It doesn't seem like a particularly appealing subject at first glance, but it quickly becomes apparent that we can thank our all-time favorite fungus, Yeast, for most of humanity's less sober moments. Yeast's only job in life is to munch through sugar and (literally) excrete alcohol, CO2, and some other lovely flavors. It's also everywhere. You're covered in millions of microscopic yeast cells right now, we promise. Also...yeast isn't red, for the record. We're doing this for effect.
If you haven't checked out our page on beer styles yet, go read that first, because this will make more sense after you read that.
Remember that the type of yeast used in the brewing process defines the beer. We're emphasizing this because it's important. As we've learned, there are two categories of beer in the whole world. Ales and Lagers.
There are thousands of identified kinds of yeast in the word, but we're only concerned with two broad categories in brewing:
Technically speaking, Ale yeasts are more broadly defined as "top-fermenting yeast". This means that the yeast goes to work metabolizing and fermenting the beer at the top of the fermentation tank (near the surface of the wort). We'll get into more details shortly.
Ale Styles - Pale Ale, IPA, Wheat Beers, Porters, Stouts, Saisons, etc.
Top-fermenting Yeast = Ale Yeast = Ale
Conversely, Lager yeasts are more broadly defined as "bottom-fermenting yeast". This means that the yeast metabolizes and ferments the wort at the bottom of the fermentation tank, away from the surface. Just like Ales, all Lagers are made like this.
Lager Styles - Pilsner, Bocks, Helles, etc
Bottom-fermenting Yeast = Lager Yeast = Lager
(This is a small narrative on where modern brewers think we first discovered yeast's importance in brewing beer...feel free to skip this part)
Remember how baking bread and brewing beer are very similar in process? Well we have beer to thank for bread, not the other way around. Beer came first. We can't really ever know when the first actual brewed beverage was deliberately made, but we can guess that it was quite a while ago.
We mentioned that yeast are fungus. We mentioned they're absolutely everywhere. Well...when humanity first cultivated grain thousands of years ago (this is called "farming" for those in the know), we probably left a basket of grain out in the rain somewhere while we ran back to our cave to avoid the impending storm. When we came back to the basket a few days later, the water and grain has gotten all funny. It was slightly sweet smelling, and oddly, it was bubbling happily. Well...wild yeast had infected our basket of primitive water/grain mixture (wort), and beer was born! Can you imagine being the first person to try this stuff? Took some guts, that for sure. This was the genesis of humanity's use of yeast to help make delicious stuff, and most food and drink that uses yeast has since evolved from that one rainy basket of accidentally fermented grain.
Yeast doesn't just produce alcohol and CO2. As its use in brewing has evolved over the centuries, brewers of beer came to realize that beer could be (and still is) somewhat nutritious. This was partly due to its ingredients, but also due to the yeast! Yeast produced a number of vital nutrients during fermentation, including some very essential minerals and B vitamins, to name a few. Remember the famous Guinness slogan "Guinness is good for you"? This is partly true. Throughout its history, beer has been used an important source of nourishment and hydration in many cultures and yeast continues to play an important role in that respect.
Similar to the other ingredients that make up beer, yeast has a profound impact on the final product. We've told you the certain hop and malt varieties literally define the beer being created. This is the same for yeast. Under the broad umbrella category of Ale or Lager Yeast, there are hundreds of marginally different yeast strains the impart specific flavors and aromas to the beer, aside from the primary purpose of making the beer alcoholic and carbonated.
This is what yeast actually looks like. Pretty nondescript stuff for how important it is. If you've ever made bread, this isn't surprising.
When the yeast is added to the wort (it's usually quite a bit of it too...not just a little packet like a loaf of bread), it immediately goes to work fermenting, transforming the sugar into alcohol, CO2, and other flavors and aromas. Some of these flavors and aromas are quite desirable, and some are not. These "off-flavors" could include aromas reminiscent of wet socks, wet dog, horse blanket, musty basement, and curdled milk. We're not kidding, that's the way they're described. Clearly we don't want some of these things in our beer, so brewers use their best judgment to use yeast strains that impart the fewest undesired flavors in a particular batch of beer.
