Discovering the Wild Side of Yeast

Some of the finest things on Earth—among them beer, bread, and wine—depend on yeast. But after more than 5,000 years of reliance on its powers and endless modern research into yeast genetics, we knew almost nothing until recently about its natural history. The many strains of Saccharomyces cerevisiae, the fungus we call yeast, were considered entirely domesticated—the cats and dogs of fermentation.

That began to change a few years ago, when researchers in Italy discovered that yeast had a wilder side. It summered on ripening fruit, which is how we first discovered its magical ability to turn humble grapes into wine. But it wintered, the researchers reported, in the guts of social wasps as the wasps were hibernating, sealed within the trunks of trees.

With a new study published in the journal Proceedings of the National Academy of Sciences, the natural history of yeast has just become even richer. If Saccharomyces cerevisiae, the common yeast we buy at the grocery store, is the household pet of brewing, Saccharomyces paradoxus is its wild cousin, the wolf running through the woods. Although geneticists recognized that some of the most successful yeasts in industrial fermentation represented crosses between the two, they didn’t know where the crossover came from.

Now Duccio Cavalieri and his coauthors at the University of Florence report that cerevisiae hears the call of the wild and mates with paradoxus within the intestines of hibernating wasps. Using yeasts with easily identifiable genetic markers, the researchers fed the two different species to wasps. Conditions in the foregut cause the yeasts to make reproductive spores, contained in a sort of sac called the ascus. Conditions in the lower intestine later break open the sac, releasing the spores. That’s where gametes of domestic and wild yeasts fuse, producing a hybrid.

Other scientists have already put the earlier research on wild yeast to work for the lofty purpose of brewing better beer. “Wild yeasts have always been considered a contaminant in brewing, and you want to keep them out because they create off-flavors,” John Sheppard, a bioprocessing specialist at North Carolina State University told National Geographic. “Actually, the opposite was true. We got some really nice fruity, flowery esters that gave it a nice taste and aroma combined with a moderate sourness.”

Cavalieri also believes that understanding the natural history of yeast could lead to better wine. He gathers some of the wasps for his studies from the grounds of Tuscany’s celebrated vineyards and suspects that the community of yeasts and social wasps “contribute to the end product in terms of taste and aroma. Without them, maybe Montepulicano and Montalcino would not be the same.”

What do the wasps get out of this arrangement? In the summer, said Cavalieri, the wasps are the lions of the ecosystem, and the vineyards are their savanna.

Wasps are of course attracted by sugar, as anyone who has discovered a yellow jacket in a can of soda may painfully remember. But they are also predators. “In particular, they love eating other insects. They eat Drosophila in large amounts. They eat honeybees. They eat other wasps.”

But during their long winter hibernation, the wasps are vulnerable. Their nests are hidden in oak tree trunks, and the tiny entrance is sealed shut with their saliva. There, “they must survive three months with the danger of cold weather and infection. We think that maybe yeast can protect them.” This may be why wasps are powerfully attracted to the aroma of yeast as well as to sugar. “We really do think there is some unknown interaction” for which the wasps depend on the yeast.

Cavalieri argues that understanding this natural history could change vineyard methods. One of the less romantic sides of harvesting grapes has to do with the abundance of wasps feeding on the ripe fruit. Workers routinely get stung, so vineyard operators naturally reach for the nearest pesticide. “We think there is a benefit to not killing them at harvest,” said Cavalieri. “We think the wasps provide an ecosystem service, increasing the diversity of fermented products.”

Vineyards also typically make heavy use of fungicide against various pathogens—but at the risk of reducing the diversity of yeast in the process. Some vineyards, he said, have slowly begun to experiment with organic methods. They’re “not going complete organic, but when they start to recognize the importance of biodiversity, they start to do things in a thoughtful way. They can intervene only when they really need to and not interfere with important ecological processes.”

The hidden power of the natural world—of yeasts, social wasps, and oak trees—can thus find its way into the glass of beer or wine that is one of life's more pleasurable rewards.