attenuate
Of a beer, to become less dense as a result of the conversion of sugar to alcohol.
attenuate: when yeast eats sugar and makes beer drier
In brewing, attenuation is the degree to which yeast consumes fermentable sugars and converts them into alcohol and carbon dioxide. A wort enters fermentation at a certain gravity, measured in degrees Plato or specific gravity; as yeast metabolizes sugars over days or weeks, the density drops. The difference between the original gravity and the final gravity, expressed as a percentage, is the attenuation rate. A beer that starts at 1.050 and finishes at 1.010 has attenuated by roughly 80 percent.
Attenuation depends on yeast strain, fermentation temperature, wort composition, and oxygen availability. Ale yeasts typically attenuate to 70 to 80 percent; lager yeasts often reach 75 to 85 percent. Some strains are deliberately selected for low attenuation to leave residual sweetness in the final beer. The type and proportion of malt also matters: a wort rich in unfermentable dextrins will not attenuate as far as one dominated by simple sugars.
Apparent and Real Attenuation
Brewers distinguish between apparent attenuation and real attenuation. Apparent attenuation is calculated from gravity readings taken before and after fermentation using a hydrometer or refractometer. Real attenuation accounts for the fact that alcohol has a lower density than water, so the measured drop in gravity slightly understates the actual sugar consumed. Real attenuation is typically 1 to 1.5 percentage points higher than apparent. Most breweries track apparent attenuation because it requires only a hydrometer and simple arithmetic.
Stuck fermentation, in which yeast stops metabolizing sugars prematurely, is the chief risk. This may result from temperature shock, nutrient deficiency, high alcohol stress, or poor yeast health. A beer that stalls at 1.020 when it should reach 1.010 remains sweeter and less alcoholic than intended, and may develop off flavors from stalled metabolism. Repitching with a fresh yeast strain or gradually raising temperature can sometimes restart fermentation.
The term attenuate comes from the Latin attenuare, meaning to make thin or weak. In brewing, it refers specifically to the thinning of the wort as sugars are consumed and converted. The concept became standardized in the 19th century as brewers began measuring gravity with instruments and tracking fermentation scientifically. Today attenuation is a key performance metric in production; breweries establish attenuation targets for each yeast strain and recipe, and monitor them batch to batch to maintain consistency.