London, September 11 : Geneticists at the Stanford University School of Medicine have discovered that yeast strains used to brew the present day lager have two genetic ancestors, not one as previously thought.
The researchers say that their findings may be helpful in understanding the origins of the two major categories of lager made these days - the 'Saaz' beers like Pilsner and Budweiser, and the 'Frohberg' beers such as Orangeboom and Heineken.
They already knew that the yeast Saccharomyces pastorianus, used to brew lager these days, is a hybrid produced through marriage between two yeast strains.
One was S. cerevisiae, the "brewer's yeast" on which the brewing industry is founded because it ferments sugars into alcohol so efficiently, while the other was S. bayanus, which is seldom used alone in brewing because it ferments sugar into alcohol far less efficiently.
Analysing the forensic ancestry of lager yeast, the researchers have now come to the conclusion that the same marriage happened independently at least twice, not once as previously thought, and gave rise to two broad families of lager beer.
Although both probably emerged during the Middle Ages in central Europe, their point of origin cannot be traced exactly.
"We can't say for sure when, where or by whom they were isolated," New Scientist magazine quoted Gavin Sherlock, who conducted the study with colleague Barbara Dunn, as saying.
When the researchers analysed 17 samples of lager yeast originally archived between 1883 and 1976, they found that the yeasts broadly fell into two groups.
Those in the first group were used to brew the 'Saaz'-type beers, while those in the second group were used to brew the 'Frohberg' lagers.
Sherlock and Dunn say that the two types of hybrids, though share the same parentage, differ from one another considerably.
The researchers also observed that both yeasts contained multiple copies of genes beneficial to brewing, such as those that ferment maltose. According to them, genes that mar the process had been lost.
Sherlock doubts whether the analysis will lead to ways of engineering new flavours and properties into beers.
"The pastorianus strains, being hybrids, are sterile, so you can't do genetics on them in a straightforward way. Rather, beer makers have been doing this via natural selection over the past several centuries, selecting those strains for further use that produced the beers they most enjoyed drinking," he says.
A research article describing the study has been published in the journal Genome Research.