Here we present the S. cerevisiaestrain IBB10B05 which was enabled to xylose fermentation by a combination of metabolic engineering (introduction of the xylose reductase and xylitol dehydrogenase) and laboratory evolution. A crucial novelty is the engineered xylose reductase, whose cofactor preference was altered, to create a largely redox neutral xylose assimilation pathway.
Two industrially significant, un-detoxified lignocellulosic waste streams were used as substrates. Firstly, spent sulfite liquor (SSL), a by-product of the paper industry. Secondly, wheat straw hydrolyzate (WS-H) generated by enzymatic hydrolysis. Enzymes were produced in-house by the optimized Trichoderma reesei strain SVG17, allowing a complete mass balance analysis of the separate hydrolysis and co-fermentation (SHCF) process.
Utilizing IBB10B05, it was possible to co-ferment glucose and xylose in WS-H, SSL and a combination thereof with high ethanol yields (~0.45 gethanol/gsugars) and high sugar consumption rates (qxylose ≤0.7 g/gCDW/h; qglucose ~2.9 g/gCDW/h). In total, 0.12 L ethanol per kg dry mass wheat straw was produced by SHCF. Despite high substrate loadings (15% dry mass), high conversion efficiencies were attained, making IBB10B05 a robust strain for lignocellulose-to-bioethanol processes.