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

Brewers today want to improve yeast, and genetics is helping them to understand their origins and how the yeast can be bettered.

Carlsberg master brewer Jeppes Andersen says it takes about 20 days to make beer. Behind him, in stainless steel, is his miniature research brewery in Copenhagen. There are all sorts of ways new yeast strains could help the business. Fermentation takes 15 days, but a “super yeast”, if discovered, could shave off one or two days and ramp up a brewer’s efficiency, for instance.

A matter of taste

Carlsberg recently introduced new barley after screening 20,000 varieties. All Carlsberg’s pilsner beers are brewed with this new variety, which reportedly gives a better flavour, a longer shelf life and a better head.

“This is a good job,” says Andersen. “Beer is a good product to work with.” Taste matters. It’s not all science; there’s craft too.

Back in Trinity, Dr Bond’s lab has hosted “beer parties” for the benefit of science, and extracted yeasts from various craft beers. They sequence the yeast DNA and look at the brewing processes.

“The yeast and the process very much influence the taste.” Her lab is now writing up a paper to sketch out the lager yeast’s family tree.

BIRTH OF A BEER: HOW LAGER CAME TO BE

Lager is made from sprouted barley seeds [malted barley] or other grain added to water and heated. Hops are then added. Those who first made lager were unaware that yeast was also a vital ingredient, fermenting the “wort” mixture into an alcoholic beverage.

Almost 500 years after lager yeast first made lager beer, scientists are beginning to work out its origins.

“It seems two yeast species fused together to produce a new species, lager yeast. It was only in 2011 when one of the parents was discovered. It was not found in Europe. It was found in Patagonia on [beech] tree bark,” Dr Ursula Bond, brewer yeast expert at Trinity College Dublin, explains. The ease with which it was found in South America suggests that the species may have come to Europe only after transantlantic trade began, so it seems there is a New World spin to European lager.

The hybrid was probably created in the 1500s. “The species is still very new and has plenty of plasticity in its genome, with lots of genes in multiple copies from both parents. We know that a lot of those genes will be lost over time, but right now in evolutionary terms it is brand new,” says Dr Bond.

Beer as a product can be sensitive to heat and light, which is why it is traditionally kept in dark bottles. Hops, the flowers of the hops plant,was added for its bitter, tangy flavours, but also has antibacterial properties.

Dr Bond is searching for natural compounds that could be added to improve beer preservation. “We are trying to discover natural antimicrobial agents that could be added to the brewing process to prevent beer spoilage by bacteria. We have engineered yeast strains with such compounds, though this is a proof of principle.”

The brewing industry steers clear of genetic modification.

Bacterial contamination continues to be a costly problem in breweries around the world and Bond hopes her lab can help save good beer from going bad.