Rochester biologist Justin Fay studies Saccharomyces cerevisiae, also known as baker’s yeast or brewer’s yeast.
The COVID-19 stay-at-home orders have fostered a trend of at-home baking, in which amateur breadmakers, like master bakers and brewers, are beginning to experiment with various strains of baker’s yeast and sourdough starters.
The strains of yeast used to make beer, bread, and wine come from the species of yeast called Saccharomyces cerevisiae. S. cerevisiae is known as “brewer’s yeast” or “baker’s yeast” for good reason: it’s responsible for the fermentation that makes beer alcoholic and allows a lump of dough to rise into a loaf of bread. In the presence of oxygen, S. cerevisiae converts sugars from a carbohydrate—such as flour involved in breadmaking or barley involved in beermaking—into carbon dioxide. The carbon dioxide inflates air bubbles within the dough, causing the dough to rise.
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Justin Fay, a professor of biology at the University of Rochester, studies the complex ancestry of S. cerevisiae in order to tackle big questions about evolutionary biology, including how species differentiate.
“S. cerevisiae is a fantastic model organism,” Fay says. “It was the first eukaryote to have its genome sequenced. That, combined with the awesome power of yeast genetics, make it one of the most attractive organisms to understand the genetic basis of evolutionary change.”
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But understanding S. cerevisiae may also help your at-home baking rise to a new level.
Baker’s yeast, domesticated from wild strains, is derived from a combination of the yeast strains used to make European grape wine and the strains used to make Asian rice wine.
Just as wolves became domesticated dogs and wheat became a domesticated crop with the spread of agriculture, baker’s yeast was domesticated from its wild ancestors.
Human-associated migration and mixing with wild populations of yeast had a strong impact on S. cerevisiae, but its historical origins have been difficult to determine because yeast was around long before people discovered and studied microorganisms. Fay and his colleagues have discovered that the genome of S. cerevisiae is derived from a combination of the yeast strains used to make European grape wine and the yeasts used to make Asian rice wine. They conjecture that modern baker’s yeast is derived from a melting pot of fermentation technology, resulting from an East-West transfer similar to the spread of domesticated plants and animals by way of the Silk Route, thousands of years ago.
Baker’s yeast has been domesticated in ways that aid the fermentation process (and results in better flavor).
“Most organisms do not ferment sugar into alcohol, but yeast does,” Fay says. “Yeast’s ability to do this makes it incredibly useful and also raises questions about how yeasts evolved this ability.”
While wild yeast strains can also ferment sugar, they have many undesirable characteristics that often making them an unpopular choice for making beer, wine, and bread. Wild yeast strains are genetically different than domesticated baker’s yeast. One difference is the ways in which their cells stick together and divide, determining how fast the fermentation process occurs and whether the yeast will rise to the surface or sink to the bottom during fermentation. Wild yeast often leads to unpredictable fermentation, which can then result in unfavorable flavors and aromas.
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Humans have put selection pressures on domesticated baker’s and brewer’s yeast to allow it to begin the fermentation process more quickly, to more efficiently withstand processing conditions such as freezing and drying, and to more efficiently break down and metabolize the complex sugar maltose. Maltose is the primary sugar present in beer wort—also known as the “beer starter,” before the beer is fermented—and in baked products.
“Wine strains are resistant to sulfites, which is helpful to avoiding spoilage,” Fay says. “Beer and baking strains have increased maltose metabolism.”
Not all S. cerevisiae strains are the same; different strains are used to make various flavors of beer, bread, and wine.
Since S. cerevisiae has been domesticated, it has itself evolved into various strains. The baker’s yeast typically used in bread dough comes from a handful of domesticated strains of S. cerevisiae manufactured by yeast companies such as Fleischmann’s, Red Star, and SAF. The “starter” used to make sourdough breads, however, can be more varied and is typically a mix of commercial, domesticated strains of S. cerevisiae and wild strains.
A 2020 study published in the journal Frontiers in Genetics found that sourdough and commercial baking strains of yeast produce significantly more carbon dioxide than wild strains and achieved fermentation faster, affecting aroma and taste.
A yeast’s environment can also greatly affect its genetic diversity and lead to more desirable traits. Beer yeasts are often kept indoors in a brewing environment, limiting their contact with wild yeast. They are reharvested after the fermentation process, resulting in continuous genetic selection according to the indoor brewing environment. The different strains of beer yeast can determine the flavor of, for instance, a lager versus an ale. Wine yeasts, on the other hand, spend most of the time outside in and around vineyards, resulting in more hybridization with wild yeasts.
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