Tuesday, May 3, 2011
Previously, our laboratory conducted the first compositional analyses of water-soluble materials in representative corn stover, switchgrass and sorghum biomass. One finding of particular interest was that water-soluble, fermentable sugars (primarily glucose, fructose, and sucrose) represented up to 17% and 13% of the dry weight of corn stover and switchgrass materials, and up to 29% of the dry weight of sorghum. These sugars are not currently quantified in typical analyses of potential biofuel feedstocks. In the current study, the effect of extractives on ethanol yield resulting from simultaneous saccharification and fermentation (SSF) was evaluated using two representative sorghum materials containing different amounts of water-soluble carbohydrates. Three bench-scale conversion paradigms were evaluated for each feedstock in a side-by-side comparison. First, water-soluble materials were removed via accelerated solvent extraction. The residual feedstock was subsequently pretreated with dilute acid and both it and the initial aqueous extract were independently fermented by E. coli. In a second experiment, the extract and pretreated feedstock were recombined prior to SSF. Ethanol yields were compared with those resulting from SSF of a non-extracted, pretreated feedstock. In both tests, removal of water-soluble materials prior to pretreatment resulted in improvements (ranging from 70-130%) of mean ethanol yield, relative to the yield observed for SSF of the non-extracted feedstock. Similar levels of improvement were observed when water-soluble materials were fermented independently or recombined with the pretreated feedstock. These results suggest water-soluble sugars contribute a net positive to overall ethanol yield when extractives are isolated from the feedstock prior to chemical pretreatment.