The Oxford Companion to Beer definition of
is a process using yeast strains that work effectively at lower temperatures 5°C–10°C (41°F–50°F), causing the yeast to work less vigorously and create carbon dioxide more slowly. This results in less turbulence in the beer and yeast precipitating early in its life cycle. Bottom fermentation is usually associated with lager yeasts.
The term “bottom fermentation” was first used in Bavaria in 1420. Traditional beers of the time were ales. The ale’s warm 17°C–25°C (63°F–77°F) and turbulent top fermentation carried the yeast to the foam on top of the beer where it often formed a thick mat and was harvested and used to start the next batch. Brewers in Bavaria, however, found it advantageous to attempt fermentation and storage in cool caves at the foothills of the Alps, where it was possible to ferment beer even in summertime. Until this development, warm weather meant that brewing had to cease, as bacteria overwhelmed the yeast in warm fermentations. In the caves, a different yeast started to emerge—a yeast that could ferment at cold temperatures, that is, temperatures at which spoilage bacteria struggled. This new type of yeast fermented more slowly and less vigorously than ale yeast, never formed much foam on the surface, and when finished, sunk quickly to the bottom of the vessel. From there it was collected and used in the next batch of beer. Over time, selection of yeast from the bottom of the vessel naturally favored yeast types that precipitated well, and these became known as “bottom-fermenting” yeasts. This type of yeast was finally isolated in a pure culture by Dr Emil Christian Hansen in 1883 and named Saccharomyces carlsbergensis.
While warm fermentations by ale yeast strains can be very rapid—as short as a few days—the cold temperatures of bottom fermentations required longer fermentation times, often 10 to 14 days. Lower temperatures of bottom fermentation slow down the rate at which the yeast consumes sugars in the beer. Further, only the top layer of the settled yeast comes into contact with the beer and is able to continue fermenting it. After active fermentation is completed, the beers tend to have immature flavors and need a period of cold storage referred to as lagering. Breweries specializing in bottom fermentation often use fermenting vessels that are short and wide, and will sometimes install platforms in the vessel to catch precipitating yeast, thus increasing contact between beer and yeast. This is the true purpose of the “beechwood strips” made famous in advertisements for AB-InBev’s Budweiser beer.
Over the past few decades many breweries and laboratories have developed ale yeast strains that ferment quickly at warm temperatures, create typical ale flavor and aroma profiles, then drop to the bottom of the fermentation vessel, easing their collection from cylindro-conical tanks. This has blurred the distinction between “top-fermentation” and “bottom-fermentation.” Furthermore, yeasts that conduct cold bottom fermentation were previously considered to be a different species from those that work by top fermentation. The species name for bottom-fermenting yeast, carlsbergensis, gave way to the name uvarum. Taxonomists have now decreed all lager yeast to be Saccharomyces pastorianus, an organism that seemingly arose from a coming together of S. cerevisiae and S. bayanus.