The genetics of beer yeast is far more complex than previously thought, a detailed analysis of the world’s favourite fungus has found.
In a paper published in the journal Nature Ecology and Evolution, researchers led by Brigida Gallone from the VIB–KU Leuven Centre for Microbiology in Belgium reveal that Saccharomyces cerevisiae – commonly known as brewers’ yeast – and three members of the same genus have swapped genetic material so rapidly and comprehensively that some European breweries harbour unique strains.
Saccharomyces cerevisiae has been the microbial driver in the production of ale for millennia – indeed, for a very long time before its existence and role were known.
At some point in the fourteenth century in Nuremberg, Germany, the species hybridised with another from the same genus – the identity of which was only confirmed less than a decade ago – to produce a new species later classified as Saccharomyces pastorianus.
This turned out to be immensely important, because left to its own devices S. cerevisiae turns barley and hops into ale, but S. pastorianus churns out lager.
The fact that lager-yeast was a hybrid was discovered around 1985, but the identity of the other genetic contributor was the subject of hot debate until 2011, when a group of geneticists identified it conclusively as Saccharomyces eubanus.
It was a discovery that in some ways raised more questions than it answered. Specifically, wild S. eubanus was known only to occur in Argentina, which meant it must somehow have reached Germany significantly before Europeans set foot in South America.
In 2014, however, the species was also found in China and Mongolia, suggesting a rather more plausible route into Europe along the Silk Road.
Two other Saccharomyces species have also been found to be involved in the fermentation process – S.uvarum, more closely associated with wine and cider, and S. kudriavzevii, linked to wine and ales. The ancestry of both is still uncertain.
In the latest research Gallone and colleagues shed light on why their heritage is such a complicated matter.
For several years the group collected yeasts from different industrial niches – including very many fermentation tanks in very many breweries – and then sequenced the haul.
As expected, most of their samples contained S. cerevisiae and the hybrid S. pastorianus was well in evidence in the lager ones. To their surprise, however, they found that roughly 25% of the samples were more complex hybrids. And these freshly identified yeast strains cropped up not just in lagers, where hybridisation had occurred before.
“Many were collected from other beer niches, such as Trappist beers, spontaneously fermented ‘Lambic’ beers and old beer bottles or equipment,” the researchers write.
Indeed, Belgian beers, such as those made over centuries by monks, feature the greatest diversity of previously unknown yeast hybrids. Fermentation in many of them is driven by yeast that combines genetic material from two, three or all four of the Saccharomyces species known to be associated with brewing.
This, say the researchers, reveals how microbes adapt to the challenges presented by the novel ecological niches contained in human-made environments – specifically, in these cases, the environmental conditions found inside breweries.
The hybridisations occurred as challenges arose – yeast genomes operating in colder environments, for instance, would favour genes that optimised performance in low temperatures, while breweries in more southern, warmer areas of Europe would offer opportunities to strains better adapted to higher temperatures.
All of this makes Belgium the primary hotspot for yeast diversity.
“Traditional Belgian beer styles harbour a remarkably diverse array of yeasts, probably as a result of the continued use of old beer brewing practices,” Gallone and colleagues write.
“For example, the production of Belgian Lambic beers has remained unchanged for centuries. The use of medieval brewing technologies, such as open-air inoculation and long-term fermentation and storage in barrels stored in the brewery’s cellar, promoted the survival of the unique yeast hybrids presented in this study, and could therefore provide a source of new biodiversity for industrial applications.”
Modern beers, they note with a faintly detectable touch of sadness, are brewed using yeasts produced in sealed, tightly controlled bioreactors, and are thus highly homogenous. This has, they write, “probably contributed to the decline of the natural beer yeast diversity”.