In brewing, attenuation refers to the conversion of
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose
Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
s into
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
and
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
by the
fermentation
Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
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 Ethanol fermentation, fermented by the brewing yeast to prod ...
.
Attenuation can be quantified by comparing the
specific gravity
Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...
— 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
A hydrometer or lactometer is an instrument used for measuring density or relative density of liquids based on the concept of buoyancy. They are typically Calibration, calibrated and Graduation (instrument), graduated with one or more scales suc ...
. 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