Ale Yeast is almost definitely the original yeast used in brewing. Brewers (probably German brewers) didn't isolate the lager strains that we have today until as late as the early 1800's, and this was a result of better temperature control. The original lagers probably weren't very good (bad temperature control and low quality ingredients), and only achieved their modern lovely crisp flavor after some serious experimentation.
Here's a handy-dandy breakdown on Ale and Lager Yeast.
Fermentation temperature - Generally speaking, anywhere between 60-75 degrees is a good bet for most ale styles. Yeast will create different chemicals and ferment at different speeds given the temperature of the surrounding wort. Ale yeast will rise to the top of the fermentation vessel and create a thick, foamy layer of bubbles during fermentation.
Flavors and Aromas - Generally speaking, ale yeasts impart richer, fruitier, and more aromatic "other things" to beer. What are these things? Read on of course! Ales can generally be counted on for widely varied palate, offering sweet, rich notes and a complex mouthfeel to a lot of your favorite beer styles.
Let's talk about Brett. We mentioned earlier on this page that certain flavors are unwanted in beer. Well, Brett is something of a wild card when it comes to this idea. Brettanomyces Lambicus, or "Brett" for short, is a popular wild yeast strain in modern craft brewing. A wild yeast is simply a yeast that brewers have not actively cultivated over the centuries into modern brewing practices, and thus they're not used as a primary fermentation agent in the brewing process. Wild yeasts can be found on you right now (ew!...not really). Historically, Brett wasn't a yeast that we wanted in our beer. During fermentation, it produced a musty, funky taste and smell in the beer. Well...it's come back into fashion in the past 10 years or so, and you'll find a lot of specialty one-off craft beers that are "brewed with Brett". While it's definitely an acquired taste, these beers can be extremely interesting to taste and breakdown. And trust us, once you taste something fermented with Brett, you'll never forget it.
Fermentation temperature - Generally speaking, anywhere between 45-60 degrees is a good bet for most lager styles. Because of the cooler temperatures, lagers ferment more slowly than ales (it can take up to twice as long to ferment a lager). Because the cooler temperatures essentially dial back or decrease their metabolic processes (remember...yeast are living creatures) they also produce CO2 less quickly, and thus you see less foam in the fermentation vessel.
Flavors and Aromas - Generally speaking, lager yeasts impart a crisp, clean, and less complex flavor profile to a given beer. When you think of lagers, you think of a highly drinkable, golden beer. This is, for the most part, true. There are numerous lager styles that are malt-forward and rich (think dunkels and schwarzbiers), but generally lagers possess a lighter body and more straightforward aroma.
We alluded to this, but those "other things" that yeast impart to beer besides CO2 and alcohol are important. These other things are chemical compounds that represent the byproduct of the fermentation process, and impart very specific and noticeable flavors. A few of these things fall into the following buckets:
Esters are sometimes confused for actual fruit. Esters are the chemical compounds in beer and wine that impart certain fruity flavors (banana, blackberry, cherry, apple, etc). Just like wine, these fruits are not actually present in the beer. The wineries and breweries aren't putting bananas and cherries in their products. Instead, the exact same compounds found in the fruits, which give the fruits their signature flavor, are created by the yeast during fermentation.
Phenols are an interesting chemical compound and represent a wide variety of flavors and aromas produced by yeast. If something is described as "phenolic", you can expect flavors reminiscent of cough syrup, mouthwash, smoky wood/burnt stuff/charcoal, and certain spicy smells like peppercorn and clove. It's hard to strictly define certain compounds, and the more beer you drink, the more accurately you'll be able to place phenolic notes in your beer.
We'll go ahead and admit that we made this category up, but it's an effective descriptor. Some of these yeast compounds can be musty, or acidic, or even putrid (rotten eggs/sulfurous). Other flavors include an astringent, briny taste or medicinal flavor, similar to some phenolic compounds. Basically this is a catch-all for a bunch of stuff that yeast could produce in the right circumstance.
And that's yeast for the most part! Nice work.