Commercial yeast producers and others publish attenuation figures for each yeast. Are these figures obtained using a standard protocol within the company publishing the results? Are these figures obtained using a standard protocol across the industry? If so, how does that protocol relate to the vast array of wort compositions and fermentation conditions that are encountered in the commercial brewery and in the home brewery?

I suspect that the reasons yeasts vary in the degree to which they attenuate wort are only partly known, but I would appreciate a brief summary of what is known about this.

-Fred L Johnson

RESPONSE:

Fred, there is no standard procedure to determine the attenuation properties of yeast strains. Usually yeast producers have internal standard wort and conditions to compare the different yeast strains. But you are absolutely right that wort composition and fermentation conditions vary from brewery to brewery, which has a huge impact on attenuation. That is why we do not give definite attenuation values for each yeast strain but put each yeast strain in an “attenuation group” (High, medium, low) based on fermentation results with our standard wort and fermentation conditions. This way the customer is still able to select a yeast strain that suits best his beer style but they have to determine the absolute attenuation under their specific conditions for themselves.

The attenuation properties of a yeast strain depend on various factors. Flocculation properties of a strain can influence attenuation; a very flocculent strain that settles before the fermentation is complete will leave more residual sugars in the beer than a strain that stays in suspension longer.

Lager yeast usually attenuates relatively high whereas ale yeasts show larger differences in attenuation depending on their metabolism. One factor is the ability to use maltotriose.

Tobias & Forbes