Massachusetts Institute of Technology scientists 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.

Yeasts are commonly used to remake sugar, corn starch and other plant materials into biofuels such as ethanol, but high concentrations of ethanol can be toxic to yeast, which has limited the production capacity of many yeast strains used in industry.

Gregory Stephanopoulos, the Willard Henry Dow Professor of Chemical Engineering at MIT said, “Toxicity is probably the single most important problem in cost-effective biofuels production.”

Stephanopoulos and colleagues at MIT and the Whitehead Institute for Biomedical Research report findings that simply altering the composition of the medium in which the yeast are grown could have a significant impact on industrial biofuel production in the current issue of the journal Science.

Saccharomyces Cerevisiae at 400x.  Image Credit: "A doubt" from Wikimedia Commons.

Saccharomyces Cerevisiae at 400x. Image Credit: “A doubt” from Wikimedia Commons.

Ethanol and other alcohols in higher concentrations will disrupt yeast cell membranes, eventually killing the cells. As the ethanol forms the concentration goes up requiring a strep of ethanol reduction before the yeast cab resume its work.

The MIT team found that adding potassium and hydroxide ions to the medium in which yeast grows can help cells compensate for the 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.

Co-senior paper author Stephanopoulos said, “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.”

Gerald Fink, an MIT professor of biology, member of the Whitehead Institute, and the paper’s other co-senior author said, “This work goes a long way to squeezing the last drop of ethanol from sugar.”

The MIT team began the project searching for a gene or group of genes that could be manipulated to make yeast more tolerant to ethanol, but this approach didn’t yield much success. However, when the researchers began to experiment with altering the medium in which yeast grow, they found some dramatic results.

By augmenting the yeast’s environment with potassium chloride, and increasing the pH with potassium hydroxide, the researchers were able to dramatically boost ethanol production. They also found that these changes did not affect the biochemical pathway used by the yeast to produce ethanol: Yeast continued to produce ethanol at the same per-cell rate as long as they remained viable. Instead, the changes influenced their electrochemical membrane gradients – differences in ion concentrations inside and outside the membrane, which produce energy that the cell can harness to control the flow of various molecules into and out of the cell.

Ethanol increases the porosity of the cell membrane, making it very difficult for cells to maintain their electrochemical gradients. Increasing the potassium concentration and pH outside the cells helps them to strengthen the gradients and survive longer; the longer they survive, the more ethanol they make.

Postdoc Felix Lam is the paper’s lead author said, “By reinforcing these gradients, we’re energizing yeast to allow them to withstand harsher conditions and continue production. What’s also exciting to us is that this could apply beyond ethanol to more advanced biofuel alcohols that upset cell membranes in the same way.”

The researchers found that they could also prolong survival, but not as much, by engineering the yeast cells to express more potassium and proton pumps, which are located in the cell membrane and pump potassium in and protons out.

Before yeast begin their work producing ethanol, the starting material, usually corn, must be broken down into the sugar glucose. A significant feature of the new MIT study is that the researchers did their experiments at very high concentrations of glucose. While many studies have examined ways to boost ethanol tolerance at low glucose levels, the MIT team used concentrations of about 300 grams per liter, similar to what would be found in an industrial biofuel fermenter.

Stephanopoulos explained, “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.”

Since the Science paper submission the MIT researchers have used this method to bump ethanol productivity even higher than reported in the Science paper.

The team is also working on using this approach to boost the ethanol yield from various industrial feedstocks that, because of starting compounds inherently toxic to yeast, now have low yields.

Please note that graduate student Adel Ghaderi also contributed to the study.

The MIT team is really on its way to helping the ethanol business get a lower cost basis to compete with other liquid fuel forms. With crude oil falling, a situation that is sure to reverse again someday and corn again back into a vast oversupply every gain in productive efficiency is going to be welcome.


1 Comment so far

  1. dmm on October 14, 2014 4:45 PM

    Buried within the Science article is the admission that this “breakthrough” does NOTHING to increase ethanol yields for fermentations of glucose carried out in standard growth media, which ALREADY are at 100% yields.

    The research will probably be useful down the road. It might lower production costs. It might increase yields from other sugars. It might lead to a stronger understanding of yeast metabolism. But this research is NOT the immediate economic blockbuster trumpeted by MIT’s press release, and strongly implied by the article’s abstract.

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