In brewing, attenuation refers to the conversion of
sugars into
alcohol and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
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
Wort () is the liquid extracted from the mashing process during the brewing of beer or whisky. Wort contains the sugars, the most important being maltose and maltotriose, that will be fermented by the brewing yeast to produce alcohol. Wort als ...
.
Attenuation can be quantified by comparing the
specific gravity
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
— 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 0.33 and 0.80, but is usually 0.75.
[Andrew Campbell, ''The Book of Beer'', 1956, p.53]
Because fermentation produces ethanol, which has a lower density than water (gravity of 0.787 at °C
), 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.
References
{{DEFAULTSORT:Attenuation
Brewing