibitor concentrations, versus 92% for a slurry with lower inhibitor concentrations. Xylose utilization exceeded 90% after SSCF for both slurries. Sequential targeting of xylose and glucose conversion sustained xylose fermentation and improved xylose utilization and ethanol yield compared with fed-batch SSCF of whole slurry.
Highlights
The production of fuel ethanol from lignocellulosic raw materials is associated with technological and economic hurdles that must be addressed to make lignocellulosic fuel ethanol competitive with first-generation fuel ethanol and gasoline
Based on the findings for the fed-batch Simultaneous saccharification and co-fermentation (SSCF) method, an alternative strategy was proposed—sequential co-fermentation with prefermentation of the hydrolyzate liquor followed by fedbatch SSCF—to improve the overall ethanol yield by increasing the consumption and conversion of xylose
The performance of prefermentation followed by fed-batch SSCF was compared with that of prefermentation of the hydrolyzate liquor coupled to SHCF [33], which was performed using the same pretreated wheat straw
Summary
The production of fuel ethanol from lignocellulosic raw materials is associated with technological and economic hurdles that must be addressed to make lignocellulosic fuel ethanol competitive with first-generation fuel ethanol and gasoline. High ethanol yield and final ethanol concentration are key factors for improved profitability [2], and the major technological challenges to achieve them are linked to the biology and chemistry of the processing steps in using the raw materials efficiently. The overall performance of the fermentation step depends largely on the tolerance of the fermenting microorganism to inhibitors and its ability to efficiently convert a variety of substrates to ethanol. Xylose is still generally fermented by S. cerevisiae to ethanol at lower rates [15] and lower yields than glucose [15, 16] This has been attributed to capacity limitations in the pentose phosphate shunt [9] and mismatched co-factor dependency during xylose catabolism in engineered XR/XDHstrains [17]. The co-factor imbalance between NAD(P)H-consuming XR and NADH-producing XDH reactions effects the production of xylitol [18, 19]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
