After millennia of ales, lagers joined the beer scene some six centuries ago. A hybrid yeast strain made it possible to brew in cold conditions — giving lagers the smooth, light flavor that has led them to represent 94 percent of the world beer market.

Until 2011, no one was able to find the species that combined with Saccharomyces cerevisiae – which brings us bread, ales and wines – to form this lager-brewing hybrid. Since the discovery of that elusive second species, named Saccharomyces eubayanas, scientists have sequenced its genome and begun to uncover the history of lager yeasts. This "genomic archaeology" has turned up evidence to settle a debate about the origins of lagers, according to a report in the journal Molecular Biology and Evolution.

"There has been a bit of a controversy over whether lager-brewing yeasts originated once or twice," said corresponding study author Chris Hittinger of the University of Wisconsin-Madison to Tech Times. "By comparing the wild S. eubayanas genome to those of different types of lager-brewing yeasts, we were able to see that those other lineages actually came from independent hybridization events — so lager yeasts actually originated two times, at least."

This marriage of yeasts is so key to creating lagers because S. eubayanas provides the hybrid with cold tolerance.

"S. cerevisiae is really optimized to grow at Mediterranean room temperature. That's why it's been with us for millennia to make bread, wine, and ale beers," Hittinger said. "But the cold temperatures needed to brew lagers created an environment in which S. cerevisiae is not able to ferment."

In terms of flavor, S. cerevisiae still does a lot of heavy lifting. It is possible to brew beer using only S. eubayanas, but it comes out sweeter than you would expect from a lager, according to Hittinger.

The strain of S. eubayanas that researchers discovered in Patagonia a few years ago is, however, genetically quite different from the hybridized S. eubayanas found in the strains of lager yeast that brewers have been propagating for centuries.

"We can see a really strong genetic signature of domestication," said Hittinger. "The S. eubayanas part of the genome, after hybridization, experienced a dramatic elevation in its rate of evolutionary change."

A lot of this evolutionary change seems to have occurred in genes related to the domestication process. One gene used to cause S. eubayanas to convert some alcohol back into food for itself. For a wild yeast strain, this makes sense. But when brewing beer, alcohol is of course one of the desired products of fermentation.

Since S. cerevisiae's long-lost partner S. eubayanas was finally found, scientists were able to blend them together to create the first new lager yeast in centuries. Better understanding how champion yeast strains such as popular hybrid lager yeasts came to be could help brewers design new strains – and delicious new beers – in the future.

Photo: Quinn Dombrowski | Flickr

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