attenuation

In fermentation, attenuation measures the percentage of fermentable sugars that yeast converts into ethanol and carbon dioxide. A wort with 10 degrees Plato of extract that ferments to 2 degrees Plato has attenuated 80 percent. The difference between starting gravity and final gravity, expressed as a proportion, tells you whether your yeast did the job thoroughly or stalled partway through.

Different yeast strains have characteristic attenuation ranges. Ale yeasts typically reach 70 to 80 percent attenuation under standard conditions; lager yeasts often push to 75 to 85 percent. A high-attenuating strain will leave less residual sugar and produce a drier beer. Low-attenuating strains leave the beer sweeter because they either cannot ferment certain sugars (like maltose, if the strain lacks the right enzyme) or simply run out of metabolic steam before all sugar is gone.

What moves attenuation up and down

Temperature, pitch rate, wort composition, and yeast health all shift attenuation. Underpit yeast struggles to finish the job. A cold fermentation may stop too early. High gravity worts with more alcohol stress the yeast before it reaches its potential. Mash profile matters: a rest at 68 to 70 degrees Celsius produces more complex, less fermentable sugars than a hotter rest, so attenuation will be lower. Yeast vigor also declines with repeated repitching and age, causing younger slurry to attenuate higher than spent yeast from many generations of use.

Brewers measure attenuation with a hydrometer or refractometer to track fermentation progress and confirm yeast performance. Stuck fermentations, where attenuation halts too soon, cost time and yield unbalanced product. Over-attenuation is rare but happens with vigorous wild yeast or certain commercial strains in warm conditions, producing excessively dry, thin beer. Knowing your yeast's attenuation profile lets you design recipes around the residual sweetness you want.