Lager Beer's Mystery Yeast

22 August 2011 3:00 pm

Stephan Zabel/iStockphoto

Bottoms up. Lager, as we know it, is likely a hybrid of S. cerevisiae and a newly discovered yeast from Patagonia.

Lager may have its roots in Bavaria, but a key ingredient arrived from halfway around the world. Scientists have discovered that the yeast used to brew this light-colored beer may hail from Argentina. Apparently, yeast cells growing in Patagonian trees made their way to Europe and into the barrels of brewers.

Through the ages, brewers have tried to make their beers better, for instance, by improving on taste or color or making them easier to keep. They selected for these traits by taking bits of good batches of beer and adding them to the next batch. Little did they know how they were influencing yeast evolution.

The key characteristic of lager beer is that it's brewed at low temperatures. Today's lager brewers use a yeast strain called Saccharomyces carlsbergensis, which scientists have long known to be a hybrid between a classic beer strain called S. cerevisiae and another species that had never been identified in the wild.

Microbiologist José Paulo Sampaio of the Universidade Nova de Lisboa in Caparica, Portugal, didn't set out to find the origins of the mystery yeast. He and his colleagues were compiling a genetic directory of different Saccharomyces species, which in nature live in oak trees. As part of the study, colleagues in Argentina collected yeast from the southern beech forests of the Patagonian mountains, which are closely related to oaks in the Northern Hemisphere, and classified them based on their genetic sequences.

When the researchers saw a partial genome sequence of one newly discovered species, which they named S. eubayanus (as it was similar to a brewing yeast called S. bayanus), Sampaio says he "clearly saw it was different" from the other Argentinean isolates. Its sequence was familiar: like that of other beer yeast.

So the researchers sequenced the entire genome of S. eubayanus and scanned libraries of other yeast genomes to find its relatives. S. eubayanus turned out to be most similar to S. carlsbergensis. In fact, S. eubayanus is 99.5% similar to the non-S. cerevisiae portion of the lager yeast, the researchers report online today in the Proceedings of the National Academy of Sciences. Digging deeper, they found that the differences between the native S. eubayanus and S. carlsbergensis are in those genes involved in fermenting sugar, which have mutated over the past few centuries as the yeast was domesticated for lager brewing.

The finding makes sense, the team says: The average temperature in the Patagonian mountains is about 6°C—so any yeast living there had cold-resistance genes that were very valuable for brewers looking to make beer at lower temperatures. Once it had entered beer barrels, its favorable characteristics may have helped it spread throughout the lager industry.

Sampaio doesn't know how and when S. eubayanus got from beech trees in South America into European breweries. Presumably, the yeast was imported on a ship, hitching a ride in some animal or living in a tree that was used to make a barrel. The species doesn't naturally occur in Europe, Japan, or North America, Sampaio says; the team scoured yeast genome libraries just to be sure.

"History and archaeology are full of surprises," says biomolecular archaeologist Patrick McGovern, who studies the history of fermented beverages at the University of Pennsylvania Museum in Philadelphia, in an e-mail. "Nowhere is this more true than in the seemingly miraculous process of fermentation."

Yeast, he adds, are "a very resilient species, and they moved around the world with humans." He finds it "intriguing" that the yeast came from an area near one of the earliest human settlements in the Americas, Monte Verde, whose residents could conceivably have made their own alcoholic drinks from it.

Sampaio—a lager fan himself—says the study has given him a taste for more. Having solved one mystery, he's interested in studying the origins of other beer yeast and wine yeasts as well—research perhaps best done with a frosty beverage in hand.

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