Yeast Management & Attenuation
Yeast is a living entity, much like ourselves. Depending on the specific organism, it requires sugar, water and oxygen to thrive. Brewers often balance a delicate act of stressing and manipulating yeast while trying to keep it as healthy and strong as possible. Thousands of strains with a plethora of characteristics exist, however a few general trends hold true across the spectrum.
Active fermentation taking place in an “open top” fermentation vessel.
Pitch rates have a large influence on fermentation characteristics, particularly yeast driven flavors. Lower pitch rates with higher levels of dissolved oxygen drive cell division, creating more flavor compounds through metabolism. Higher pitch rates with less aeration often leave a “cleaner” yeast profile, allowing malt and hops to dominate. Both pitch rate and aeration influence attenuation, the percentage of sugars the yeast will ferment. Increased attenuation directly relates to more ethanol production, however this must be balanced with the desired flavor characteristics. When using new strains or recipes, an online yeast calculator can offer a good starting point. WYeast offers the ability to calculate two-step propagations.
Raising the temperature of fermentation around 1*P before final gravity will (hopefully) increase yeast metabolism. This step, often referred to as a “D-rest”, allows yeast to increase the uptake of Vicinal Diketones (VDK). Lager beer is particularly susceptible to high levels of VDKs (Diacetyl), so the rest is often implemented. As an alternative, Alpha Acetolactate Decarboxylase (ALDC) can be directly added to the fermented wort. Check out Maturex Pro for more detail.
When re-pitching harvested yeast, traditional methods suggest a weighted pitch of 1kg per hL, often lazily converted to 2#/bbl for American brewers. Yeast should be stored under limited pressure to prevent osmotic stress. Contrary to popular belief, it is okay for stored yeast to be exposed to oxygen, so it is not necessary to pre-purge yeast brinks. Vitality and viability will decline over time, so any yeast should be used as fresh as possible, ideally within two weeks from harvest. Generations should be tracked, along with attenuation data, to confirm satisfactory production parameters over successive batches.