ETHANOL-PRODUCING YEAST
Figure 1. Oval saccharomyces cerevisiae yeast cells, magnified 400 times, can tolerate high levels of ethanol by altering medium in which cells are grown. Source: A. Doubt/Wikimedia Commons.
“Toxicity is probably the single most important problem in cost-effective biofuels production,” notes Gregory Stephanopoulos, a professor of chemical engineering at MIT and part of the team that identified changes in the fermentation environment that enhance yeast’s tolerance of ethanol.“This work goes a long way to squeezing the last drop of ethanol from sugar,” believes Gerald Fink, an MIT professor of biology and member of the team. Lonnie Ingram, director of the
Florida Center for Renewable Chemicals and Fuels at the University of Florida, Gainesville, Fla., terms the discovery “remarkable and unexpected.”The researchers originally set out to find a gene or group of genes to manipulate that could make yeast more tolerant of ethanol. Not having much success with that approach, they started to experiment with changing the medium in which the yeast grow. Ethanol and other alcohols can disrupt yeast cell membranes, eventually killing the cells, the researchers explain. They found that adding potassium chloride to the environment and raising the pH with potassium hydroxide can help compensate for that membrane damage. The altered medium fostered an about 80% increase in ethanol production without affecting the biochemical pathway used by the yeast to make ethanol. Moreover, the researchers note the approach works with commercial yeast strains and with other types of alcohols, including propanol and butanol, which are even more toxic to yeast. More details on the approach appear in a recent article in Science. Unlike other academic studies, the trials used high concentrations of glucose, about 300 g/L, a level comparable to that in an industrial biofuels fermenter. “If you really want to be relevant, you’ve got to go to these levels. Otherwise, what you learn at low ethanol levels is not likely to translate to industrial production,” says Stephanopoulos. However, performance in industrial fermenters with plant-derived feedstocks is still a complete unknown, notes Felix Lam, another member of the team. Larger-scale laboratory or pilot-plant tests will depend upon collaboration with industry, he says.If such trials take place and give positive results, implementing the approach at existing bioethanol plants should be fairly straightforward, given the simplicity of how tolerance can be modulated, and will be aided by improved yeast strains, Lam believes. The method may not require pH set-point control, he adds; preventing drops below pH 3 may suffice.The researchers now have raised productivity beyond the level reported in the
Science article. They also are working to boost yields from various industrial feedstocks containing high levels of aldehydes and acetic acid, which are inherently toxic to yeast, that currently have low yields.
