Yeast’s epic journey gave rise to lager beer

In the 15th century, when Europeans first began moving people and goods across the Atlantic, a microscopic stowaway somehow made its way to the caves and monasteries of Bavaria. Orange-coloured galls, from the beech tree forests of Patagonia have been found to harbour the yeast that makes lager beer possible.

The 15th century stowaway, a yeast that may have been transported from a distant Patagonian shore on a piece of wood or in the stomach of a fruit fly, was destined for great things. In the dank caves and monastery cellars where 15th century brewmeisters stored their product, the newly arrived yeast fused with a distant relative, the domesticated yeast used for millennia to make leavened bread and ferment wine and ale. The resulting hybrid - representing a marriage of species as evolutionarily separated as humans and chickens - would give us lager, the clear, cold-fermented beer first brewed by 15th century Bavarians and that today is among the most popular - if not the most popular - alcoholic beverages in the world.

And while scientists and brewers have long known that the yeast that gives beer the capacity to ferment at cold temperatures was a hybrid, only one player was known: Saccharomyces cerevisiae, the yeast used to make leavened bread and ferment wine and ale. Its partner, which conferred on beer the ability to ferment in the cold, remained a puzzle, as scientists were unable to find it among the 1,000 or so species of yeast known to science.

An international team of researchers believes it has now identified the wild yeast that, in the age of sail, apparently travelled more than 7,000 miles to those Bavarian caves to make a fortuitous microbial match that today underpins the $250 billion a year lager beer industry.

Writing in the Proceedings of the National Academy of Sciences, researchers from Portugal, Argentina and the US describe the discovery of a wild yeast in the beech forests of Patagonia, the alpine region at the tip of South America, that apparently solves the age-old mystery of the origin of the yeast that made cold-temperature fermentation and lager beer possible.

“People have been hunting for this thing for decades,” explains Chris Todd Hittinger, a University of Wisconsin-Madison genetics professor and a co-author of the new study. “And now we’ve found it. It is clearly the missing species. The only thing we can’t say is if it also exists elsewhere (in the wild) and hasn’t been found.”

Expanding the search to other parts of the world, however, finally paid dividends when collaborator Diego Libkind of the Institute for Biodiversity and Environment Research (CONICET) in Bariloche, Argentina, found a candidate species whose genetic material seemed to be a close match to the missing half of the lager yeast. Orange-coloured galls, from the beech tree forests of Patagonia were found to harbour the yeast that makes lager beer possible.

“Beech galls are very rich in simple sugars. It’s a sugar-rich habitat that yeast seem to love,” notes Hittinger.

The yeast is so active in the galls, according to Libkind, that they spontaneously ferment. “When over-mature, they fall all together to the (forest) floor where they often form a thick carpet that has an intense ethanol odour, most probably due to the hard work of our new Saccharomyces eubayanus.”

The new yeast was hustled off to the University of Colorado School of Medicine, where a team that included Hittinger, Jim Dover and Mark Johnston sequenced its genome. “It proved to be distinct from every known wild species of yeast, but was 99.5% identical to the non-ale yeast portion of the lager genome,” says Hittinger.

The Colorado team also identified genetic mutations in the lager yeast hybrid distinctive from the genome of the wild lager yeast. Those changes - taking place in a brewing environment where evolution can be ramped up by the abundance of yeast - accumulated since those first immigrant yeasts melded with their ale cousins 500 years ago have refined the lager yeast’s ability to metabolise sugar and malt and to produce sulfites, transforming an organism that evolved on beech trees into a lean, mean beer-making machine.

“Our discovery suggests that hybridisation instantaneously formed an imperfect ‘proto-lager’ yeast that was more cold-tolerant than ale yeast and ideal for the cool Bavarian lagering process,” Hittinger asserts. “After adding some new variation for brewers to exploit, its sugar metabolism probably became more like ale yeast and better at producing beer.”