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Why Yeast?


Establishing GTA


Dr. Nancy Ho


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The Saccharomyces yeast, also known as bakerís yeast, is the most effective microorganism for the fermentation of sugars to ethanol. This yeast has been the only microorganism used for the large-scale industrial production of ethanol, and has been used to make wine and bake bread since the dawn of civilization. The traditional feedstocks for ethanol production - cornstarch and cane sugar - contain polymers of glucose or simpler molecules made of glucose and fructose, which can all be effectively fermented by the Saccharomyces yeast to produce ethanol. Saccharomyces yeast has other characteristics as well that make it the most productive and desirable industrial microorganism for the production of ethanol. (See
Why Yeast?)

In the 1970s, the world suffered its first energy crisis.  As a result, governments worldwide, particularly the US Government, strongly supported the development of alternative fuels for transportation, particularly a renewable liquid fuel that could be produced from domestically available renewable resources. Ethanol has been proven to be a desirable renewable liquid fuel for transportation. In particular, ethanol can be produced by not only fermenting sugars derived from food crops - such as cornstarch and cane sugar - but also from cellulosic biomass.  Cellulosic biomass (corn stover, rice straw, wood, grasses, waste papers, etc.) is the largest renewable resource in the world and is the most attractive feedstock for the production of ethanol fuel via microbial fermentation of its sugar molecules. 

The United States and many other parts of the world have tremendous amounts of cellulosic biomass, and more than 70% of this resource can be converted to sugars and fermented to ethanol by microorganisms, preferably Saccharomyces yeast.  However, the conversion of cellulosic biomass to ethanol requires the development of new technologies.  In particular, cellulosic biomass contains polymers comprised of two major sugars - glucose and xylose.

Unfortunately, the natural Saccharomyces yeast is unable to ferment xylose to ethanol. In the 1970s, research efforts were thus carried out worldwide to search for new microorganisms that could ferment both glucose and xylose to ethanol.  However, no such natural microorganisms were discovered. Therefore, in the early 1980s, a concerted effort was made by scientists in various countries, particularly in the United States, Europe, and Japan, to use recombinant DNA techniques to modify the Saccharomyces yeast to ferment xylose to ethanol. In the beginning, there were nearly ten groups worldwide focused on developing such recombinant yeast with at least half of the groups in the US. One such group was the Molecular Genetics Group at Purdue University (West Lafayette, Indiana) led by Dr. Nancy Ho. Not only was her group the smallest, but she also had to obtain her own funding to support this project.

This task turned out to be far more difficult than initially anticipated. One by one, the research groups fell by the wayside. Most experts concluded that it might not be possible to engineer the Saccharomyces yeast to ferment xylose.  By the end of the 1980s, there were only four groups worldwide that continued in this endeavor. Dr. Hoís group was the only US group that persevered.  According to her analysis and design, she was certain that this task could be accomplished. She was also passionate that renewable cellulosic biomass be utilized, and thus, persisted with great determination. Compounding this task was the fact that after the oil crisis eased in the mid-1980s, it became far more difficult to obtain funding (in the US) for research to develop alternative fuels. On several occasions, she nearly had to terminate this project for lack of funding.


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