Scientists have cracked the mystery behind lager beer after years of research, and found the yeast's genomic foundation, paving the way for new types of designer beers.

The parent yeast of lager is believed to have journeyed from Patagonia to Bavaria, in what is now Germany, giving birth to the most popular alcoholic beverage of today.

Scientists are still unable to find out how yeast traveled from South America to the caves and monasteries of Bavaria where lager beer was born. But lager beer brewing began in the 16th century, about the same time as the rise of trans-Atlantic trade, so the yeast, a microscopic stowaway destined for great things, may have hitched a ride on a sailing ship, perhaps on a piece of wood or in the stomach of a fruit fly.   

The newfound yeast fused with a distant relative, Saccharomyces cerevisiae, which was used for millennia to make leavened bread and fermented wine and ale.

The resulting hybrid, representing a marriage of species as evolutionarily separate as humans and chickens, would give us lager, the clear, cold-fermented beer first brewed by 16th century Bavarians.

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. 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.

Now, researchers have identified the wild yeast that, in the age of sail, apparently traveled 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.

Researchers from Portugal, Argentina and the United States teamed up in the hunt for the yeast, dubbed Saccharomyces eubayanus, whose genome was sequenced and confirmed as a near-perfect match as a parent of the lager yeast hybrid that made cold-temperature fermentation and lager beer possible.

People have been hunting for this thing for decades, said Chris Todd Hittinger, a University of Wisconsin-Madison genetics professor. 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.

Scientists said the gene of the newfound yeast, which is prevalent in the southern beech forests of Patagonia and distinct from every known wild species of yeast, was 99.5 percent identical to the missing half of the lager yeast.

Beech galls are very rich in simple sugars. It's a sugar-rich habitat that yeast seem to love, adds Hittinger.

The researchers didn't set out to find out where lager beer came from. But the secret ingredient in the beer popped up in a broader search for new yeasts.

The detective work began in Europe, which seemed logical, but no yeasts there fit the profile. Instead, they found what turned out to be the missing piece an ocean away.

Researchers also identified genetic mutations in the lager yeast hybrid distinctive from the genome of the wild lager yeast. Those changes accumulated since those first immigrant yeasts melded with their ale cousins 500 years ago, and refined the lager yeast's ability to metabolize sugar and malt and to produce sulfites, transforming an organism that evolved on beech trees into a beer-making machine.

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

That cold-loving hybrid then evolved into the modern lager yeasts that are used today in breweries throughout the world, acquiring several genetic changes that altered their sugar and sulfur metabolism.  

It was fun trying to track down the mystery yeast, but it also was important, said Mark Johnston, chairman of the department of biochemistry and molecular genetics at the University of Colorado School of Medicine.