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

Abstract

The production of second generation (2G) ethanol remains an interesting proposition for the implementation of sustainable and net carbon-neutral energy systems. 2G makes use of renewable lignocellulosic feedstocks, generating fermentable sugars that are converted to ethanol or other bio-based products. To be economically viable, 2G biore neries must make use of all processing streams, including the less desirable C5 sugar stream. In this work, a strategy of sequential acid and alkaline pretreatment of the lignocellulosic feedstock switchgrass for improvement of fermentable sugar yield, and the subsequent utilization of wild yeasts for co-fermentation of its C5-C6 sugar streams are presented. Hemicellulose-enriched hydrolysates, obtained by dilute acid pretreatment of switchgrass, were fermented by a newly-isolated wild Scheffersomyces parashehatae strain-UFMG-HM-60.1b; corresponding ethanol yield (Y PS ) and volumetric productivity (Q P ) were 0.19 g/g and 0.16 g/L h, respectively. Afterwards, the remaining switchgrass cellulignin fraction was subjected to optimized alkaline deligni cation at 152 ºC for 30 min. The deligni ed solid fraction was subjected to contiguous enzymatic sacchari cation and fermentation, releasing a C6 sugar stream in which Saccharomyces cerevisiae 174 strain displayed a productivity of 0.46 g/g (Y PS ) and 0.70 g/L h (Q P ), whereas the S. parashehatae UFMG-HM-60.1b 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 S. parashehatae strain provided higher yield (94%) than the generic S. cerevisiae (90%). Henceforth, our integrated pretreatment and co-fermentation process provides a pathway for maximum utilization of the switchgrass carbohydrates for 2G ethanol production.