Integration of Sucrose Extraction, Physical Fractionation, and Pretreatment of Energy Sorghum for High-Titer Mixed Sugar Production

Sorghum (Sorghum bicolor L. Moench) offers substantial potential as a feedstock for sugar-derived biofuels or biochemical production from both extractable sugars and cell wall polysaccharides.  In this work, two “energy” sorghum lines (Della and TX08001) exhibiting a combination of high biomass productivity and high sugar accumulation were evaluated for their potential for integration into an alkaline pretreatment scheme. This includes determination and comparison of cell wall composition of extractable sucrose, fructose, glucose, starch, and pectic polysaccharides as well as tissue-specific (pith parenchyma, inner rind/vascular bundles, and outer rind/epidermis as a function of internode) composition and the response of these fractions to enzymatic hydrolysis following either no pretreatment or pretreatment with mild alkali or liquid hot water. Additionally, a novel approach was investigated that may be able to integrate with current diffuser-type extraction systems for sugar extraction.  This includes combining counter-current sucrose extraction with mild alkaline pretreatment which yields both a clean, pretreated bagasse and a high-concentration mixed sugar stream.  Following hydrolysis, these combined hydrolysates derived from cellulosic sugars and extractable sugars were found to be fermentable to high ethanol titers (>5%) without detoxification using a Saccharomyces cerevisiae strain metabolically engineered and evolved to ferment xylose.