Cellulose dreams: the search for new means and materials for making ethanol

Science News,  August 25, 2007  by Corinna Wu

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Alternative energy is hitting the headlines. Last year, former Vice President Al Gore scored a surprise hit with his climate-change documentary An Inconvenient Truth. Currently, drivers are steeling themselves against gasoline prices that could shoot well past $3 per gallon. The war in Iraq continues to draw attention to the United States' dependence on imported oil and has prompted calls for a shift toward domestic sources of fuel.

More and more, policy makers are touting a homegrown solution--literally--to the nation's energy and global warming problems: ethanol made from plants. Mixing ethanol into gasoline reduces overall greenhouse-gas emissions from vehicles because plants recycle carbon: The fuel that they yield produces carbon dioxide, just as fossil fuels do, but the plants consume carbon as they grow. And because ethanol molecules contain oxygen, their presence makes gasoline burn more completely, reducing carbon monoxide and other harmful tailpipe emissions.

Ethanol figures significantly in the Bush Administration's promotion of biofuel. Currently, the United States produces about 6 billion gallons of ethanol annually, mostly from corn. The President's Twenty in Ten initiative sets a goal to reduce gasoline usage by 20 percent in 10 years--in part by increasing the production of renewable fuels to 35 billion gallons per year by 2017.

But to reach that goal, corn alone won't do. Researchers are looking to trees, grasses, and waste organic matter as possible raw materials for ethanol production. The basic idea is to extract the cellulose locked up in plants' cell walls, break it down into its component sugars, and ferment those sugars into ethanol.

In February, the Department of Energy (DOE) announced that it would spend up to $385 million over the next 4 years to work with commercial partners on six ethanol pilot plants. Then in June, DOE granted $375 million to fund three new Bioenergy Research Centers to develop technology for cellulosic ethanol and other biofuels.

"I equate what we're doing to society saying, 'We're going to the moon, or 'We're going to sequence the human genome,'" says Tim Donohue, a bacteriologist at the University of Wisconsin-Madison. "To me, this is a critically grand scientific mission that we're just setting off on today."

BILLION-TON VISION People figured out long ago how to make alcohol from grains, and now a similar process is used to turn corn into ethanol for fuel. First, corn kernels are ground into a coarse flour and combined with water and the enzymes alphaamylase and glucoamylase, which convert starch into sugar.

After this mash is cooked and sterilized--to destroy the two amylase enzymes--yeast is added to ferment the sugars into ethanol. The final step, distillation, separates the ethanol from water, solids, and other chemical products of fermentation.

Corn sugar is almost entirely glucose, which yeast readily ferments into ethanol, Donohue says. But because corn is a foodstuff for people and animals, diverting large amounts of it into ethanol production could push up prices and even cause shortages. That's why plants that aren't currently used in other ways are attractive alternatives to corn.

In 2005, the Oak Ridge National Laboratory in Tennessee issued a report for the Departments of Energy and Agriculture estimating that the United States could produce 1.3 billion tons of plant matter that, if turned into ethanol, could fill more than 30 percent of the nation's petroleum needs. Agricultural waste forms a large part of that estimate.

But in order to reach that ambitious billion-ton goal without impinging on food supplies, high-cellulose crops, such as poplar, switchgrass, and wheatgrass, must also be grown specifically for ethanol production. All three plants are relatively undomesticated, so there's plenty of opportunity for breeding them to improve their value as cellulose sources, says Brian Davison of the Oak Ridge lab.

The nagging problem with these plants and others is that the cellulose in their cell walls is hard to get out, a problem that researchers call "recalcitrance of biomass."

"Nature developed plants so they're not easily degraded," says Martin Keller, also at Oak Ridge. The rigid cell wall has a complex structure built from three polymers: cellulose, hemicellulose, and lignin. Cellulose consists of long chains of glucose molecules (simple sugars with six carbon atoms) organized into tiny fibers. These fibers form a scaffold that supports hemicellulose, a polymer composed mostly of xylose (simple sugars with five carbon atoms). Lignin is a compound of various polymers that gives the plant strength and rigidity, but how it links with cellulose and hemicellulose is not well understood.

Current ethanol-making strategies require a number of difficult steps to dismantle a cell wall. Treatment with heat, pressure, or acids first removes hemicellulose and lignin from the long cellulose fibers. The cellulose and hemicellulose are then separately processed into ethanol, though doing so is challenging. The cellulose fibers don't dissolve well in water, making it difficult for the amylase enzymes to access the cellulose and break it down into glucose. Microbes are used to break hemicellulose into its component sugars and ferment them, but they produce a lot of by-products and not much ethanol.