Brewing Arts Program

Microbiology Essentials

Module 05: Microbiology Essentials

Module 5 brought me right back to chemistry class once again, as we dove into the microbiology behind brewing – aka what is yeast, how does it work, and how does it affect beer?

We start with your basic microbiology and biochemistry (which I will not cover here) with things like cell theory and how a living organism is made up of atoms, which are held together into molecules by positive and negative bonds – how these basic molecules are structured and their main functions. When it comes to beer, sugars and proteins are primary molecules, helping with energy storage and structure (sugar) and enzymes (proteins). This, of course, is extremely important when we are talking about yeast and fermentation.

Fun fact – yeast is NOT bacteria! They are actually eukaryotes, made up of more complex cell structures than prokaryotic bacteria. Yeast is a member of the Kingdom Fungi and based on their cell and DNA sequences are technically more closely tied to animals than to plants! Yeast used for brewing falls within the Order Saccharomycetales. The Family Sacharomycetaceae encompasses the Genus Saccharomyces, which is then broken down into Saccharomyces cerevisiae, ale yeast, or Saccharomyces pastorianus, lager yeast.

Flocculation & Attentuation

First, let’s discuss some important pieces about yeast post-fermentation that we should understand prior to actually adding yeast to wort. Once the sugar in a wort has been fermented into beer, the brewing yeast begins to clump together, or flocculate. As it flocculates, CO2 will adhere to the clumps and the yeast will either rise to the top of the fermenter or sink to the bottom, depending on the strain. Higher flocculation reduces attentuation, the percentage of sugars consumed. Yeast is characterized into three levels of flocculation – high (flocculating by day 3 – 5, less attenuation), medium (6 – 15 days, more attenuation), or low (does not flocculate by day 15, cloudy). Ale yeast is considered top-fermenting, with medium flocculation, and an ideal fermentation range in the mid-upper 60’s F. Typically this yeast is described as clean, fruity, phenolic, eccentric, or a hybrid. Lager yeast is a bottom fermenting yeast with lower flocculation. It ferments at a lower temperature, typically 50 – 55 F, and creates a slower fermentation process. It is characterized as dry and full. 

Pitching Yeast

Now that we better understand our post-fermentation flocculation, let’s bounce back to the start of fermentation and how we determine what and how much yeast to add to our wort. The act of “pitching” is inoculating wort with yeast, “pitching rate” is the concentration of yeast cells in wort at the start of fermentation, and “pitch” is the amount of yeast needed to reach an accurate pitch rate in the wort.

The pitchable amount of yeast depends on the volume, original gravity, and yeast strain. Pitching rates for ale yeast are 1 million cells per mL per degree Plato. Lager yeast is double that at 2 million cells per mL per degree Plato. From this, you can calculate how many yeast cells are needed for the amount of wort you have. As an alternative to counting yeast cells, yeast pitching is often measured by weight (rule of thumb: 1 lb slurry equals 12.5 million cells/mL per bbl of wort [Standards of Brewing]) or by volume (50 mL sample of slurry in graduated tube and wait to see packed yeast and liquid separate). A low pitch rate (underpitching) can increase off flavors, have higher terminal gravities (aka your beer will be less alcoholic), and can lead to your fermentation process stalling out, which can then lead to infection. A high pitch rate (overpitching) can lead to a thin mouthfeel and yeasty flavors in the beer.

Yeast During Fermentation

During fermentation, your microbiology knowledge really comes into play as the yeast goes through several phases. During the first phase, lag phase (0-15 hours after pitching), the yeast is taking in oxygen, amino acids, minerals, and vitamins from the wort and will begin to build proteins and make important enzymes. The Exponential Phase (4 hours – 4 days after pitching), yeast is actively growing, consuming sugars, and producing CO2. The third phase is the Stationary Phase (typically 3-10 days after pitch) where reproduction is slowing down. Flavors have been produced but the balance may not quite be matured yet. The yeast needs time to settle and reabsorb some compounds. At this stage, the attenuation (via gravity measurement) should be checked to confirm if fermentation is complete. It may be necessary to rouse the yeast, re-pitch, or raise the temperature if gravity is not correct. If attenuation and gravity are where you want them to be, the beer can then be crashed.

Storage and Reusing Yeast

It’s also important to store yeast correctly to decrease chances of bacterial infection. Yeast should be stored at cold temperatures (and cooled as quickly as possible) in a container with no abrasions. Leave headspace to purge with CO2 at the top and have a way to release pressure. Yeast can also be reused multiple times but it’s important to keep detailed notes about how it affects the quality of the beer. Most breweries can use yeast through about 10 generations (gone through fermentation 10 times) before needing a new yeast source. Keeping track of the generation number, length in storage, viability, and its effect on the fermentation time, flavors, flocculation, and vitality are key.

Isn’t microbiology fun?! Stay tuned for module six where we cover systems used for and during fermentation! And if you missed the last module all about equipment and the brewhouse – check it out!

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