Next time you’re lifting a glass of crisp, sudsy lager, send a quick mental thanks to Saccharomyces eubayanus.

This is the organism, says a new study in the Proceedings of the National Academy of Sciences, that made it possible for 15th century Bavarian monks to develop lager, the beer that today is one of the most—if not the most—popular alcoholic beverage in the world.

Brewers and scientists who study beer had long known that the yeast that gave lager the capacity to ferment in the cold (unlike ales, which can only do so at warmer temperatures) was a hybrid, but only one parent was known: Saccharomyces cerevisiae, which is used to make bread rise and to ferment wine or ales. While about 1000 other species of yeast are known, none of them were a match for the rest of the genetic fingerprint.

Until now.

"People have been hunting for this thing for decades," Chris Todd Hittinger, a University of Wisconsin-Madison genetics professor and a co-author of the new study, told ScienceDaily. "And now we've found it.”

The new discovery was the result of an exhaustive, international five-year search which began with a Portuguese team of researchers led by the New University of Lisbon's José Paulo Sampaio and Paula Gonçalves fruitlessly scouring scientific literature, poring through European yeast collections and gathering new yeasts from the continent.

The breakthrough came when Diego Libkind of the Institute for Biodiversity and Environment Research (CONICET) in Bariloche, Argentina, discovered a possible candidate hidden in galls that infect beech trees in the forests of Patagonia.

"Beech galls are very rich in simple sugars,” Hittinger told ScienceDaily. "Yeast seem to love [it]." In fact, when overly mature, the galls tend to fall to the forest floor and form a carpet with "an intense ethanol odor.”

Samples were sent to the University of Colorado School of Medicine, where a team including Hittinger sequenced that yeast’s  genome.

"It proved to be distinct from every known wild species of yeast, but was 99.5 percent identical to the non-ale yeast portion of the lager genome," said Hittinger, now at UW-Madison.

The mystery remains how this yeast made its way halfway around the world, from the tip of South America to the dank monasteries of beer-brewing monks in Bavaria, where it mated with its ale-ish cousin. An unlikely match, it must be said, since ScienceDaily describes the two yeasts as “as evolutionarily separated as humans and chickens.”

For the moment, the theory is that, at the dawn of the cross-Atlantic trade, a speck of yeast—small enough to fit in the stomach of a fruitfly—made its way across the ocean. Of course, there are more prosaic explanations, too.

“It is clearly the missing species,” says Hittinger. “The only thing we can't say is if it also exists elsewhere (in the wild) and hasn't been found."

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