Sustainable Second-Generation Ethanol Production from Switchgrass Biomass via Co-fermentation of Pentoses and Hexoses Using Novel Wild Yeasts

The production of second-generation (2G) ethanol remains an interesting proposition for the implementation of sustainable and net carbon–neutral energy systems. To be economically viable, 2G biorefineries must make use of all processing streams, including the less desirable pentose (C5) sugar stream. In this work, a strategy of sequential dilute acid and alkaline pretreatment of the lignocellulosic feedstock, switchgrass, was implemented for improving the fermentable sugar yield. The hemicellulose-enriched hydrolysate obtained after dilute acid pretreatment was fermented by a newly isolated wild Scheffersomyces parashehatae strain—UFMG-HM-60.1b; the corresponding ethanol yield (Y PS ) and volumetric productivity (Q P ) were 0.19 g/g and 0.16 g/L h, respectively. The remaining switchgrass cellulignin fraction was subjected to optimized alkaline delignification at 152 ºC for 30 min. Then, the delignified solid fraction was subjected to contiguous enzymatic saccharification and fermentation releasing a glucose (C6) sugar stream. The control yeast strain, Saccharomyces cerevisiae 174, displayed an ethanol Y PS of 0.46 g/g and Q P of 0.70 g/L h for the C6 sugar stream, whereas the above-mentioned wild strain presented Y PS and Q P of 0.29 g/g and 0.38 g/L h, respectively. Upon combining the conversion of hemicellulose (37%) and cellulose-derived sugars (57%), the wild S. parashehatae strain provided higher yield (94%) than the generic S. cerevisiae (90%). Henceforth, our sequential two-stage pretreatment and fermentation of C5 and C6 sugar streams provides a pathway for maximum utilization of switchgrass carbohydrates for 2G ethanol production.