Attenuation (brewing)


In brewing, attenuation refers to the conversion of sugars into alcohol and carbon dioxide by the fermentation process; the greater the attenuation, the more sugar has been converted into alcohol. A more attenuated beer is drier and more alcoholic than a less attenuated beer made from the same wort.
Attenuation can be quantified by comparing the specific gravity — the density of a solution, relative to pure water — of the extract before and after fermentation, quantities termed the original and final gravities. Specific gravity can be measured by buoyancy, with a hydrometer. The higher the specific gravity of a solution, the higher the hydrometer floats.
Apparent attenuation is calculated using the equation:

where AA is apparent attenuation and OG and FG are the original and final gravities. For example, if a beer's OG is 1.05 and its FG is 1.01, then the apparent attenuation is:

Attenuation can range between two thirds and four fifths, but is usually three quarters.
Because fermentation produces ethanol, which has a lower density than water, the apparent attenuation overestimates the actual percentage of sugars consumed. Brewers generally refer to this apparent attenuation when using the word without qualification, although the measurement of real attenuation — the actual percentage of sugar consumed by the yeast — is an important indicator of yeast health and for producing certain styles of beer. A beer which does not attenuate to the expected level in fermentation will have more residual sugar and thus be sweeter and heavier-bodied.