Studies Steadily Advance Cellulosic Ethanol Prospects

By Ann Perry
October 14, 2014

The potential for producing cost-effective cellulosic ethanol that uses plentiful and sustainable cellulosic plant biomass continues to grow, thanks to research at the U.S. Department of Agriculture (USDA). USDA Agricultural Research Service (ARS) scientists at the Bioenergy Research Unit in Peoria, Illinois, have recently completed studies on multiple approaches that could help streamline cellulosic ethanol production. ARS is USDA's chief intramural scientific research agency, and this work supports USDA's priority of finding new sources for producing bioenergy.

In one study, a team led by ARS chemical engineer Bruce Dien looked at using switchgrass, a perennial grass native to the prairie, for ethanol production. The team concluded that biomass producers could optimize cellulosic ethanol production by planting Kanlow variety—a lowland ecotype—and harvesting at either mid-season or post frost. Results from this study were published in Environmental Technology in 2013.

ARS chemist Michael Bowman led another study of switchgrass xylans, which is challenging to convert to sugars with enzymes because of its complex chemical structure. Bowman determined that structural features of xylan remained the same as the plant matures, even though the amount of xylan changed with maturity. This is good news for biorefiners, because it suggests that they can use the same biomass hydrolyzing enzymes to break down xylans in all switchgrass biomass, no matter when the crop is harvested. Results from this study were published in Metabolites in 2012.

ARS molecular biologist Ronald Hector led work on the microorganisms needed to ferment xylose—molecules that make up xylans—into ethanol. Distiller's yeast used by corn ethanol producers does not ferment xylose. An enzyme called D-xylose isomerase, or XI, catalyzes the missing metabolic step for fermentation of xylose to ethanol. Hector's team isolated four novel XI genes encoding the enzyme from rumen and intestinal bacteria and expressed them in distiller's type yeast strains, conferring the ability for them to ferment xylose.

Then the scientists took the most promising yeast strain from this first round of trials and improved its growth and fermenting capacities through further adaptations. The result was a yeast strain that grew almost four times faster than other strains that contained XI enzymes and one that could produce ethanol at significantly greater yields than other yeasts engineered to ferment xylose to ethanol. The scientists published their findings in Biotechnology for Biofuels in 2013.

Read more about this work in the October 2014 issue of Agricultural Research magazine.