Bottom fermentation

Bottom fermentation is a slow alcoholic fermentation during which the yeast cells collect at the bottom of the fermenting liquid, which takes place at a temperature of 4 to 10° C. This results in less turbulence in the beer and yeast precipitating early in its life cycle. At these temperatures, few aroma substances, also called esters, are produced by the yeast. It gives a fairly-neutral tasting and smelling end product.

The term “bottom fermentation” was first used in Bavaria in 1420. According to history, a group of Bavarian monks began storing their beer in icy caves for better preservation and, as a result, the wort attracted a cooler temperature yeast, which, by happy accident, produced an entirely different beer: lager.

Bottom fermented beer relies on the cooler weather yeast called Saccharomyces pastorianus while top fermented beer relies on that warm weather yeast called Saccharomyces cerevisiae.

Saccharomyces pastorianus forms foam at the bottom of the wort. These yeasts ferment more sugars producing a crisper taste which is often found in lager-type beers. Yeasts are fungi which grow as single cells, and create daughters through budding or binary fission.

The fermentation using Saccharomyces pastorianus requires very low temperatures compared to other industrial fermentation processes. This group of organisms has benefited from both the whole-genome duplication in its ancestral lineage and the subsequent hybridization event between S. cerevisiae and S. eubayanus, resulting in strong fermentative ability.

Examples of bottom-fermented beers are: pilsner, lager, bock beer (bokbier is sometimes also brewed top-fermented in the Netherlands, but the original beer style is bottom-fermented).
Bottom fermentation