West Lafayette, Indiana
April 26, 2007
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A magnified image of a cornstalk particle
shows the many tiny pores that pretreatment - a
phase of the ethanol production process - opens
up. These pores create more surface area for
subsequent reactions to take place and give
enzymes better access to cellulose, the source
for cellulosic ethanol. Researchers said this
information could help in establishing an
economic method for industrial production of
cellulosic ethanol.
(Purdue University photo/Meijuan Zeng) |
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Tiny pores within plant cells may
hold promise for green fuels.
Researchers have discovered that
particles from cornstalks undergo previously unknown structural
changes when processed to produce ethanol, an insight they said
will help establish a viable method for large-scale production
of ethanol from plant matter.
Their research demonstrates that pretreating corn plant tissue
with hot water - an accepted practice that increases ethanol
yields 3 to 4 times - works by exposing minute pores of the
plant's cell walls, thus increasing surface area for additional
reactions that help break down the cell wall.
"This brings together the tools
that link the processing technology to the plant tissue
physiology," said Nathan Mosier, an assistant professor of
agricultural and biological engineering at
Purdue University. "It helps
us understand, on a fundamental level, what the processing is
doing and how we can improve it."
Mosier said that research, further described in a study
published Thursday (April 26) in the journal
Biotechnology and Bioengineering, applies to cellulosic
ethanol, or ethanol produced from cellulose, which is a key
component of plant's cell walls.
Using high-resolution imaging and chemical analyses, the
researchers determined that pretreatment opens reactive areas
within the cells of the corn stover - another name for
postharvest corn remnants, like leaves and stalks - that were
previously overlooked. In the next step of processing, these
enlarged pores are more easily attacked by enzymes that convert
cellulose into glucose, which is in turn fermented into ethanol
by yeast, Mosier said.
Producing ethanol from cellulose would be advantageous over
existing industrial processes in several ways, said Michael
Ladisch, the study's co-author and a professor of agricultural
and biological engineering.
Currently, almost all industrial ethanol derives from either
starch found in corn grain or from sugar cane. This limits U.S.
ethanol production, which is almost entirely from corn grain, to
a grain supply that already is in demand for a variety of uses.
"Cellulosic ethanol would allow industry to expand beyond the
limits brought about by corn's other uses, like sweetener
production, animal feed and grain exports," Ladisch said.
For these reasons, he said, cellulosic ethanol also would likely
put less pressure on food prices.
The new process has the potential to become more efficient, with
a larger potential supply of plants that can be grown more
economically than traditional row crops. What's more, research
in plant science has yielded ? and will likely continue to yield
- new types of energy crops with larger pools of usable
cellulose.
However, the catch is that cellulose is not easily freed from
the cell wall's complex, rigid structure, and, to date,
cellulosic ethanol has not been commercially viable. Ladisch
said this study should help change that.
"This study will help us translate science from the lab to an
industrial setting and will help produce cellulosic ethanol
economically," he said.
Plant's cell walls are rigid structures made up of a variety of
polymers, including cellulose and hemicellulose, which can be
converted into sugars that are then made into ethanol. However,
cellulose and hemicellulose are held in place by a variety of
compounds like lignin, a strong cellular glue that resists
treatment and protects cellulose from being broken down. Mosier
and Ladisch found that after pretreatment opens corn's tiny
pores, enzymes not only removed more cellulose and hemicellulose
from the cell wall, but also removed it at a faster rate.
Cellulosic ethanol comes from plant biomass, another term for
the tissue of recently dead plants, or plants that grow and die
annually. This distinguishes the current supply of plant biomass
- to be used for cellulosic ethanol - from plant matter that
died eons ago and through time created our current supply of
carbon fuels, namely coal and oil. This is why plant biomass is
often labeled as renewable, since it can be grown each year, and
why petroleum is referred to as non-renewable ? once it's gone,
it cannot be replaced.
Mosier and Ladisch are currently at work on a variety of
projects related to ethanol production, such as how to best
scale up from laboratory operations.
They have conducted research in this area for years. The hot
liquid water pretreatment process used in this study was
originally developed in the Laboratory of Renewable Resources
Engineering at Purdue, which Ladisch directs.
Ladisch's graduate student, Meijuan Zeng, was the paper's first
author.
This study was funded by the U.S. Department of Energy, U.S.
Department of Agriculture and Purdue Agriculture.
Writer: Douglas M Main
Microscopic Examination of Changes of Plant Cell Structure in
Corn Stover Due to Hot Water Pretreatment and Enzymatic
Hydrolysis
Meijuan Zeng, Nathan S. Mosier, Chia-Ping Huang, Debra M.
Sherman, Michael R. Ladisch
ABSTRACT
Particle size associated with
accessible surface area has a significant impact on the
saccharification of plant cell walls by cellulolytic enzymes.
Small particle sizes of untreated cellulosic substrate are more
readily hydrolyzed than large ones because of higher specific
surface area. Pretreatment enlarges accessible and susceptible
surface area leading to enhanced cellulose hydrolysis. These
hypotheses were tested using ground corn stover in the size
ranges of 425-710 and 53-75 micrometers. Ultrastructural changes
in these particles were imaged after treatment with cellulolytic
enzymes before and after liquid hot water pretreatment. The
smaller 53-75 micrometer corn stover particles are 1.5X more
susceptible to hydrolysis than 425-710 micrometer corn stover
particles. This difference between the two particle size ranges
is eliminated when the stover is pretreated with liquid hot
water pretreatment at 190 degrees C for 15 min, at pH between
4.3 and 6.2. This pretreatment causes ultrastructural changes
and formation of micron-sized pores that make the cellulose more
accessible to hydrolytic enzymes. |
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