Adding a pretreatment step would allow producers to get more ethanol from switchgrass harvested in the fall, according to a Purdue University study.
Michael Ladisch, a distinguished professor of agricultural and biological engineering, and Youngmi Kim, a research scientist, compared switchgrass based on growing location, harvest time and whether it was given a pretreatment step. They found that location wasn't important, but the other two factors could significantly increase the amount of ethanol obtained from the feedstock.
"Switchgrass harvested in the spring had more cellulose, but also more lignin," said Kim, whose findings were published in the early online version of the journal Bioresource Technology. "You do not get the advantage of the increased cellulose content because it's more difficult to extract those sugars because of the lignin."
Lignin, a rigid substance found in plant cell walls, is one of the most significant problems with cellulosic ethanol production. Besides the harvest time, a pretreatment step - cooking switchgrass in hot water under pressure for about 10 minutes - would also help work around lignin.
Before pretreatment, Kim said about 10 percent of cellulose was converted to glucose, the yeast-fermentable sugar that produces ethanol. After pretreatment, that number jumped to as much as 90 percent. The pretreatment dissolves hemicellulose, which bonds cellulose and lignin in the plant. Once it is gone, there is more access to the sugars contained in the cellulose.
"There is more surface area for the enzymes to digest cellulose," Kim said.
Ladisch said advancements in techniques to work around lignin could make spring switchgrass more attractive. But he said that fall switchgrass given a pretreatment and fermentation with special yeast shows potential to give as much as 800-1,000 gallons of ethanol per acre per year, compared with 150-250 gallons per year without pretreatment. Ladisch said corn ethanol from grain produces about 500-600 gallons per acre per year.
"This shows that we can improve the processes and increase the amount of ethanol we get from switchgrass," Ladisch said.
Ladisch is chief technology officer at Mascoma, a renewable fuels company based in New Hampshire. He received no funding from the company for this research, which was supported by the U.S. Department of Energy.
The work is part of a concerted research effort on pretreatments by a consortium made up of the University of California Riverside, Auburn University, Texas A&M University, Michigan State University, Genencor and the National Renewable Energy Laboratory together with Purdue University.