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Newly Uncovered Enzymes Turn Corn Plant Waste into Biofuel

Cellulose-loving fungi can cut biofuel costs by enabling existing corn ethanol plants to process cheaper, woody feedstocks such as corn stover
cellulosic biofuel enzymes corn stover



WIKIMEDIA COMMONS/ROYALBROIL

"Visualize three tons of moldy bread." It's not the most appealing image, perhaps, but it's a description of the moist mound of growth media tended by bioscientist Cliff Bradley and his partner, chemical engineer Bob Kearns at their biofuel facility in Butte, Mont., that could help cut ethanol costs at the fuel pump.

Selected soil fungi that eat cellulose—the hard-to-digest, structural component of woody plants—thrive on the big pile of putrefaction from which Bradley and Kearns harvest certain powerful enzymes. The special enzymes allow standard biofuel plants to produce ethanol at lower cost by replacing some of the high-priced corn (starch) they process with cheaper corn stover "waste"—the leaves, stalks, husks and cobs of the maize plant itself.

Replacing 35 percent of the corn (which goes for $4.28 a bushel) now used in a typical ethanol plant with inexpensive corn stover (at $65 per ton) could save a quarter on each a gallon of ethanol the facility produces, the researchers calculate. And that's before any blender's credit or tax benefits from government for processing cellulose. Bradley and Kearns say that the basic integrated starch–cellulose process also works for biofuels produced in Brazil where ethanol is distilled from sugarcane and bagasse, or highly cellulosic cane plant residue.

Supporting development of the promising new technology is Cupertino, Calif.–based AE Biofuels, which has constructed a commercial pilot facility in Butte, where the pair demonstrates their integrated fermentation technology to potential licensing customers. The patent pending process "can be a bridge to cellulosic ethanol," says Andy Foster, executive vice president at AE Biofuels. The use of cellulosic feedstocks effectively enables farmers and producers to squeeze more ethanol from each acre of farmland, he states.

AE Biofuels is one of several companies in the U.S. that is trying to jump-start progress toward greener biofuels made from nonfood feedstocks with high cellulose content. But most of the demonstration efforts have slowed or halted "since the banking meltdown which made it very tough to arrange capital," says biofuels expert George W. Huber, a chemical engineer at the University of Massachusetts Amherst. Despite last year's economic turmoil, however, new pilot cellulosic biofuel plants were opened by KL Energy, Verenium Corp., and POET, LLC, he notes.

For the past few decades, Bradley and Kearns—self-styled "industrial fermentation guys"—have focused on developing effective ways to raise hard-to-cultivate soil fungi that secrete the crucial enzymes. Unlike their competitors, they grow fungi on the moist surfaces of solid nutrient particles. Standard large-scale fermentation processes, in contrast, take place in water-filled tanks. "They put an organism in a tank where everything's in a water solution," Kearns explains, "and then they try to get enough oxygen in there to make the aerobic fungi happy." Rather than "trying to adapt the organism to a desired environment," the two researchers created an environment that suits the organism.

One of the pair's special enzymes readily degrades cellulose and another has the unique ability to break down corn starch at ambient temperatures, a talent that enables existing corn ethanol plants to incorporate cellulosic feedstocks into their standard starch fermentation processes. "The integrated process uses the same equipment, which is important now that capital financing is so hard to get," Bradley says.

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