How yeast works: A layperson’s guide to saccharomyces

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

Barley, wheat, rye, hop cones, and hop pellets all have immediately tangible, readily identifiable qualities. You can inspect a jar of these raw materials on a brewery tour and instantly get a good idea of what and how they contribute to your beer.

Yeast, on the other hand, is mysterious and intangible. While the brewer interacts more or less manually with malt and hops, yeast works alone in the dark. It’s microscopic; you can’t really show it off to a tour group, and you can’t hold a flask full of pasty slurry and have that same intuitive understanding of what it will do for the finished product.

But without yeast there’s no beer. If beer were a car, malt would be the frame and wheels and gas tank; hops would be the rims and spoiler and airbrushed dragons. But yeast is the engine. And since that’s how we get anywhere, it’s worth learning a bit more about what it is.

What is yeast?

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Photo by Aaron Davidson

Yeast is a single-celled eukaryotic (all genetic material contained in the nucleus) fungus, and it’s a damn life-affirming everyday miracle because it turns sugar into alcohol. Some anthropologists hypothesize that brewing beer was the reason ancient humans transitioned from hunter/gatherer to agrarian societies, paving the way for civilization itself—which puts the domestication of yeast right up there with the discovery of fire, wheels, and Led Zeppelin.

When introduced to a food source like beer wort, yeast cells transfer sugar molecules across their cell walls and, through a series of complex metabolic pathways, break them down for energy and create ethanol and CO2 gas as byproducts. Cells reproduce by budding; that is, daughter cells form on the wall of a mother cell, receiving a portion of the nucleus before splitting off and starting the life cycle anew.

Yeast is omnipresent and can be found on plants, animals, porous surfaces—even, as Rogue famously demonstrated a few years ago, in their founder’s beard. Before its life cycle could be observed and understood, yeast’s introduction into beer wort would have been accidental, or at least incidental.

A history in three paragraphs

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Yeast at Northern Brewer // Photo by Aaron Davidson

Prior to the modern age and microscopy, yeast was not recognized as a living organism, but its action was certainly observable. The word fermentation is rooted in the Latin “fevere” (to boil), which, if you’ve ever been to a brewery and seen a big conical venting into a bucket of water, you’ll agree is an apt description.

Brewers in the Dark Ages knew that some external force was at work in their wort, and explanations tended toward the mystical: the mysterious transformative agent was called “godisgood,” and quasi-magical “barm sticks” were transferred from batch to batch. Unbeknownst to the brewers, the wood—of the brewing vessels as well as the barm sticks—actually housed colonies of yeast, inoculating the fresh wort that came into contact with them. Over time, these populations of yeast would have selectively mutated to adapt to their specialized environment, gradually developing tolerance to a certain temperatures and alcohol levels, passing along desirable sedimentation habits and other behaviors we value and expect from our brewing yeast today.

In the Middle Ages, yeast cells were first observed but not understood; alchemists believed that ethanol was already present, but had to be separated from “impurities” like yeast and CO2. That theory held sway for hundreds of years, until the 19th century when microbiology was established as a science and proved that yeast was the agent of fermentation, not a byproduct.

A little Latin

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Carboys of fermenting beer at Northern Brewer // Photo by Aaron Davidson

There are two great families of beer: ale and lager, with membership determined by the type of yeast used during fermentation.

Saccharomyces cerevisiae is sometimes used as a blanket term or general synonym for “brewing yeast,” but it specifically refers to ale yeast. S. cerevisiae encompasses a vast range of characteristics—everything from the clean and crisp (Sierra Nevada’s Chico, German kölsch) to buttery and fruity (English) to phenolic and funky (hefeweizen, Belgian). Unlike lager yeast, ale strains do not seem to share a recent common origin or have similar genetic fingerprints. They will happily ferment and grow at temperatures up to 98°F or above, but slow significantly or go dormant below about 50°F.

Lager yeast belongs to Saccharomyces pastorianus, a more recent hybrid of S. cerevisiae and S. eubayanus. It only shares 60 percent of its genome with S. cerevisiae and seems to have inherited its cold tolerance from S. eubayanus; it can continue to ferment and reproduce at temperatures that would send S. cerevisiae into dormancy. Genome sequencing has shown that lager yeasts originated from one or two common sources, which makes any two strains of S. pastorianus more closely related than any two strains of S. cerevisiae are to one another.

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