The ancestor to modern brewing yeast has been found hiding in Ireland

In 1516, the duchy of Bavaria in Germany imposed a law on its beer brewers meant to reserve ingredients like wheat and rye for the baking of bread. The decree restricted brewers to using only barley, hops, water and yeast to make their libations, and set the prices for beer depending on the time of year. The law inadvertently limited brewing to the winter, which favored a cold-tolerant yeast called Saccharomyces pastorianus, which brews lager, over the more common S. cerevisiae, which brews ale.

S. pastorianus is a hybrid, produced from the mating of S. cerevisiae with another yeast called S. eubayanus. Despite lager’s European origins, S. eubayanus hadn’t actually been found there and was only first discovered in 2011, in the Patagonia region of South America (SN: 8/23/11). Now, thanks to a research project carried out by undergraduate students, S. eubayanus has been found living in European soil — fittingly, in the beer-loving nation of Ireland.

“Since the discovery of S. eubayanus [more than] 10 years ago, it’s been a fun puzzle putting together where the species is actually found,” says Quinn Langdon, a biologist at Stanford University, who was not involved with the study.

A leading theory is that S. eubayanus originated in Patagonia and then spread around the world, eventually mating with S. cerevisiae in European breweries to make S. pastorianus.

Geraldine Butler, a geneticist at University College Dublin and leader of the project, always thought that teaching genome-sequencing techniques by having students scour soils for yeast could turn up S. eubayanus. Still, she says, she couldn’t contain her excitement when she saw the first hint of the microbe. “I was sitting by the sequencer waiting for the results to come out,” she says.

One of Butler’s students, Stephen Allen, found two local strains of S. eubayanus hiding in plain sight on the Belfield campus of University College Dublin. The team has since gone back and found the yeast again, Butler says, suggesting that there is a stable population of the yeast living in the Irish soil.

The new discovery was published December 7 in FEMS Yeast Research.

Butler hopes this discovery will brew interest elsewhere in Europe to search for S. eubayanus, including in Bavaria, where lager brewing is thought to have first started. She is also looking for commercial partners to try making beer with the Irish strains.

Langdon isn’t confident that the new microbes will lead to tasty brews because there are other S. eubayanus strains that don’t grow well on maltose, the sugar that needs to be digested by yeasts during the brewing process. Still, Langdon says, “it’d be fun to brew with them.”

Whether the newly discovered Irish strains of S. pastorianus’ missing parent taste good or not, there’s no denying that their discovery helps solve a little piece of the puzzle of lager brewing’s origins. That 16th century shift from S. cerevisiae to S. pastorianus led to a global shift that continues to this day — more than 90 percent of beer sold worldwide today is lager.

Fungi are the “forgotten kingdom,” Langdon says, not getting as much attention as plants or animals, despite playing an outsize role in human history. “Yeasts are just single cells living in the soil, and they’re doing really important things.”