Beer Basics: Yeast & Fermentation
We’ve looked recently at the basics of hops, malt, and malt enzymes as we walk through the foundational ingredients of beer, and today we're going to take a look at yeast and fermentation!
After the boil is complete (learn more about that in the hops primer), the beer will be separated from the coagulated proteins and the vegetal hop matter—collectively known as trub—and then chilled down and moved into fermenters. Then our third major beer ingredient takes over.
The overwhelming majority of beer is fermented by one of two species of yeast from the same genus. Saccharomyces cerevisiae ferments ales, and Saccharomyces pastorianus ferments lagers. Saccharomyces yeast is a microscopic fungus that consumes simple sugars and creates ethanol and carbon dioxide as byproducts.
Once the brewer has cooled the unfermented beer down to the desired temperature for the yeast they’re using, they add the yeast to the fermenter and let it do its thing. Fermentation will take anywhere from a few days to a couple weeks.
During fermentation, the yeast will convert sugar into alcohol and CO2, but there are a host of other microbiological processes happening alongside this. The yeast is also absorbing oxygen early in the process, dividing into new yeast cells to multiply, absorbing nutrients such as nitrogen and zinc to keep itself healthy, consuming trace amounts of other compounds in the beer, and, depending on the strain of yeast being used, creating varying levels of many different aromatic compounds that will influence the flavor of the finished beer. Depending on the desired result, the health of the yeast, and how well the process is planned and monitored, these flavors might be desirable or undesirable.
When the yeast has finished fermenting the beer, it will usually settle to the bottom or the tank. In modern breweries, fermentation is done in vessels that have a cone-shaped bottom, allowing the brewer to harvest the yeast from a batch of beer and use it again if desired.
There are a lot of flavor compounds yeast can potentially create in beer, some good and some bad. Esters will generally have fruity or floral aromas. If you’ve ever enjoyed a Hefeweizen, the banana flavor you get in that beer is from a yeast ester. Phenols generally have spicy, earthy, or even medicinal aromas. Going back to that Hefeweizen, the clove spice in that beer is from a yeast phenol. Some esters and phenols are pleasant, some are unpleasant, and some are pleasant on their own but shouldn’t be present in certain styles. Some compounds the yeast can create are unpleasant or simply aren’t desired in high concentrations or in certain styles. It’s all about context.
Earlier I mentioned there are two main yeast species—one for ales and one for lagers—so let’s spend a moment talking more about that.
There are a lot of misconceptions about the difference between lager and ale. Thanks to mass-produced brands advertising bland, watery beers as the highest level of lager brewing, the average consumer thinks lager is code for flavorless or cheap. The truth is, lagers can be brewed with all the character and quality of ales, aren’t necessarily weaker or milder beers than ales, and certainly aren’t easier to brew.
Ale yeast—Saccharomyces cerevisiae—prefers to work at a higher brewing temperature than lager yeast, Saccharomyces pastorianus. The lower temperature for lager means the yeast generally produces fewer aroma and flavor compounds. From a sensory standpoint, this means there are generally fewer yeast-derived aromas and flavors in lagers than in ales. Lager yeast does its work without leaving much trace of itself behind. Within both species, there are many strains, and they perform slightly differently, allowing a brewer to select the strain that will work best for the beer they’re brewing.
Beyond that, lagers can be every bit as characterful as ales, so if you hear lager and think boring, think again.
After fermentation is complete, the beer will undergo a resting period at low temperatures, a stage called conditioning or secondary fermentation. This rest allows flavors in the beer to come together, some solids to drop out, and rough edges to be sanded off. This lasts from days to months, and generally speaking, lagers condition longer than ales.
We have often talked about sour beers on Bean to Barstool, particularly in a pairing context. Sour and mixed-fermentation beers undergo a fermentation by microorganisms beyond the two yeast species we talked about earlier. Usually a conventional Saccharomyces fermentation happens before or alongside these alternative fermentation processes, but it is possible in some cases to brew with Saccharomyces at all.
Other microorganisms that are sometimes used in brewing include bacteria like Lactobacillus, Pediococcus, and Acetobacter, which produce sour acidity in the beer, and yeasts like Brettanomyces, which can produce a wide range of fruity or funky flavors. Due to various technicalities, traditions, and variations, not everyone agrees on what to call all of these beers, and some are not even sour. You might hear terms like kettle sour, spontaneous fermentation, or wild ale used with different beers in this group, and not everyone used the same terms (yes, it gets annoying).
Common traditional sour styles include Berliner Weisse, Gose, Lambic, Gueuze, Flanders Red, and Oud Bruin, though there are many others, and many modern beers that don’t fall under an easy style umbrella.
The biochemistry at work during beer fermentation can be overwhelmingly complex, and there are aspects of the process we are only now beginning to understand (for example, the impact of yeast on hop flavors and the role of yeast in haze production for Hazy IPAs). It’s an exciting time!