Dilute Acid Pretreatment of Oven-dried Switchgrass Germplasms for Bioethanol Production

Bioethanol production potential of three oven-dried switchgrass germplasms (St6-1, St6-3E,and St6-3F) containing 26.65-29.28% glucan, 17.92-19.37% xylan, and 17.74-19.23% lignin (dry matter basis) was investigated. Evaluation of the effect of three acid concentrations (0.5, 1.0, and 1.5% w/v) and residence times (30, 45. and 60 min) on composition of all germplasms indicated significant hemicellulose solublization relying greatly on pretreatment intensity. No apparent delignification was observed during pretreatment. Pretreated samples with the least lignin content or greatest hemicellulose solubilization within each germplasm were selected for hydrolysis and fermentation. Enzymatic hydrolysis at cellulase activities of 0, 15. and 30 FPU (filter paper units)/g dry biomass indicated that addition of cellulase significantly improved glucan hydrolysis (P < 0.05) but supplementation with xylanase did not statistically improve hydrolysis (P > 0.05). Glucan-to-glucose conversion was enhanced by acid pretreatments, especially those resulting in greater hemicellulose solubilization. The greatest glucan conversion of 91.8% was obtained from 60 min/1.5% acid pretreated St6-3E switchgrass hydrolyzed at 30 FPU cellulase/g dry biomass supplemented with xylanase. Fermentation of hydrolyzates by Saccharomyces cerevisiae (ATCC 24859) resulted in nearly complete utilization of glucose. The highest ethanol yield of 0.082 g ethanol/g raw St6-3E switchgrass corresponded with 53.5% of theoretical yield based on glucose fermentation. These results demonstrate that the new switchgrass germplasms are potential energy crops for bioethanol production through appropriate processing.