In brewing, attenuation is the percentage that measures the conversion of sugars into alcohol and carbon dioxide by the fermentation process; a more attenuated beer will generally be drier and more alcoholic than a less attenuated beer made from the same wort. The percentage is calculated by comparing weight or specific gravity of the extract before and after fermentation.
Attenuation = 100 % * (starting extract − current extract) / (starting extract)This formula works with extract given in weight percentages or degree Plato. Extract refers to all the non water substances (sugars, dextrins, proteins, vitamins, minerals, etc.) that are present in the wort. The percent extract or Plato scale is a measure in percent of how much of the wort’s weight is extract. Since, at least in the wort and beer gravities that most brewers work with, an almost linear relationship between (specific gravity − 1) and extract percentages exists, the above formula can also be expressed as:
Attenuation = 100 % * (starting gravity − current gravity) / (starting gravity − 1)for brewers who prefer to work with specific gravity.
Real vs. apparent attenuation
Because fermentation produces ethanol, which has a lower density than water (gravity of 0.787 at 25°C ), using the formula given above will give a higher value than the actual percentage of sugars consumed. Brewers are generally referring to apparent attenuation when using the word without qualification, though commercial breweries do concern themselves with the real attenuation—the actual percentage of sugar consumed by the yeast.
Measurement of this value is important in brewing because it is an indicator of yeast health and because specific attenuation levels are important for certain styles of beer. A beer which does not attenuate to the expected level in fermentation will have more residual sugar and will thus be sweeter and heavier-bodied than planned.