The Secret to Belgian Ale? Medieval Super-Yeasts

(CN) – Classic Belgian beers, some of the most famous ales in the world, are fermented with rare, hybrid yeast species, according to a study published Monday in Nature Ecology and Evolution.

A team of researchers have recently discovered that the secret to true Belgian specialty beers, specifically Gueuze and Trappist beers, is a special strain of yeasts created when two entirely separate yeast species crossed DNA. The researchers found the two yeast species, the common ale yeast Saccharomyces cerevisiae and the cold-tolerant wild yeast Sascharomyces kudriavzevii, crossed to form “medieval super yeasts” that have been used by Belgian brewers in their signature ales for centuries.

Kevin Verstrepen, Professor at the University of Leuven and one of the leaders of the study, noted this kind of hybridization in nature is a remarkably uncommon occurrence, and almost never results in a species that can live as long as these super yeasts.

“Crosses between species are rare in nature, and they normally represent an evolutionary dead end because the hybrids are not fertile and thus cannot produce offspring (see for example mule [donkey x horse], liger [lion x tiger], wolphin [whale x dolphin]),” Verstrepen said in an email. “With yeast, however, things are different because they can reproduce sexually (necessary to make the cross in the first place) and a-sexually (i.e. by making clones of themselves, like bacteria). The latter is still possible for the hybrids, and thus they were able to survive.”

Researchers found that when the two different yeasts crossed with one another, the resulting yeast type carried the best characteristics of both of its parents. With the Saccharomyces cerevisiae offering up some much-needed fermentation capabilities and the Saccharomyces kudriavzevii’s natural stress resistance and aromatic characteristics, the hybrid yeasts born from this combination are equipped with certain genetic advantages not normally found in other yeasts.

Verstrepen reports that when this discovery was made, it came as a sizeable surprise to everyone involved – including the brewers.

“Funnily, no one, including the brewers, knew that they were using such special yeasts – it was a big surprise to everyone.  When we started sequencing the complete DNA of hundreds of beer yeasts, we simply noticed that some yeasts seem to contain DNA of two separate species.  First, we thought it was an error or contamination in our experiments, but we now know that these yeasts indeed contain DNA of two different species because they are a cross between two species,” Verstrepen said.

The study reports that many mysteries continue to surround these hybrid yeasts, and that future research is certain to shed further light on the nature of these uncommonly super species.

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