Yeast strains used today to brew lager have two genetic ancestors, not one as previously thought.

The discovery may explain the origins of the two major categories of lager today, described in the trade as the “Saaz” beers such as Pilsner and Budweiser, and the “Frohberg” beers such as Orangeboom and Heineken.

It turns out that both probably owe their origins to laws in 16th-century Bavaria that banned brewing in the summer because scorching heat ruined the ale that was brewed before the emergence of lager.

Forced to produce their beer in the winter, brewers accidentally created conditions favouring the emergence of a hybrid yeast better suited to the cold. Researchers already knew that Saccharomyces pastorianus, now used to brew lager, is a hybrid produced through marriage between two yeast strains.


Brewing’s founder

One was S. cerevisiae, the “brewer’s yeast” on which the brewing industry is founded because it ferments sugars into alcohol so efficiently. The other was S. bayanus, a yeast strain seldom used alone in brewing because it ferments sugar into alcohol far less efficiently.

Now an analysis of the forensic ancestry of lager yeast has established that this same marriage happened independently at least twice, not once as previously thought, giving 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,” says Gavin Sherlock of Stanford University, who conducted the study with colleague Barbara Dunn.

They discovered the double emergence of an ancestor after analysing 17 samples of lager yeast originally archived between 1883 and 1976. They found that the yeasts broadly fell into two groups.

Those in Group 1 were used to brew the “Saaz”-type

beers. Those in Group 2 were used to brew the “Frohberg” lagers.

Family differences

Sherlock and Dunn report that although the two types of hybrids shared the same parentage, they differed from one another considerably.

The Group 1 yeasts, for example, were true hybrids, with one set of genes from each parent. The Group 2 yeasts had an extra copy of S. cerevisiae, making them “triploid” hybrids.

Very little genetic material from S. bayanus has been lost from either hybrid, probably because its capacity to produce energy at low temperatures from mitochondria turned out to be an indispensable asset in the cold beer cellars of 16th-century Bavaria.

By contrast, S. cerevisiae had jettisoned much of its genetic material in the Group 1 yeasts, maybe to reduce unnecessary energy expenditure.

In Group 2 yeasts, however, S. cerevisiae remained more intact, maybe because two copies of the genome were available to cushion losses of individual genes.

The researchers also noticed that both yeasts contained multiple copies of genes beneficial to brewing, such as those that ferment maltose. Likewise, 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,” he says. “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.”

Journal reference: Genome Research DOI: 10.1101/gr.076075.108

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