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Yeast is yeast? Not so when it comes to beer flavours

Researchers at Carlsberg Labs and in Trinity College are examining the genetic make-up of lager yeast in an effort to improve beer quality and develop new flavours

Beer disease was the curse of brewers and the drinking classes in the 19th century. Large amounts of lager could be brewed, but quality was erratic. The problem was solved in 1893 when Emil Christian Hansen of the Carlsberg Laboratory in Copenhagen realised that contamination from wild yeast was to blame.

“He made a pure lager yeast by diluting the yeast so much that there was only one cell left,” explains Dr Marie Bojstrup, senior scientist at the Carlsberg Lab. There was a marked difference in the very first brew that used this method, she says proudly. Famously, Carlsberg then made its pure yeast, Saccharomyces carlsbergensis , freely available to competitors.

Today, European lagers are often made with just malted barley , water , hops and yeast derived from the Carlsberg strain. Scientists who want to improve beer quality and develop new flavours look to the genetics of yeast, the single-celled maestros that convert carbohydrates to carbon dioxide and alcohols.

Magic ingredient

Microbiologist Dr Ursula Bond at Trinity College Dublin says different strains of yeast contribute to the different kinds of beer. Craft beers can range from cloudy gold to clear amber, taste malty with raspberry tones or perfumed with a hint of nutmeg, yet only the yeast is different. And yeast can conjure up hundreds of flavour and aroma compounds.

Dr Bond's lab is probing the genetic make-up of lager yeast and trying to understand strains' origins. “We're trying to tease out what makes them unique and useful for lager beer,” she says. “Ale yeast is a different kind, and we recently discovered that the yeasts used for making stouts are different again.” The brewer yeast does not live in the wild, but hundreds of strains evolved in breweries. Yeasts belong to the fungi kingdom.

Fermentation by yeast turns a stodgy mix of grain and water called “wort” into an alcoholic drink with light fizz: lager. Unlike ales, lagers require slow, low-temperature fermentations. Lager brewing began in Bavaria in the 15th century and was not permitted in summer, so lager yeast evolved for cold conditions.

“We have discovered specific regions of the genome that allow us to distinguish different groups of yeast,” Dr Bond explains.

“Altering or improving the flavour of beer is something people would like to do, but it is not easy. There are many genes controlling the myriad of enzymes required to produce the end flavours in beer, and subtle changes in enzyme levels can have unexpected effects.”

It was recently discovered that the enigmatic brewer's yeast Saccharomyces carlsbergensis (or S pastornianus ) is actually a hybrid (see panel). Carlsberg confirmed this in a report this summer.

They also probed the genetics behind a traditional split in lager yeasts into a Bavarian and an eastern European type – group I (Czech and Carlsberg beer) and group II (Weihenstephan and Heineken). The founder of Carlsberg had travelled to Munich in 1845 to obtain his yeast.

In the name of science, an 1886 bottle of Carlsberg beer was uncorked to obtain the original strain of lager yeast used. The beer tasted like port wine, says yeast scientist Dr Andrea Walther at Carlsberg Labs. “Our tasting panel said it was sherry-like, chocolate, with strong fruity aromas.”

When they tried the old strain, the beer was not so good. But after using a handwritten 19th-century protocol, “it was one of the best beers I've ever had”, Dr Walther recalls. The old way was to ferment at 4 degrees, not 17 degrees like today.