Cellulosic ethanol research in Peoria

October 28, 2014 – At the Agricultural Research Service’s
Bioenergy Research Unit in Peoria, Illinois, field work and bench
investigations keep ARS scientists on the scientific front lines of converting
biomass into cellulosic ethanol.

For instance, one recent research focus has been on
determining how switchgrass plant maturity at harvest affects ethanol yields.
Chemical engineer Bruce Dien led a study that evaluated samples of two
different switchgrass varieties that were harvested at three different points
in plant development and then pretreated with diluted ammonia. This approach is
similar to a treatment used sometimes for enhancing forage quality.

Dien’s team observed that even though plant glucose and
ethanol conversion efficiencies decreased as the plants matured, overall
ethanol yields were relatively consistent—between 176 and 202 liters per metric
tonne (42 and 48 gallons per ton) of biomass. After evaluating the different
yields obtained from the two varieties, the scientists concluded that biomass
producers could optimize ethanol production from their crops by planting the
variety Kanlow—a lowland switchgrass type—and harvesting either at mid-season or
after a frost. Results from this study were published in Environmental
Technology in 2013.

Chemist Michael Bowman led another study that focused
specifically on switchgrass xylans. Xylans are polymer chains composed
primarily of the sugar xylose. Bowman studied xylan levels at three different
stages in switchgrass development to see whether xylan structures change as the
plant matures.

Bowman determined that structural features of xylan remained
the same throughout different stages of maturity, even though the amount of
xylan differed from one stage to another. This is good news for biorefiners
because it suggests that they can use the same enzyme mix to break down xylans
for all switchgrass biomass, no matter when the crop is harvested. Results from
this study were published in Metabolites in 2012.

Molecular biologist Ron Hector, meanwhile, led work on the microorganisms
needed to ferment xylose into ethanol. Xylose is more difficult to convert to
ethanol, compared to glucose. Scientists already knew that an enzyme called
“D-xylose isomerase,” or XI, is one of several enzymes required to convert
xylose into ethanol. But, to date, it has been difficult to harness XI’s
conversion potential because of the difficulty of expressing XI in yeast
strains and other technical issues.

However, Hector and his colleagues isolated the XI enzyme
from several different rumen and intestinal bacteria and used them to engineer
yeast strains that were able to ferment xylose into ethanol. Then they took the
most promising yeast strain from this first round of trials—which contained the
XI enzyme from the rumen bacterium Prevotella ruminicola—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 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, and a U.S. patent was recently issued for the XI enzyme
described in this article.