Conversion of switchgrass to ethanol using dilute ammonium hydroxide pretreatment: influence of ecotype and harvest maturity

Abstract

Switchgrass (Panicum virgatum L.) is a perennial C4 grass that is being developed as a bioenergy crop because it has high production yields and suitable agronomic traits. Five switchgrass biomass samples from upland and lowland switchgrass ecotypes harvested at different stages or maturity were used in this study. Switchgrass samples contained 317.0–385.0 g glucans/kg switchgrass dry basis (db) and 579.3–660.2 g total structural carbohydrates/kg switchgrass, db. Carbohydrate contents were greater for the upland ecotype versus lowland ecotype and increased with harvest maturity. Pretreatment of switchgrass with dilute ammonium hydroxide (8% w/w ammonium loading) at 170°C for 20 min was determined to be effective for preparing switchgrass for enzymatic conversion to monosaccharides; glucose recoveries were 66.9–90.5% and xylose recoveries 60.1–84.2% of maximum and decreased with increased maturity at harvest. Subsequently, pretreated switchgrass samples were converted to ethanol by simultaneous saccharification and fermentation using engineered xylose-fermenting Saccharomyces cerevisiae strain YRH400. Ethanol yields were 176.2–202.0 l/Mg of switchgrass (db) and followed a similar trend as observed for enzymatic sugar yields.

Keywords:

• switchgrass
• bioethanol
• simultaneous saccharification and fermentation
• pretreatment
• bioenergy crop

Acknowledgements

We thank Genencor, Inc. for the generous donation of the enzymes and Victoria Nguyen for excellent technical assistance.

Funding

This work was supported in part by Agriculture and Food Research Initiative Competitive from the USDA National Institute of Food and Agriculture [grant number 2011 68005-30411].

Notes

^†Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.