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As you fill up your car this summer and tighten your belt around your bank account you may wonder whether or not there is any relief in sight from high gasoline prices. Unfortunately, immediate relief is not in sight, but scientists are working on a way to solve the problem and that solution, they say, is ethanol made from cellulose.
Conventional ethanol is typically made from corn kernels. Corn kernels are composed of starch and simple sugars that dissolve easily in water. Once the sugars are dissolved they can be fermented by yeast to form ethanol. Cellulose is part of what's left over after you harvest the corn kernels. For example, the remaining leaves and stalks, the part of the plant we think of as crop residues, are made up of cellulose. These residues can also be used to make ethanol. However, because cellulose makes up the skeleton of the plant, it is physically much tougher, and extracting the sugars is much more difficult.
As recently as 2001, making ethanol from cellulose was simply too costly at $5.00 per gallon for any commercial venture. Then, enzyme producers Genencor International and Novozymes A/S partnered with the United States Department of Energy to reduce the cost of enzyme-based production of ethanol from cellulose.
Cellulose can be broken down into its component sugars in two ways. One way is to treat the cellulose with chemicals and the other is to treat it with enzymes. Enzymes work by breaking the chemical bonds between molecules. "One way to think of it," says Bill Dean, Vice President of Development for Genencor International, "is that cellulose is a line of ping-pong balls hooked together, where the ping-pong balls are the sugars. Then the enzymes go in and cut them apart into simple sugars that are accessible by the yeast." The yeast or other microbes can then ferment the sugars to ethanol, bioplastics or other value-added chemicals. Sugars are the agricultural equivalent of "crude oil" that we will use in modern biorefineries.
Because of the high cost of the enzymes, two of the goals of Genencor scientists wereto reduce the amount of enzyme necessary to cut apart the ping-pong balls and to be able to make it more cheaply. The scientists chose as their manufacturing plant Trichoderma reesei, a common soil fungus that makes lots of enzymes that break down cellulose. Their strategy was to modify T. reesei to make its enzymes work faster while also making these enzymes more cheaply. To make them work faster the scientists had to find enzymes out in the natural world that worked at a higher temperature than the ones currently produced by T. reesei. The high temperature was important because chemical reactions, such as breaking down cellulose, proceed more quickly at higher temperatures. The goal was to insert the genes that made these enzymes into T. reesei. "That way," says Dean, "our fungus becomes our production organism that breaks the cellulose down into sugars" that can be fed to the yeast.
Industrial biotech provides an elegant solution to making fuel ethanol from cellulose. The sugar product created by enzymes is "cleaner" than that produced by chemical treatment of cellulose. "If you're treating [cellulose] with extreme chemical conditions, you're going to end up with a very complex and very noxious mixture. Then you're feeding this mixture to an organism that's making ethanol. So you want the most efficient process, and to get that you want that yeast to be really happy. That yeast is not going to be really happy if you're feeding them sugars mixed with lots of toxins."
The end result? Biotechnology results in happier more efficient yeast, and those yeast will one day make a difference for us at the pump. Thanks to the development of these enzymes, the cost of making ethanol from cellulose today is estimated to be $1.50 to $2.50 a gallon, making it a viable alternative to gasoline at $3.00 a gallon at the pump today. In the future scientists predict that cost will be lowered to 90 cents a gallon.
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