Bierfermentation

Bierfermentation, or beer fermentation, is the biochemical process in which yeast metabolizes fermentable sugars in wort to produce ethanol and carbon dioxide, giving beer its alcohol content and carbonation. Fermentation takes place in a closed vessel under controlled temperatures and sanitation to influence flavor, aroma, and clarity. The yeast strain and the fermentation temperature largely determine the character of the final beer.

Two principal categories of beer yeast are used in brewing: ale yeasts (Saccharomyces cerevisiae) that ferment at warmer temperatures and typically rise to the surface during fermentation (top-fermenting), and lager yeasts (Saccharomyces pastorianus) that ferment at cooler temperatures and work at the bottom (bottom-fermenting). Ales generally complete fermentation within several days, while lagers require longer, cooler conditioning. Some traditions employ spontaneous fermentation with wild yeasts and bacteria, as in lambics.

During fermentation, yeast consumes sugars such as glucose and maltose derived from malted barley, producing ethanol, carbon dioxide, and a range of flavor compounds including esters, phenols, and diacetyl. Temperature, oxygen availability at pitching, and yeast health influence flavor balance and aroma. A diacetyl rest, a temporary rise in temperature, may be used to remove buttery diacetyl flavors.

After primary fermentation, beers may undergo conditioning or lagering to mature flavors and improve stability, followed by packaging in bottles, cans, or kegs with carbonation either natural or forced. Proper sanitation and yeast management are essential to prevent contamination by spoilage organisms and ensure consistent quality.