Yeast are commonly used to transform corn and other plant materials into biofuels such as ethanol. However, large concentrations of ethanol can be toxic to yeast, which has limited the production capacity of many yeast strains used in industry. "Toxicity is probably the single most important problem in cost-effective biofuels production," says Gregory Stephanopoulos, the Willard Henry Dow Professor of Chemical Engineering at MIT.

Now Stephanopoulos and colleagues at MIT and the Whitehead Institute for Biomedical Research have identified a new way to boost yeast tolerance to ethanol by simply altering the composition of the medium in which the yeast are grown. They report the findings, which they believe could have a significant impact on industrial biofuel production, in today's issue of the journal Science.

Ethanol and other alcohols can disrupt yeast cell membranes, eventually killing the cells. The MIT team found that adding potassium and hydroxide ions to the medium in which yeast grow can help cells compensate for that membrane damage. By making these changes, the researchers were able to boost yeast's ethanol production by about 80 percent. They also showed that this approach works with commercial yeast strains and other types of alcohols, including propanol and butanol, which are even more toxic to yeast.

"The more we understand about why a molecule is toxic, and methods that will make these organisms more tolerant, the more people will get ideas about how to attack other, more severe problems of toxicity," says Stephanopoulos, one of the senior authors of the Science paper.

"This work goes a long way to squeezing the last drop of ethanol from sugar," adds Gerald Fink, an MIT professor of biology, member of the Whitehead Institute, and the paper's other senior author. Postdoc Felix Lam is the paper's lead author, and graduate student Adel Ghaderi also contributed to the study.