Chemical engineers and biologists at MIT have found a simple way to make yeast produce more ethanol from sugars: Spike the mixture they're growing on with two common chemicals.
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Adding potassium and an acidity-reducing compound helps the yeast tolerate higher concentrations of the ethanol they're making without dying. Aided by those "supplements," traditionally underperforming laboratory yeast made more ethanol than did industrial strains genetically evolved for ethanol tolerance.
The supplements also enabled lab yeast to tolerate higher doses of high-energy alcohols such as butanol, a direct gasoline substitute. In other "firsts," the researchers described the mechanism by which alcohols poison yeast; they defined two genes that control ethanol tolerance; and they modified those genes in lab yeast to make them out-produce the industrial strains—even without the supplements.
Manufacturers worldwide rely on yeast to convert sugars from corn or sugar cane into ethanol, a biofuel now blended with gasoline in cars and trucks. But there's a problem: At certain concentrations, the ethanol kills the yeast that make it. As a result, a given batch of yeast can produce only so much ethanol.
"The biggest limitation on cost-effective biofuels production is the toxic effect of alcohols such as ethanol on yeast," says Gregory Stephanopoulos, the Willard Henry Dow Professor of Chemical Engineering at MIT. "Ethanol is a byproduct of their natural metabolic process, as carbon dioxide is a byproduct of ours. In both cases, high doses of those byproducts are lethal."
Potassium, calcium, and ammonium are all critical to the nutrition and functioning of living cells, so what made the potassium special? And what about the phosphate?
After much testing, they realized that the change wasn't due to metabolic processes—or any processes—occurring inside the cell. The impact of the potassium on ethanol tolerance occurred too quickly for cells to be mounting a biological response.
And further testing showed that the role of the phosphate, a natural pH buffer, was simply to reduce the acidity of the mixture.
"It had little to do with the biology inside the cell but rather more with the chemistry outside it," Lam says. Experiments confirmed their reinterpretation: Just adding potassium and reducing acidity—with no phosphate present—gave the same remarkable boost in ethanol production. ■