Purdue Yeast Makes Ethanol from Agricultural Waste More Effectively
A strain of yeast developed at Purdue University more effectively makes ethanol from agricultural residues that would otherwise be discarded or used as animal feed, and the first license for the yeast has been issued to the biotechnology company Iogen Corp.
Purdues genetically altered yeast allows about 40 percent more ethanol to be made from sugars derived from agricultural residues, such as corn stalks and wheat straw, compared with "wild-type" yeasts that occur in nature.
The agricultural residues are primarily made up of cellulose and "hemicellulose," which are known as cellulosic materials. Unlike traditional ethanol feedstocks, such as corn kernels, the cellulosic materials contain two major sugars, glucose and xylose, which cannot both be fermented into ethanol by natural Saccharomyces yeast, the microorganism used by industry to produce ethanol, said Nancy Ho, a senior research scientist and leader of the molecular genetics group in Purdues Laboratory of Renewable Resources Engineering, or LORRE. Iogen specializes in producing ethanol from cellulosic material.
A team led by Ho developed the more efficient yeast during the 1980s and 1990s. Conventional yeast can ferment glucose to ethanol, but it cannot ferment xylose. Xylose makes up about 30 percent of the sugar from agricultural residues, and the inability to ferment xylose would represent a major loss of ethanol yield, Ho said.
The Purdue researchers altered the genetic structure of the yeast so that it now contains three additional genes that make it possible to simultaneously convert glucose and xylose to ethanol. The ability to ferment xylose increases the yield of ethanol from straw by about 40 percent. Being able to simultaneously ferment glucose and xylose is important because both sugars are found together in agricultural residues, Ho said.
"It would cost too much money to separate the two sugars before proceeding with fermentation to ethanol, so being able to ferment both sugars together to ethanol is critical," she said. "To be more cost competitive with gasoline, the two sugars have to be converted together to ethanol.
"Until we developed our yeast, no suitable microorganism could convert these two sugars together."
Iogen has obtained a non-exclusive license from the Purdue Research Foundation for the yeast and related patents. Its Ottawa, Canada, demonstration facility is the first plant in the world to produce ethanol from cellulosic materials. Iogen is using the Purdue yeast to produce ethanol from the sugars the company derives from wheat straw.
"We have confirmed that Purdues recombinant glucose- and xylose-fermenting yeast is the most effective microorganism available for the production of ethanol from cellulosic materials," said Jeffrey S. Tolan, senior research scientist for Iogen. "The ethanol yield and productivity from the Purdue yeast in our plant matches that obtained by Dr. Hos group in the lab at Purdue. The Purdue yeast is also easy to work with and is favored by our plant operators because of this."
The ethanol made in Iogens plant is blended into gasoline at the Petro-Canada refinery in Montreal. Cars use the ethanol-gasoline blend without any modifications; typically, drivers are not even aware of the presence of the ethanol, except for the label on the gas pump. The Ottawa plant represents the latest step toward Iogens goal of making ethanol from cellulosic materials widely available as a fuel, Tolan said.
In Iogens process, about two-thirds of the straw is converted to ethanol, with a yield of about 75 gallons of ethanol per ton of straw. Most of the remaining one-third of the agricultural residue, which cannot be fermented, is burned to generate power to run the plant, and there is little waste or use of fossil fuels, he said.
"The use of cellulose ethanol offers advantages to the environment that are not obtained with other transportation fuels that are available," Tolan said.
Also known as ethyl alcohol, ethanol can be used as fuel by itself or blended with gasoline. The Purdue yeast is used in combination with other technologies under development that first convert agricultural materials to xylose and glucose, said Purdues Michael Ladisch, Distinguished Professor of Agricultural and Biological Engineering and director of LORRE.
"Iogens efforts are beneficial to companies in Indiana and elsewhere in the United States by providing an industrial test bed for the fermentation part of the cellulose conversion technology, hence speeding its development for uses on a range of crops and crop residues once the other technologies, including preprocessing of the cellulosic materials and converting these materials to sugars, are developed and proven in the industry," Ladisch said.
Ethanol is environmentally friendly and a cleaner fuel than gasoline, he said.
"The carbon dioxide that is generated from burning ethanol is recycled back into plant material because plants incorporate CO2 into cellulose as part of the photosynthesis cycle," Ladisch said. "This reduces the net generation of the greenhouse gas since part of it is recycled."
Ethanol currently is produced when yeast ferments glucose and related hexose - or six-carbon sugars in food crops such as cane sugar, corn and other starch-rich grains. However, Ho said, because these crops are expensive and in relatively limited supply, they cant yield sufficient amounts of ethanol for transportation needs.
Cellulosic materials represent an opportunity to address this problem, she said. Cellulosic materials cost only about half as much as corn per ton but are more difficult than corn to convert to ethanol. Part of the difficulty is the fermentation to ethanol of the xylose, which is a five-carbon sugar. This sugar is not naturally fermented by yeast or other microorganisms.
"Corn-based ethanol production in the United States currently is about 3 billion gallons per year," Ho said. "According to conservative estimates, 30 percent of the residue left behind in the cornfield after harvest could produce another 4 billion to 5 billion gallons annually.
"The use of cellulosic materials also could open up new markets for crops such as grasses, which can be grown on marginal lands, creating jobs and providing more energy independence."
An added advantage of yeast strains developed by Ho is that they are based on environmentally safe Saccharomyces yeast, which has been used for centuries to make wine and bread and is the only microorganism used by industry for large-scale ethanol production from glucose.
Ho has worked for 20 years to produce and perfect a yeast that can effectively convert more of the sugars in plant matter - corn stalks, tree leaves, wood chips, grass clippings, and even cardboard - into ethanol.
"Ethanol produced from cellulosic materials is an ideal, domestically available fuel," Ho said.
In 1993, Hos group became the first in the world to produce a genetically engineered Saccharomyces yeast that can effectively ferment both glucose and xylose.
Hos research has been funded by the U.S. Department of Agriculture, the Department of Energy, the Consortium for Plant Biotechnology Research Inc., the U.S. Environmental Protection Agency and industry sources.
Ho holds a doctoral degree in molecular biology from Purdue, a masters degree in organic chemistry from Temple University and a bachelors degree in chemical engineering from the National Taiwan University.