Conversion of high-solids hydrothermally pretreated bioenergy sorghum to lipids and ethanol using yeast cultures

Authors: CHENG, MING-HSUN - University Of Illinois; Dien, Bruce; JIN, YONG-SU - University Of Illinois; Thompson, Stephanie; SHIN, JONGHYEOK - University Of Illinois; Slininger, Patricia - Pat; Qureshi, Nasib; SINGH, VIJAY - University Of Illinois

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2021
Publication Date: 6/14/2021
Citation: Cheng, M., Dien, B.S., Jin, Y.S., Thompson, S., Shin, J., Slininger, P.J., Qureshi, N., Singh, V. 2021. Conversion of high-solids hydrothermally pretreated bioenergy sorghum to lipids and ethanol using yeast cultures. ACS Sustainable Chemistry & Engineering. 9(25):8515–8525. https://doi.org/10.1021/acssuschemeng.1c01629.
DOI: https://doi.org/10.1021/acssuschemeng.1c01629

Interpretive Summary: This study compares conversion of sugars extracted from biomass sorghum into either oil or ethanol. Biomass sorghum holds promise as a crop for production of sugars because of its high yield, ability to grow on marginal lands, and low inputs. Unlike in a traditional sugar or grain crop, the sugars are extracted from the stalks and leaves of the plant. In this case, the biomass sorghum was processed at an industrial testing facility located at the University of Illinois. The end sugar syrup contained over 20% sugars by weight. In this study, we tested new yeast based processes to convert these sugars into either ethanol or oil. The end result was that both processes worked well. Notably, the ethanol concentration (48 g/l), yield (67% of the maximum), and rate (0.80 g/l/h) are of commercial interest. U.S. Department of Energy calculates that establishment of a second generation biofuel industry would create 1.5 million jobs, supply up to 50 billion gallons of biofuels, and reduce annual carbon dioxide emissions by 550 million tons. Stakeholders who would directly benefit from this research are farmers who own marginal farmland and agricultural based refiners interesting in sourcing new feedstocks.

Technical Abstract: Glucose and xylose are the major sugars present in cellulosic hydrolysates. The cellulosic sugars can be used for the production of platform chemicals. In this study, productions of lipid and ethanol by yeasts were compared for concentrated bioenergy sorghum syrup. Bioenergy sorghum was hydrothermally pretreated at 50% w/w solids in a continuous industrial reactor and sequentially mechanically refined using a burr mill to improve biomass accessibility for hydrolysis. Fed-batch enzymatic hydrolysis was conducted with 50% w/v solids loading and cellulase cocktail (50 FPU/g biomass) to achieve 230 g/L sugar concentration. Various strains of Rhodosporidium toruloides were evaluated for converting sugars into lipids, and strain Y-6987 had the highest lipid titer (9.2 g/L). The lipid titer was improved to 19.0 g/L by implementing a two-stage culture scheme, where the first stage was optimized for yeast growth and the second for lipid production. For ethanol production, the engineered Saccharomyces cerevisiae SR8'ADH6 was utilized to coferment glucose and xylose. Ethanol fermentation was optimized for media nutrients (YP, YNB/urea, and urea), cellulosic sugar concentration, and sulfite conditioning to maximize the ethanol concentration from sorghum syrups. Fermentation of 70% v/v concentrated hydrolysate conditioned with sulfite produces 50.1 g/L ethanol from 141 g/L of sugars.