After cooled wort is oxygenated and pitched, yeast consumes simple sugars and produces ethanol, carbon dioxide, heat and hundreds of secondary compounds. Fermentation is therefore both a conversion process and a major stage of flavour creation.
Ale, lager and mixed cultures
Ale strains generally work warmer and can express fruit-like esters or spicy phenols. Lager strains ferment cooler and are selected for a clean profile, although they can produce sulphur compounds that diminish during maturation. Sour and mixed fermentations may involve lactic-acid bacteria and Brettanomyces as well as conventional brewer’s yeast.
Control points
Brewers manage yeast vitality, cell count, oxygen, nutrients, pressure and temperature. Too little healthy yeast can cause slow or incomplete attenuation; excessive heat can create unwanted solvent-like alcohols or esters. Near the end, a controlled temperature rise may help yeast reduce buttery diacetyl.
When is fermentation finished?
A stable gravity reading is more reliable than bubbles in an airlock. Brewers also evaluate flavour, pH and the expected attenuation of the strain before cooling or transferring the beer.