This week I take a look at the yeast flocculation, what it means and how it affects your finished beer.
What is Yeast Flocculation?
Flocculation is technically the tendency of yeast cells to clump and aggregate together as fermentation winds down and eventually falling out of the beer to form a sediment at the bottom of your fermenter. As the yeast flocculates out it increases clarity in the beer and also makes things like filtration easier.
The chemistry and mechanics of flocculation is actually quite complex. Flocculent cells actually have tiny hairs or spines on the cell walls that are visible with an electron microscope. The cells themselves have a small negative surface charge, so they tend to repel each other. Flocculent cell walls, however, overcome this surface charge and will actually stick together. Over time many cells can form clumps which are called “flocs” that eventually fall out of solution. Some yeast types can even be non-flocculent, and their yeast walls will appear smooth so they do not join together to form flocs.
If you want to read more about the current theory about how yeast cells flocculate, start with the section on the Lectin Hypothesis on this Wyeast article. For most brewers, it is sufficient to know that some yeast strains flocculate much better than others, though a variety of factors including yeast health, nutrients, temperature, alcohol percentage, pH, and other environmental considerations can impact how well yeast cells flocculate. I will highlight a few of these environmental factors below, but as a starting point we will focus on the yeast strain itself.
Yeast Strain Flocculation Values
If you’ve read my article on How to Read a Yeast Data sheet, you probably already know that yeast labs list flocculation broadly as “High”, “Medium” and “Low” though there are also some yeast strains like Wyeast German Ale 1007 that are non-flocculent. At a broad level yeasts that are highly flocculent will clump together into flocs, and fall out of the beer the fastest, while low flocculent and non-flocculent yeasts can remain in suspension for an extended period of time, and may need assistance either from fining additives or cold crashing your beer to achieve good clarity.
Choosing a yeast to match the style of beer you are brewing is important. For example low flocculation yeasts may actually be desirable in something like a Hazy IPA or Bavarian Hefeweizen where suspended yeast is actually part of the style. Highly flocculent yeasts may work best when production schedules are tight, clarity is needed and may aid in filtering your beer.
As with anything, flocculation is a tradeoff. A yeast that flocculates too rapidly may actually result in an incomplete fermentation and low attenuation or off flavors like acetaldehyde or diacytl. You can offset this by recirculating the yeast back into suspension. However highly flocculent yeasts can be good for cask ales or rapid production where you want the yeast to fall out quickly.
Conversely low flocculation yeast can create clarity issues in a finished beer, and also be a problem in situations where refermentation can occur for example through hop creep or when shelf stability is poor in an unfiltered beer.
Environmental Factors Affecting Flocculation
While the Wyeast article linked above has more detail on the brewing environment, I want to highlight just a few of the key environmental factors which can affect how well your yeast flocculates at the end of brewing:
- Yeast Health and Oxygenation: If your yeast cells are old, damaged, strained, underpitched or not properly aerated, it can result in an early and incomplete flocculation. Be sure to pitch enough healthy yeast and aerate your wort before pitching.
- Temperature: During fermentation, you should stick to the recommended range for your strain as it is yeast strain dependent, but strains generally perform best within the recommended temperature range. Reducing temperature or cold crashing after fermentation is complete will generally aid in both flocculation and sedimentation.
- Alcohol and pH Levels: Both the pH and alcohol levels will also affect flocculation though the exact effects are yeast strain dependent. However, trying to brew a beer that is close to the alcohol tolerance for a given yeast strain will put stress on the yeast, resulting in poorer flocculation.
- Pitch Rates: Higher pitch rates generally increase the rate of flocculation, and a good pitch rate is important for overall yeast health.
Flocculation is not the most important factor when selecting a yeast strain, but it is one consideration to take into account if you are targeting a particular style or trying to shorten your brewing timeline. I hope you enjoyed this week’s article on yeast flocculation. Thanks for joining me on the BeerSmith Home Brewing Blog. If you want to take the guesswork out of brewing, please try my BeerSmith recipe software from BeerSmith.com. Be sure to sign up for my newsletter or my podcast (also on itunes and youtube) for more great tips on homebrewing.