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Elusive ancestor of lager yeast found in Ireland

Undergrad students at the University College Dublin have made an unexpected discovery while working on their projects to identify wild yeasts and sequence their genomes. They have found populations of the yeast parent that produced a hybrid yeast currently used all over the world to produce lager beer. This parental species had been identified in the South American Andes and a few other places around the world, but not in Europe – which is strange, given the European predilection for lager beer.

Humans have been brewing fermented beverages for thousands of years, making this one of the oldest human industries. Evidence of fermented beverages from China dates to between 7,000 and 8,000 years ago, while in Israel this date is far older (13,000 years ago). Today, beer is the most widely consumed fermented drink in the world, but its origins are most likely linked to the days when cereals were first domesticated and the seeds stored for later consumption. 

Historically, all beers were fermented with one particular strain of yeast, identified as Saccharomyces cerevisiae, which has been used in beer brewing for literally thousands of years. This is a “top-fermenting” yeast (floats to the top during fermentation) that takes effect rapidly and likes warm conditions. It produces a particular flavor to the beer and is associated with the production of ale-type beers. Today, S. cerevisiae continues to be used to make ales, wines and bread. 

However, in the Middle Ages in Europe, a new beer began to do the rounds. Known as lager, it had a subtly different flavor and was produced with a new (but similar) strain of yeast – Saccharomyces pastorianus. This yeast is known as a “bottom-fermenting” yeast because it sinks to the bottom during the fermentation process. In addition, it works optimally in cold conditions and ferments the sugars relatively slowly. At this time in Bavaria (13th century), beer could only be brewed in the cooler months of the year, making conditions unsuitable for the use of S. cerevisiae, the ale-type yeast. 

Although lager brewing started off as a small affair carried out in private homes for consumption by family members, lager beer spread worldwide in the 19th century, and today lagers represent more than 90 percent of beers sold.

But where did this new species of yeast come from? Genetic analysis has indicated that S. pastorianus is a hybrid of two parents, one of which is S. cerevisiae. However, the other parent eluded scientists for a long time. It was isolated for the first time in 2011, from the Patagonian Andes in South America – it is named Saccharomyces eubayanus. Like S. pastorianusS. eubayanus is cold-tolerant and scientists believe that the lager-style of cold brewing selected for the formation of the S. pastorianus hybrid yeast from an ale strain of S. cerevisiae and a wild S. eubayanus isolate.

Apart from wondering how S. eubayanus made its way from South America to Europe to form the hybrid yeast, scientists were puzzled by the fact that no isolates of S. eubayanus had ever been found in Europe, the epicenter of lager brewing origins. Isolates of S. eubayanus had been identified in North America, the Tibetan Himalayas, Sichuan and West China, and New Zealand, but not in Europe. This begged the question of whether S. eubayanus had, in fact, ever been in Europe, and, if not, where had the lager yeast S. pastorianus actually come from?

All this changed when the undergrad students from University College Dublin identified this elusive parent species for the first time from Irish soils right on the University’s campus. They identified two different S. eubayanus strains from soil samples taken 17 m apart on the Belfield campus in September 2021. The genome sequences of these two isolates showed that they are related to the ancestral S. eubayanus strain that initially crossed with S. cerevisiae to form the hybrid lager yeast, S. pastorianus. The details of the discovery are published in the journal FEMS Yeast Research, produced by Oxford University Press.

The discovery of S. eubayanus in Ireland shows that this yeast is native to Europe and makes it possible that there were natural populations of the yeast in southern Germany in the Middle Ages, at the time when lager brewing first began. Thus the hybridization event could have taken place locally. It remains to be seen whether natural populations of these ancient yeasts still exist hidden somewhere in the forests of Bavaria.

“This discovery is a fantastic example of research-led teaching,” said study lead author Geraldine Butler. “Our undergraduates have found more than a hundred yeast species in Irish soil samples over the past five years, and we’re delighted to stumble across S. eubayanus on our own doorstep. We’re hoping to find a commercial partner to brew with it so we can find out what it tastes like!”

By Alison Bosman, Staff Writer

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