Researchers find plant cells key to cellulosic ethanol

Could the tiny pores within plant cells hold promise for less reliance on grain for ethanol production? Research at Purdue University seems to point in that direction.

Researchers there have discovered that particles from cornstalks undergo previously unknown structural changes when processed to produce ethanol, an insight they say will help establish a viable method for large-scale production of ethanol from plant matter.

Their research demonstrates that pre-treating corn plant tissue with hot water, an already-accepted practice that increases ethanol yields three to four 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," says 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."

Enzymes attack enlarged pores

Mosier said that research applies to cellulosic ethanol, or ethanol produced from cellulose, which is a key component of a 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 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 many ways, said Michael Ladisch, the study's co-author and a professor of agricultural and biological engineering. It would be less limiting on U.S. ethanol production which is almost entirely reliant on grain that is already in demand for livestock feeding and other purposes.

"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 may also become more efficient, relying on a broader supply of plants that can be grown more economically than traditional row crops. Research has already begun to yield new types of energy crops with larger pools of usable cellulose. The problem has been in easily freeing the cellulose from the cell wall's complex, rigid structure, which means that to date, cellulosic ethanol production has not been commercially viable. But Ladisch says this latest work could change that.

Economical production

"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, as opposed to plant matter that died eons ago and over time created our current supply of carbon fuels, namely coal and oil.

Mosier and Ladisch are currently at work on a variety of projects related to ethanol, including the ability to move beyond laboratory operations for their current findings. 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.