Image caption The yeast was isolated from fungus growing on Patagonian beech trees

Scientists have identified a yeast that led to the discovery of lager.

The researchers isolated the new species in the frozen forests of Patagonia in South America.

Their discovery suggests that this yeast crossed the Atlantic hundreds of years ago and combined with one traditionally used in Europe to make ale.

The discovery is described in the Proceedings of the National Academy of Sciences.

A lucky find

The workhorse of brewing, the yeast Saccharomyces cerevisiae, is used worldwide to ferment fruit and grains to make wine, cider and ale.

Lager, which is fermented more slowly and at lower temperatures than ale, is presumed to be a later invention, and was likely stumbled upon when Bavarian monks moved their beer barrels into caves for storage.

In those caves, Saccharomyces cerevisiae, which prefers to grow just above room temperature, is presumed to have been outcompeted in the fermenting beer by a species that thrived at cooler climes.

The modern-day lager-brewing yeast, Saccharomyces pastorianus, which is a fully domesticated species, is probably a hybrid of this cool-loving strain and the ale-brewing species, and survives because brewers keep back a little of the lager each time to seed the next batch with the same yeast.

Lager's cradle

"The hybrid almost definitely formed accidentally and people adopted it because the beer came out differently," said evolutionary biologist Chris Hittinger from the University of Wisconsin in Madison, US, who was one of the team behind the discovery.

Image caption The UK consumes 20 million barrels of lager each year

But researchers have long wondered where the original cool-loving yeast species came from.

That is until Dr Hittinger and his colleagues isolated it from a beech tree in the forests of Patagonia this year.

These forests, where daily lows average around -2C, are the perfect cradle for modern-day lager-brewing yeast. The species has been designated Saccharomyces eubayanus.

"I personally prefer lagers to ales, and I am very grateful that these two distant cousins met up in a Bavarian cellar hundreds of years ago," Dr Hittinger told BBC News.

Knowing the ancestral strain to the modern day lager-brewing yeast will help scientists pinpoint the effects of domestication in the genome of brewing yeasts.

And there is also the possibility that there are other undiscovered species of yeast in those Patagonia forests that could become the next best brew.