Thermotolerant yeast will change the face of biofuel production


Researchers looking to improve processes for "brewing up" a batch of biofuel say they're developing a new ingredient to help them; a more heat-tolerant strain of yeast.

Yeast is essential for producing ethanol fuel through fermentation -- brewers have been depending on the process for millennia -- but the heat created during the process, and even the resulting ethanol, can be damaging or even deadly to the yeast.

That's why in current ethanol production, the fermentation process has to be kept cooled to no higher than 86 degrees Fahrenheit. That's an ideal temperature for yeast, but not the optimal for ethanol production, where 104 degrees would be a better temperature for breaking down starches and sugars to yield ethanol through fermentation.

That has led researchers on a search for ways to improve the heat tolerance of yeast.

At Chalmers University of Technology in Sweden, scientists led by biologist Jens Nielsen have attempted to "evolve" such strains of yeast in a process known as adaptive laboratory evolution.

"We said, 'Why don't we try to do nature's approach and try to do an evolution of this?'" Nielsen said.

Writing in the journal Science, the researchers described exposing cultivations of numerous strains of yeast to temperatures of about 104 degrees Fahrenheit for long periods of time to track any resulting genetic changes.

After months of work with hundreds of generations of yeasts, strains began to emerge that grew well at the elevated temperatures they were being exposed to, the researchers say.

"It's really Darwinian survival of the fittest," Nielsen says. "we found quite a large number of new mutations."

Transferring the genetic mutation into industrial yeast strains could lead to less-costly and more efficient large-scale production of biofuels, the researchers said.

Meanwhile, U.S. researchers have been involved in work to make yeast more tolerant to the toxic effects on the yeast of ethanol itself.

Scientists at MIT and the Whitehead Institute of Biomedical Research discovered that potassium chloride and potassium hydroxide added to their yeast-growing medium increased its tolerance to alcohol and lengthened the time individual yeast cells could continue to produce ethanol.

"Toxicity is probably the single most important problem in cost-effective biofuels production," says MIT chemical engineering Professor Gregory Stephanopoulos.

Nielsen said the work of his team and of the U.S. researchers could be combined to create yeast strains that are tolerant of both heat and alcohol.

"The two could easily be combined; I don't see any technical hindrance to combining the two approaches," he said.

Such an outcome could be a boon to the U.S. ethanol fuel industry, which produced 13.3 billion gallons of ethanol in 2013.

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