Microbes, Too, Can Play a
Making biofuels, such as ethanol, economically from the whole crop instead
of just the grain is the long-range goal of scientists in the Fermentation
Biochemistry Research Unit at ARS' National Center for Agricultural Utilization
Research, in Peoria, Illinois.
"But our starting point is researching fermentation of fiber in just
the corn kernel," says ARS microbiologist Rodney J. Bothast, who leads the
project. Currently, the kernel fiber is separated out and used as inexpensive
cattle feed that is valued for protein, not fiber. If technology were developed
to break down the different polymers in kernel fiber to simple sugars, about 10
percent more ethanol could be produced from each bushel of wet-milled corn.
Bothast collaborates with scientists at ERRC and in the Department of Wood
Science at the University of British Columbia, Vancouver, in research on the
physical and chemical pretreatment of fiber. Pretreatment frees the cellulose
from hemicellulose, starch, and lignin components.
Cellulose fragments are more readily converted into sugars that can be
fermented to make ethanol. The lignin component is not fermented but can be
burned to produce energy.
"So far, the most effective way we've found to break down the fiber is
to pretreat it with mild acid and then with alkaline hydrogen peroxide,"
The pretreated fiber contains sugars, mainly arabinose and xylose and some
glucose. Normally, ethanol-producing microbes eat the glucose first, leaving
little appetite for the other sugars. Nancy N. Nichols, microbiologist, and
Bruce S. Dien, chemical engineer, have developed genetically engineered
microorganisms that consume the sugars at nearly equal rates.
These researchers are collaborating with others at the University of British
Columbia, Purdue University, and Williams Energy Service, in Pekin,
Illinoisthe second largest ethanol producer in the countryto test
these new microbes on kernel fiber converted to sugars by industrial processes.
As the scientists seek ways to increase ethanol production efficiency,
they're mindful of coproducts that might help make ethanol crops more
economically successful. For example, other microbes developed by Nichols and
Dien convert the sugars derived from kernel fiber into lactic acid. Biobased
companies use lactic acid to produce solvents and biodegradable plastics.
Badal Saha, an ARS chemist, and microbiologist Timothy Leathers have
developed yeasts that convert the xylose derived from corn fiber into xylitol,
a low-calorie sweetener. Xylitol, which has a minty-cool taste, is used in some
mints and gum and sells for about $3 per pound. It's made from birch wood by an
expensive, energy-intensive process.
Saha and Leathers have also discovered fungi that produce enzymes especially
well suited for converting corn fiber into sugars. Use of enzymes decreases the
amount of acid needed to convert corn fiber to sugars, and that makes ethanol
an even more environmentally friendly fuel.
And Out on the Range . . .
Instead of making ethanol from the sugars and starches in plants, Ken Vogel,
with ARS in Lincoln, Nebraska, is experimenting with using cellulose and
hemicellulose from switchgrass as another source of ethanol. Vogel's hope is
that farmers might be able to grow this native prairie grass on highly erodible
soilsincluding those set aside for USDA's Conservation Reserve
Programharvest the grass periodically for ethanol production, and reap
conservation benefits, such as reduced soil erosion and enhanced carbon
Vogel and colleagues are breeding new switchgrasses for biofuel use. They're
genetically improving the grass for conversion to ethanol and conducting
on-farm trials to obtain economic information on production costs. Ron Follett,
at Fort Collins, Colorado, is working with Vogel and ARS scientists at Mandan,
North Dakota, to study carbon storage on lands grown for biofuel crops.
Plant geneticist JoAnn Lamb and colleagues at the ARS Plant Science Research
Unit in St. Paul, Minnesota, are looking at alfalfa as another cellulose source
for producing ethanol. They received $288,000 from ARS' new $2.4 million in
funding for developing bioenergy crops.
They are breeding a new alfalfa variety specifically to double as a
high-quality livestock feed and a bioenergy crop. They'll incorporate genes
from southern European varieties to give the plant a thicker, almost woody,
stem. This means more cellulose for ethanol production.
The humid East might prefer alfalfa to switchgrass as an ethanol source, but
switchgrass is ideally suited for the arid West, because it needs very little
rainfall to grow. Both alfalfa and switchgrass can also be burned to generate
There are obstacles to overcome when making ethanol from cellulose in plants
like switchgrass or alfalfa, such as finding ways to convert the complex sugars
in cellulose into simple ones that can be fermented to produce ethanol.
Facilities to do this conversion will have to be built. Equipment for this
purpose could be tested at the new Illinois pilot ethanol plant when it is up
and running, as could equipment for the new continuous fermentation stripping
Comis, Agricultural Research Service Information Staff. Ben
Hardin and Kathryn Barry Stelljes, both formerly with ARS, also
contributed to this story.
This research is part of Quality and Utilization of Agricultural Products
(#306) and Bioenergy and Energy Alternatives (#307), two ARS National Programs
described on the World Wide Web at http://www.nps.ars.usda.gov.
To reach scientists mentioned in this article, contact Don Comis,
USDA-ARS Information Staff, 5601
Sunnyside Ave., Beltsville, MD 20705-5129; phone (301) 504-1625, fax (301)
Biodiesel is a clean-burning alternative fuel that can be made from
materials such as vegetable oils, animal fats, and spent cooking greases.
Typically, biodiesel is prepared by the reaction of fat or oil with alcohol
under alkaline conditions. Soy-based biodiesel is the most commonly used form.
Ethanol is an alcohol-based fuel produced by fermenting sugars from
crop starches. Currently, 95 percent of ethanol is produced from corn kernels.
About 5 percent of U.S. ethanol is made from sugar- and starch-containing
materials other than corn. These include wheat, barley, and sorghum grains;
sugarcane; cheese whey; and wastes from paper mills, potato processing plants,
breweries, and beverage manufacturersor some combination of these
Originally, most ethanol was made through wet-milling, which means
the starch is separated from the corn germ and fiber and liquefied by cooking.
The liquefying creates sugars in a form that can be fermented with yeast to
produce ethanol and carbon dioxide. The ethanol is then removed from the
The number of ethanol plants has surged in the past few years, and
dry-milling is now the method used for over half of the ethanol
currently produced. In this process, kernels are ground to a fine powder, and
all of it is cooked to liquefy it, without removing the germ or fiber.
Different enzymes are added at different stages and temperatures as the mash
cools, producing ethanol and carbon dioxide.By