Yeast terminology attenuation and flocculation

Flocculation describes the process of yeast sticking and clumping together once all the nutrients have been used. As the yeast gathers, they become too heavy to remain in suspension in the beer, and fall to the bottom of the fermenter, mixing with the sediment and nutrients. Home-brewed beer can use yeast to carbonate bottles (aka bottle conditioning), so this process also occurs in the bottles, causing the yeast and ‘trub’ to form a thin layer along the bottom of the bottle.

The strength of this “clumping” is determined by a host of factors from O2 levels in the beer, temperature, and nutrient levels, but the primary factor seems to be the strain of yeast itself (in truth, flocculation is still being researched, and has a few mysteries remaining.) Flocculation occurs near the end of the yeast life cycle, so a yeast that flocculates early will not convert as much sugar, and result in a lower-than-expected alcohol content. Strong flocculation gives a clearer beer, while weaker flocculation can lead to a cloudy beer, sometimes with a yeasty taste. Hefeweizen yeast is a good example of a strain that has very low flocculation, meaning a lot of it is left behind in suspension. Traditional non filtered hefeweizens should look cloudy from the yeast (not the wheat as some might think).  The clove like signature flavor in a hefeweizen comes from the yeast.

Attenuation describes the overall efficiency of a yeast strain in converting sugars into alcohol under a specific set of conditions. Higher gravity beers require the use of yeasts with higher attenuation rates. Attenuation is listed as a percentage, meaning the percentage of sugars present in the wort that get converted into alcohol. Most beer yeasts have a 65-80% attenuation rate. Wine & mead yeasts have a higher attenuation rate. The exact attenuation rate can be found for a given beer by comparing the starting (pre-yeast) original gravity (OG) to the final gravity (FG) of the finished beer.

When selecting yeasts, the beer style should be considered. When brewing a Belgian Witbier, a yeast strain with a 65-70% attenuation rate would be appropriate. A “stronger” yeast (higher attenuation) could be used, resulting in a higher alcohol content, but the overall flavor & style of the beer would be “off” from the standard. The attenuation for yeast for an English ale would be lower than the yeast for a Belgian brown.

The attenuation of a particular batch is affected by anything that would affect the health of the yeast. Unexpected flocculation, changes in temperature and insufficient nutrients can all lower attenuation.