Uh oh. Your homebrewing buddy just said something about “brett” and is asking your opinion about buying a stir plate. This conversation is getting dangerously yeasty.
But that’s OK! I’m here to help put the “you” back in “Eukaryote” with a primer about yeast, and why it’s so damn important to beer.
Much like the other posts in this series, this primer will cover the basics (yes, I left quite a bit out) for those who want to write (or speak) with a little more confidence. If you’re looking for a journey to the center of fermentation, check out Chris White and from Brewer’s Publications.
Yeast as a Living Thing
Yeast is literally everywhere. You breathed some in just now. You probably ate some that was resting on your lunch. The little buggers are all up in your shit (literally), and play an important bit part in maintaining your body’s homeostasis. Fret not; it’s an integral part of our immune system and you’d have to ingest a very large amount of it to experience any ill effects (see: auto-brewery syndrome).
Biologically, yeast falls under the Fungi kingdom (here’s a quick reference if you forgot your high school taxonomy). They are technically eukaryotic (meaning their cells contain a nucleus that houses genetic information), but are the only single-cell eukaryote ever described by science. Despite any deeply romantic feelings you may have developed for your favorite IPA, yeast reproduces asexually, through the very painful-looking process of mitosis.
It’s tricky to organize yeast because they don’t all fit under one taxonomic group. But generally (please don’t kill me, biologists reading this) the yeast we use to brew can be classified by species, which are often sold to brewers as strains. Homebrewers and bakers will be familiar with Saccharomyces cerevisiae, which is probably the mostly commonly used yeast in ale. Lagers use Saccharomyces pastorianus. Then there’s the popular Brettanomyces, which is known for its distinctive and sort of gross qualities.
But that’s just a few, easy to recognize examples. There are ~1500 described strains of yeast, many of which we don’t use in brewing. The yeast in our bodies – often responsible for a number of nasty infections – is called Candida albicans. In healthy humans, this yeast is kept in check by bacteria. Fun fact: lactobacillus, a bacteria use to make some kinds of sour beer and sourdough bread, is one of the natural counter-balances to the yeast that grows in our guts.
Somewhat amazingly, we didn’t even know that yeast was a thing until one very cool French dude named Louis Pasteur described yeast and what is does in 1857. Although a scientist named Leeuwenhoeck (yea, I have no idea how to pronounce that, either) visually saw yeast in 1680, he didn’t really know what is was. Prior to Pasteur’s badass book, “The Diseases of Beer, Their Causes, and the Means of Preventing Them” some people assumed fermentation was spontaneous, and as White and
Wooden brewing paddles were passed down through generations of brewers, all of who were apparently oblivious to the fact that wood was porous, and that the yeast from previous batches of beer were hiding deep inside all of their tools, just waiting to inoculate the next batch.
Yeast as a Brewing Ingredient
There’s a classic quote beer writers should know:
“We brewers don’t make beer, we just get all the ingredients together and the beer makes itself.” — Fritz Maytag
Yeast is going to do its thing regardless of what we do. The brewer’s job is more interior decorator than creator: she needs to turn the wort into a welcome, clean, inviting home that the yeast want to move into to start their family. But the yeast aren’t picky; they’ll move into any home that’s got plenty of sugar to eat, even one infested with other nasty tenants of less reputable backgrounds. The brewer has to do everything she can to make sure the yeast and its family are the only ones living in the house, and that they’re as healthy and comfortable as possible.
Yeast can come from third party labs as dry cells, or ready-to-use liquid. While pre-packaged yeast can be used (I’ve used it dozens of times), many brewers will create a yeast “starter.” This is basically a sugary proto-beer that kick starts the growth of the yeast. A starter ensures you’ve got plenty of healthy yeast to begin and maintain a strong primary fermentation. Some companies sell “smack packs” which are a sort of all-in-one starter (that includes an activator) where you just “smack” the bag of yeast to mix up the contents and create a mini early fermentation before pitching it into the wort.
Logistically, yeast is added after the wort has been boiled, hops have been added, and the combined concoction has been cooled. The drop in temperature in very important: yeast are living things, and adding them to hot liquid can easily injure or kill them. To properly reproduce, yeast need oxygen, so wort is aerated. This is tricky, because oxygen is a mortal enemy to fermented beer.
Oxygen before yeast? Good! Oxygen after yeast? Bad!
Yeast’s primary role is to eat the sugars extracted from the base malts during mash, and turn them into ethyl alcohol and carbon dioxide (C02). That’s an incredible oversimplification though; the amount, type, and length of sugars, the temperature of the fermenting beer, and the type of yeast used all dictate how the yeast will perform. Fermentation is what makes beer taste like beer; you couldn’t just add alcohol to hopped-wort and expect beer. Yeast is responsible for hundreds of other compounds that produce flavors we’re all familiar with (banana and clove and fruit esters, oh my!)
Yeast is the prime mover for the Original Gravity (OG) and Final Gravity (FG) equation. By measuring the original amount of sugar in the beer, and the comparing it to the final amount when fermentation in done, a brewer can calculate how much sugar is left in the beer, how much was eaten by the yeast, and how much alcohol it created. The amount of sugar the yeast ate is also called the amount of “attenuation.”
The trick to remembering the difference between ale and lager is that they are brewed using different yeasts (see above). Ale yeast ferments “on top” of the beer, while lager yeast ferments “on the bottom.” This is not a perfect rule. Yeast generally moves through the entire body of the fermenting beer, but this describes where “most” of the fermentation activity occurs.
More important than where they ferment is how they ferment; ale yeasts prefer warmer temperatures (55-70° F), while lager yeasts prefer colder temperatures (40° F). Ale yeast would go dormant and sleepy at such cold temperatures, but certain strains of lager yeast can and will ferment at higher temperatures, resulting in estery, fruity lagers a la “Steam Beer.”
Yeast as a Word
Yeast is almost always a noun. While I’m sure some intrepid wordworker could use yeast as a verb (I may be guilty of that), “yeasted” and “yeasting” don’t exist in a traditional vocabulary.
While it can be used as an adjective (yeasty) I’d warn against using it too often, because like “malty” or “hoppy,” it’s not overly descriptive. It functions perfectly well as a general label, but different yeasts perform and taste different, so when describing it, try to pull out words that capture the essence of what the yeast has done to the beer, not just that it is in fact, in there.
Writing about yeast tends to get biological very quickly, so be sure to balance your diction appropriately. No one wants to read a text book, but no one wants juicy scientific details left out either. Above all, respect yeast’s role in making beer, and remember that even though it’s not as glamorized and talked about as hops (or even malt), it’s (arguably) the single most taste-defining ingredient in the entire brewing process.
Don’t believe me? Try drinking straight, uncarbonated wort.
TL;DR – Remember that yeast is the “living” part of beer, ales and lagers are classified as such by their yeast strains, and the scientific names are always italicized.
Literature and Libation 
