'New World' yeast for perfect pint
For lager lovers worldwide, it is probably the most highly prized micro-organism in the world, but the identity of the yeast that made beer possible has been a long-standing mystery.
Now scientists believe they have unmasked the microbe - a stowaway that sailed to Europe from the New World 500 years ago.
The yeast ended up in the caves and monasteries where beer was brewed in Bavaria. There it crossed with a distant relative, a yeast used for millennia to make bread and ferment wine and ale. The rest was history.
Lager, first brewed in 15th-century Germany, is now one of the world's most popular alcoholic beverages.
The yeast conferred characteristics that for the first time meant that beer could ferment in the cold.
Writing in the journal Proceedings of the National Academy of Sciences, an international team of researchers describes how they discovered the organism after an exhaustive search.
Dubbed Saccharomyces eubayanus, the yeast was traced to Patagonian beech forests at the tip of South America. It lives on sugar within beech galls, causing spontaneous fermentation that generates alcohol.
Analysis showed it was unlike any other known species of wild yeast, but 99.5% similar to the unidentified half of the lager hybrid.
"People have been hunting for this thing for decades," said Professor Chris Hittinger, from the University of Wisconsin-Madison in the US. "And now we've found it. It is clearly the missing species."
The yeast may have been transported to Europe on a piece of wood or in the stomach of a fruit fly, the researchers believe. Genetic mutations accelerated by the brewing process refined the lager yeast's ability to produce cold beer.