How yeast works: A layperson’s guide to saccharomyces

What is a strain?

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Yeast cells under a microscope // Photo courtesy of Northern Brewer

We often hear brewers refer to different types of S. cerevisiae or S. pastorianus as a “strain” of yeast. A yeast strain would be defined by a microbiologist as a genetic isolate that can be distinguished from other isolates of the same species by DNA variations that express themselves through different, specific characteristics.

What this means for brewers is that these characteristics—flavors, aromas, and behavior like flocculation or alcohol tolerance, which are passed from mother cell to daughter cell—can be replicated from batch to batch, allowing for predictable fermentations and consistent beer.

Creator of flavor

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A yeast starter at Northern Brewer // Photo by Aaron Davidson

It’s difficult to understate the importance of yeast to the finished beer we pour into our glasses: this little fungus can generate over 500 flavor-active compounds during fermentation, and the vast majority of beer aromas and flavors we experience are at least affected by, if not created by, yeast.

In their mission to survive by glycolysis, the cells produce more than just ethanol and CO2. A small percentage of their output is comprised of a range of those 500-odd flavor-active compounds: esters, phenols, diacetyl, fusels, aldehydes, and more. How perceptible these byproducts are depends greatly on the brewer’s choice of yeast strain and the environment into which it’s introduced; whether they’re desirable depends greatly on the beer style and the drinker.

There’s both nature and nurture at play here. Yeast strains like those for Belgian saison or Bavarian hefeweizen have a gene that triggers production of phenolic compounds as part of their metabolic process, which gives those beers their signature spicy, peppery character. In lager strains, this gene is set to “off,” and so their fermentations will lack those phenolic compounds.

Being a living organism and sensitive to its environment, yeast’s fermentation byproducts are affected by environmental factors: warm fermentation causes rapid growth, which leads to higher ester production, while excess oxygen can cause higher levels of fusels.

Today, many beers are filtered or fined to separate yeast, so we just taste what they’ve left behind. Unfiltered cask ales or bottle-conditioned beers, on the other hand, are “live” products, with further flavor development fostered by the yeast that’s still in contact with, and working on, the packaged beer. An extreme example are weissebiers or some Belgian ales, where sedimented yeast is roused and reintroduced to create a cloudy appearance and nutty, bready flavor.

Tens of trillions of yeast cells likely went into the tank that produced the beer I hope you’re enjoying right now, so let’s pour a little out for all our microscopic homies.

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