has brewed wine and beer from dozens of different types of yeast. But not necessarily for drinking pleasure. It鈥檚 all in the name of scientific research.
Fay, an associate professor of , studies yeast in order to tackle bigger questions about evolutionary biology. For example: What is the genetic basis of evolutionary change? What is the basis of differences among species or among individuals within a species?
鈥淲hether that鈥檚 differences in temperature preferences or differences in certain domesticated brewing characteristics, we want to know what genes are involved,鈥� Fay says.
In a new paper published in the journal , Fay and his colleagues report some intriguing findings about a specialized strain of commercial yeast called Saccharomyces cerevisiae, also known as brewer鈥檚 yeast. S. cerevisiae has been used to make beer for thousands of years and is genetically distinct from wild populations of yeast. But while brewer’s yeast has specialized characteristics, its historical origins have been difficult to determine because yeast was around long before people discovered microorganisms.
Fay and his colleagues set out to unravel the complex pedigree of brewer鈥檚 yeast by analyzing its genome. What they found was surprising: the genome of brewer鈥檚 yeast is actually derived from a combination of the yeast strains used to make European grape wine and the yeasts used to make Asian rice wine. The results provide information about the domestication of organisms and may offer brewers insights that can lead to novel, new beer flavors.
A deep dive into beer yeast
Just as wolves became domesticated dogs and wheat became a domesticated crop with the spread of agriculture, beer yeast or brewer鈥檚 yeast was domesticated from its wild ancestors.
Brewer鈥檚 yeast is one of three essential ingredients鈥攁long with water and a carbohydrate such as barley鈥攖hat make up beer. Brewer鈥檚 yeast is known for its strong fermentative characteristics that, in the presence of oxygen, allow it to convert sugars from the carbohydrate into alcohol; it has gained the ability to competitively dominate other species in high-sugar, low-nutrient environments.
Most beers, especially Western beers, are one of two types鈥攁les or lagers. Cell division within the yeast鈥攁nd the temperature鈥攁re essential in determining the beer鈥檚 flavor.
鈥淕enome changes that occurred during cell divisions have had a clear impact on subsequent beer strain diversity and have likely played an important role in specialization to various brewing styles,鈥� Fay says.
Wild yeast strains, for example, work in completely different ways than traditional brewer鈥檚 yeast, from the temperatures at which their mitochondria undergo fermentation to the ways their cells stick together鈥攄etermining whether the yeast will rise to the surface or sink to the bottom during fermentation. These characteristics, in turn, affect a beer鈥檚 taste.
Tracing the origin of beer yeast
It can be difficult for researchers to trace the origins of domesticated organisms because an organism鈥檚 lineage is often clouded by migration, gene flow, and intermixing with other groups. But the researchers found that brewer鈥檚 yeast has several unique characteristics that allowed it to remain relatively isolated from other species. For one, brewer鈥檚 yeast is polyploid, meaning it has three or four copies of the genes in its genome. Human diploid cells, by comparison, have two copies of 23 chromosomes (46 chromosomes total) and human haploid gametes (egg and sperm) have only one copy of 23 chromosomes.
Because it is polyploid, brewer鈥檚 yeast does not easily combine naturally and exchange genetic material with its wild ancestors. That leads to less contamination, allowing researchers to more easily trace the strain鈥檚 lineage. 鈥淧olyploidy provided the yeast strain with a means of remaining isolated from other populations and provided us with a living relic of its ancestors,鈥� Fay says.
To reconstruct the history of modern ale strains, Fay and his colleagues sequenced and compared the genomes of modern brewer鈥檚 yeast to a panel of reference strains. That鈥檚 how they discovered that the genomes of the modern beer strains were similar to the genomes of European grape wine strains and Asian rice wine strains. The researchers conjecture that modern brewer鈥檚 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.
More unique flavors
While yeast has traveled the world, brewers have used different strains of S. cerevisiae in their beers. Fay鈥檚 research may open doors for even more unique brews.
鈥淏eermakers are experimental, always wanting to try new things and make their beers distinct,鈥� he says. 鈥淰ery recently鈥攁nd our study will add to this鈥攂eermakers have been exploring using other yeast strains besides the typical commercial beer strains.鈥� These experimental techniques have included open-casket fermentation, which involves a mixture of bacteria and yeast species, and utilizing wild strains such as in Heineken鈥檚 wild lager series.
鈥淩ight now you can make a beer out of the large number of brewing strains that are available. If you really want to be creative and do something different, you could go out and use a wild strain of yeast, but you need to be able to combine the beneficial characteristics of the yeast strains that are commercially available with novel characteristics.
鈥淜nowing more about where beer strains came from will help facilitate that.鈥�
