Submitted to: Energy and Fuels
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/29/2009
Publication Date: 6/15/2009
Citation: Yang, Y., Sharma-Shivappa, R., Burns, J.C., Cheng, J. 2009. Dilute Acid Pretreatment of Oven-dried Switchgrass Germplasms for Bioethanol Production. Energy and Fuels. 23:3759-3766. Interpretive Summary: The need exists to reduce the cell wall matrix of perennial grasses, as sources of biomass, into monomer sugars which can be readily converted into biofuels (bioethanol). This study compared the two recently released lowland switchgrass cultivars, ‘BoMaster’ and ‘Performer’, and an improved germplasm (ST6-1) selected for yield, for their potential as bioethanol sources. Plant tissue was first pretreated with a range of dilute sulfuric acid. The breakdown of the cell wall, or hydrolysis, was continued using the three enzymes cellulase, cellobiase, and xylanase. The resulting monomers can subsequently be fermented to ethanol. Pretreatment with mild (1.5%) sulfuric acid solubilized the hemicellulose fraction, but required residence time of 60 minutes. All glucose produced was essentially converted to ethanol. The ethanol yield ranged from 0.035 to 0.082 g/g of dry matter and was germplasm dependent. Both BoMaster and Performer have potential as energy crops for producing bioethanol. The conversion process involves that was effective includes pretreatment, hydrolysis, and subsequent fermentation and warrants the investigation of other pretreatment methods and the consideration of scale-up and related economic analyses.
Technical Abstract: Bioethanol production potential of three oven-dried switchgrass germplasms (St6-1, St6- 3E and St6-3F) containing 26.65 to 29.28% glucan, 17.92 to 19.37% xylan, and 17.74 to 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 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/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.