What is Yeast?
Since ancient times, people have cultivated yeast for both fermentation and baking. At its base, yeast is a single-celled miracle organism that turns sweet wort into delicious beer and adds wonderful complexities to your favorite sudsy beverage. It works by taking in sugars and converting them into carbonation (in the form of carbon dioxide), and ethanol (alcohol), that lovely intoxicating compound. This much, however, is common knowledge; what’s less understood are the different varieties of yeast and how they affect the flavor of beer.
What kinds are there?
Primarily, there are two families of yeast responsible for most of the world’s beer. These are: Lager yeast & Ale yeast.
Within the main lager and ale families, there are many variations (known as strains), and most breweries will even have their own unique variety. The following is a brief explanation of how each category works, and how they can (in a general sense at least) affect the flavor of beer.
The lager yeast family, called saccharomyces pastorianus, is a yeast commonly used to brew lager style beers from Pilsner to Doublebock, and everything in between.
During fermentation, lager yeast settles towards the bottom of the wort, and typically works best at lower temperatures between 40 – 45oF (4 – 7oC). For this reason, lager strains of yeast are often referred to as “bottom fermenting.”
Lower temperatures result in a slower brewing process that produces the crisp, clean taste of classic lager style beer.
Saccharomyces cerevisiae, also called top fermenting yeast and ale yeast, is the yeast family responsible for fermenting the complex and diverse world of ale beer styles. Brewing temperatures for this yeast are best above 55oF (13oC), allowing for faster chemical processes and the release of more complex flavor related compounds.
Ales comprise of a myriad of different styles from golden session beers to pitch-black stouts. Their flavors are typically fruitier and more full-bodied.
A brief note on wild yeast
In addition to the main, cultivated varieties described above, there exists other, more unwieldy varieties – referred to as “wild.” These naturally occurring airborne yeast strains can result in spontaneous results that are undesirable in most beers. However, some brewers actually want their beer to be infected with wild unwieldy yeast strains to create funky, barnyard-like flavor and aroma. Brewers of Belgian Lambic style beers have been doing it for centuries, with some delightfully quirky results. (Brettanomyces is the most commonly known wild yeast variety.)
As you’ll likely of gathered thus far, yeast is the engine behind the brewing process of beer. It’s truly incredible how you could essentially brew a beer using the same raw ingredients multiple times, but wind up with a totally different beer from each batch – simply by changing the yeast.
Now let’s take a brief look at what yeast does during the brewing process, and how some of the resulting flavor compounds can affect the final beer (for better or worse).
Brewing, Temperature, & Yeast
When added to wort, yeast will metabolize sugars into alcohol alongside a number of other flavor compounds. On average, 65-85% of sugars are metabolized before an equilibrium is reached and yeast cannot metabolize anymore.
The amount of sugar consumed has a dramatic effect on flavor. When a yeast is less inclined to metabolize sugar, the beer is usually malty and full-bodied. Conversely, higher metabolizing yeast leaves beer dry and more bitter.
In addition to yeasts varying appetite for sugar, the temperature at which yeast is brewed also immensely affects the end flavors of the beer.
It is so sensitive to heat that using the same ingredients and conditions at different fermentation temperatures can create many distinct beers. When yeast metabolizes sugars, carbon-dioxide and ethanol are released, but what most people don’t realize, is many other flavor creating compounds are created as byproducts to the process. As the fermentation temperature increases, more of these flavor compounds are created and released into the beer, for better or worse. Some of the less desirable compounds can be reabsorbed when the yeast is heated to a higher temperature for a brief period of time and turned into flavorless or less flavorful compounds. Diacetyl is a common compound that adds unwanted buttery flavor to beer and is eliminated in this way. This complex balance of the right yeast, at the right temperature, is part of what makes brewing part art, part science. It’s a skill learnt over many years of trial and error.
As mentioned above, fermentation creates several flavoring byproducts. Under normal circumstances, an appropriate yeast type is chosen and helps improve the flavor of beer. However, there are some reasons why yeast, even of the right type, will make a beer worse:
If yeast is fermented above its effective temperature range its cell membrane becomes easily permeable and allows an increased amount of byproducts seep into the wort.
It’s a common practice for yeast to be harvested and reused multiple times before being discarded. Sometimes, yeast mutates and its cell structure changes. These changes are most often for the worst and commonly leave the cells with defective cell membranes that don’t contain the right desirable flavoring byproducts.
As with mutations, this has to do with recovered yeast. After 7-10 uses, yeast tends to become stressed and is normally thrown away. If it is still used when stressed, the cell membranes deteriorate with each successive use, thinning and becoming increasingly permeable.
List of Common Fermentation Byproducts
The brewing information above is all well and good, but how do you know if your beer contains off flavors as a direct result of ill treated yeast? Below you will find some common quirkier flavor byproducts, including some off flavors best avoided.
Barnyard or woody aroma
Wanted only when brettanomyces is used in fermentation.
Wanted in weizen style beers
Green leaves, green apple aroma
Never a desired byproduct
Soy sauce, muddy aroma and flavor
Never a wanted result
Butter or butterscotch flavor and aroma
Rarely a desired result
Solvent and mild fruity aroma
Sometime wanted in small amounts to add to fruity aromas
Bananas and peanuts aroma
Small amounts can be desirable to add fruity aromas
We hope this article provides you with a handle on the basics of yeast and how it turns sweet wort into flavorful beer. Under most circumstances, and with great skill, yeast will transform a beers key ingredients (grain, hops, water) into a world of diverse, beautiful flavors. Using the notes on many of the common fermentation byproducts above, you may begin to better identify a good beer from a bad one. See if you can identify the flavor profiles described during your next tasting session. For some tips on setting up a tasting session, check out our previous post on the subject.