Why is yeast flocculation so important to brewing?
Yeast flocculation is sometimes overlooked but is very important to brewing great beer. The term flocculation refers to the clumping together of yeast cells in beer at the end of fermentation. One of the most widely accepted flocculation theories is that it occurs when lectin-like proteins on a yeast cell attach to mannose residues on another cell. These clumps will either float to the surface or sink to the bottom of the fermenter, depending on strain. Calcium ions are important in flocculation, as they allow the lectins to activate and bind to other cells. It is also reversible, by the addition of wort, which allows brewers to reuse yeast for multiple generations.
There are several factors to consider when selecting a yeast strain for a specific beer style. Flavor profile and attenuation seem to be the biggest driving factors of strain selection. Flocculation tendencies should be considered just as seriously as these other factors when selecting a strain. This yeast behavior impacts attenuation, finished beer clarity, and yeast collection for repitching.
Yeast strains are typically put into one of three flocculation behavior categories: low, medium, or high. Strains with low flocculation, like German Weizens, leave a higher percentage of cells in suspension even after fermentation has ended, leaving a haze in the beer. Medium flocculation refers to strains that start to flocculate near the middle to end of fermentation. High flocculation strains like Fullers London Ale flocculate quite early in fermentation, which can result in an incomplete fermentation.
Proper harvesting techniques are critical to maintaining a strains flocculation characteristics. If slurry is harvested from the bottom of the cone, cells that flocculate earlier in fermentation will be selected and cause problems in subsequent batches. Yeast from the middle of the “cake” should be selected for repitching and avoiding unwanted mutations. If fermentation performance starts to diminish with a strain you have reused for a few generations, it is time for a fresh pitch.
Yeast flocculation is still not totally understood but there are several parameters to focus on to ensure your strain performs as it should. Wort nutrient conditions, post boil pH, oxygen levels, fermentation temperature, and harvesting techniques are not only important to brewing great beer, but also the performance of future yeast generations.
By Jason Lima, Technical Support
References:
Stratford, M. (1989) Yeast flocculation: calcium specificity
Soares, E.V. (2010) Flocculation in Saccharomyces cerevisiae: a review
Sampermans, J. Mortier, E.V. Soares (2004) Flocculation onset in Saccharomyces cerevisiae: the role of nutrients
Verstrepen KJ, Derdelinckx G, Verachtert H, Delvaux FR. (2003) Yeast flocculation: what brewers should know