From Sugar to Alcohol
Fermentation is the metabolic engine of brewing. Saccharomyces cerevisiae (ale yeast) or S. pastorianus (lager yeast) consume the fermentable sugars produced during mashing and excrete ethanol, CO2, and hundreds of flavour-active compounds. The degree to which yeast consumes available sugars — attenuation — determines the beer's final gravity, alcohol content, body, and sweetness.
Apparent vs Real
A hydrometer measures liquid density, but ethanol is lighter than water. After fermentation, the hydrometer reads lower than the true residual sugar concentration because the alcohol pulls the reading down. Apparent attenuation overstates the true sugar consumption by about 18%. The Balling correction (multiply by 0.8192) converts apparent to real attenuation, giving the actual percentage of extract consumed.
Yeast Character and Conditions
Different yeast strains have different attenuation ranges. English ale yeasts typically attenuate 65-72% (leaving more body), while Belgian saison strains can hit 90% (bone dry). Fermentation temperature, pitch rate, oxygen levels, and nutrient availability all affect both the degree of attenuation and the flavour profile — higher temperature means more esters and faster fermentation but potentially rougher flavour.
Tracking Fermentation
This simulation models fermentation kinetics over time, showing the gravity curve declining from OG toward FG. You can see how pitch rate affects lag time, how temperature affects the fermentation rate, and how the final gravity depends on the yeast strain's attenuation capability. The animation shows yeast cells consuming sugar particles and producing ethanol bubbles in real time.