Bottomfermenting

Bottomfermenting, or bottom fermentation, is a method used in brewing in which yeast strains responsible for primary fermentation settle to the bottom of the fermenter and carry out most of the fermentation there. This approach is best known for producing lagers.

The primary yeast is Saccharomyces pastorianus (formerly S. carlsbergensis), a hybrid of S. cerevisiae and S. eubayanus. Fermentation typically occurs at cooler temperatures than top-fermenting yeasts, about 7–13°C (45–55°F). The cooler conditions promote a clean flavor profile with lower production of fruity esters and phenols. The yeast forms a dense sediment at the bottom (lees) as fermentation progresses. After primary fermentation, many lagers undergo an extended cold conditioning, or lagering, to mellow flavors and clarify the beer.

Origin and usage: Bottom-fermenting yeasts were developed and popularized in Central Europe, especially Germany and Austria, during the late medieval to early modern period. The modern lager yeast was isolated and propagated in the late 19th century by Emil Christian Hansen at Carlsberg, leading to widespread use of bottom-fermenting yeasts in commercial brewing.

How it differs from top-fermentation: Bottom-fermenting yeasts generally exhibit higher apparent flocculation and settle to the bottom, producing cleaner, crisper beers with subtler esters. Top-fermenting (ale) yeasts ferment warmer, rise to the surface, and contribute more fruity flavors. Styles typically brewed with bottom-fermenting yeasts include pilsners, helles, dunkel, Märzen (Oktoberfest), and other lagers.