Flocculation refers to a yeast strain’s tendency to clump together and drop out or fall out of suspension to the bottom of the fermenter or holding vessel. As yeast flocculates, the beer begins to clarify. Some yeast strains tend to have high flocculation, such as Wyeast Scottish – 1728, while other strains like Wyeast American Wheat – 1010 have very low flocculation. The physical appearance of the yeast cell plays a big part in its flocculation level.
It is important to choose a yeast with an appropriate flocculation profile when designing a beer; for instance you would not pair a Belgian Wit wort with a high flocculation yeast, as you want some of the yeast to stay in suspension in the finished beer.
Apparent attenuation is the measurement of the percentage of sugars that have been converted to alcohol by the yeast in a beer. Apparent attenuation is equal to the original gravity minus the final gravity divided by the original gravity, showing the percentage of conversion. A typical apparent attenuation range is between 65 and 80%.
Apparent attenuation calculation example:
Original Gravity of sample beer = 1.06
Final Gravity of sample beer = 1.012
Calculation: 1.06 – 1.012 = .048
.048 / .06 = 80% Apparent Attenuation
There are a variety of ways to impact your apparent attenuation. Some of these include the type of yeast you use, the amount of yeast you pitch, your mash temperature(s), your grain bill composition, and your mash PH. If you are doing extract brewing, then the primary impacts will come from your yeast and any unfermentable sugars that you may add, such as maltodextrin.