Beer and the Evolution of Yeast

For the UAS Whalesong

Typically, when someone thinks of yeast, they think: fermentation. Or maybe just about bread or beer and wine. There is a surprising amount of research revolving around yeast ranging from health benefits to potential biofuels. However, the focus of this article is about the evolution of yeast in relation to the beer making process.

So, what is yeast exactly? For most of us we know that it comes in little packets or in glass jars in the store and is pretty small and brown in color. Yeast is a microorganism belonging to the kingdom Fungi. They are able to convert fermentable sugars into alcohol along with other by products. According to the United States Department of Agriculture, there are roughly 1500 species of yeast. However, there are likely to be more because of the extensive biodiversity regarding yeast. Humans have been making alcohol for thousands of years. However, my focus is specifically on beer.

Beer is broken down into two categories: ales and lagers. Ales are the older of the two with their production having been able to be tracked back some 5000 years. Within the ale category, one can typically expect more intense flavors. Yeasts used in ales are commonly known as top-fermenting yeast because they will rise to the surface during the fermenting process and the result is a fairly thick yeast head. Ales are fermented at temperatures between 10-25°C.

Lagers, on the other hand, are relatively young, only having been around for several hundred years. The strains of yeast used here are fermented at relatively cool temperatures that are between 7-15°C. The yeast used here are commonly known as bottom fermenting because they settle out at the  bottom when the beer is near completion. These yeast strains are also noted to grow more slowly than those used in ales. Maybe not surprisingly, lagers brewed within the United States are often very different than their European counterparts.

When making beer, temperature and yeast strain are very important because the flavor of the beer depend heavily on those two components. Flavor and aroma of beer can be a fairly complex affair. A couple of favorable byproducts of fermentation are: acetaldehyde, which has a green apple aroma, and diacetyl, which can taste and smell buttery or like butterscotch.

One of the original ale yeasts is Saccharromyces cerevisiae.

Lagers apparently came into being when Bavarians (a region in the southern part of Germany) observed that beer that was stored in caves over winter continued to ferment. They also noticed that this beer had distinct characteristics which set it apart from ales.

In 2011 a group of researchers discovered that in lagers the yeast is a hybrid strain composed from two different species: S. cerevisiae and S. eubayanus. The lineage of these two strains, however, has remained a mystery until recently.

Using next-generation sequencing technology several researchers were able to compose a high quality genome of S. eubayanus. Then then compared this yeast genome to other strains that are used when brewing lagers. Ultimately, this allowed for the for people to compare complete genomes from parental yeast strains. What makes this even more fascinating is that they found that there were two independent origin events for S. cerevisiae and S. eubanyus hybrids.

Chris Todd Hittinger of the University of Wisconsin-Madison, a coauthor in the study analyzing the genomes noted, “Lager yeasts did not just originate once. This unlikely marriage between two species, genetically as different from one another as humans and birds, happened at least twice. Although these hybrids were different from the start, they also changed in some predictable ways during their domestication.”

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