Lagering is a form of beer maturation on the yeast that usually lasts for several weeks, if not months, at or near-freezing temperatures, after fermentation and before filtration and/or packaging of the beer. Lagering is employed almost exclusively for bottom-fermented beer styles—hence the name “lager” for these beers—and only rarely for top-fermented beer styles, that is, ales. See ale, fermentation, and lager. Perhaps the best known lagered ale styles are altbier and kölsch. Lagern is a German verb meaning “to store.” Before the days of mechanical refrigeration, German brewers stored beer in cool, deep caves, especially during the hot summer months. Eventually the lager yeast type, capable of fermentation at cold temperatures, was understood and isolated. Today, the lagering phase of cold-fermented beer production takes place in chilled tanks, and these are often horizontally oriented. During lagering, beer undergoes subtle, but significant, flavor-altering biochemical processes that are responsible for the crisp and clean taste we usually associate with lager beers. Lagering reduces any acetic and lactic acids, for instance, to fruity-tasting esters, whose effects on beer flavor tend to be marginal, because they have a much higher taste threshold to humans than do their precursors. Likewise, any residual acetaldehyde, which can contribute a raw green apple flavor to beer, can decrease by as much as 20% to 70%; 2-3-pentanedione and the volatile, butterscotch-tasting diacetyl, as well as their precursors, are reduced by as much as two-thirds. Diacetyl, which can be detected in tiny amounts, is reduced to fruity-tasting acetoin. Traditionally, when beer reaches the lagering tanks it frequently still contains a small fraction of the sugar that was originally present at the start of fermentation. About four-fifths of this residual sugar is made up of easily fermentable maltose; the rest is mostly maltotriose. During lagering, the total residual sugar content of the beer usually drops by as much as 50%. In the closed lagering tanks, the beer slowly carbonates as the yeast processes the remaining fermentable sugar. If any oxygen was introduced during the transfer to the lagering tanks, it is possible for yeast to scavenge it during the lagering phase, limiting potential damage to beer flavor and appearance.

Traditional lagering can last for up to 3 months, but modern commercial pressures have reduced lagering times substantially at most breweries. In a modern lager brewery, secondary fermentation may or may not occur during the cold storage. If there is to be no secondary fermentation, most brewers will employ a short warm phase (up to 62°F/16.5°C) at the end of primary fermentation; this allows the yeast to ferment residual sugar more completely while absorbing unwanted diacetyl. Today, lagering rarely lasts more than a month and 21 days is fairly standard.

Lagering also has a few mechanical rather than organic chemical effects. It promotes the precipitation of residual colloidal complexes, for instance, that are created in the brew kettle, when large- molecular-chain proteins link up with grain and hop phenols, such as tannins. The precipitation of these complexes has a positive effect on beer stability and reduces protein hazing in the finished product. During lagering, slowly dissipating carbon dioxide scrubs several unpleasant-tasting compounds out of the beer, including sulfur dioxide, which contributes green-beer flavors; dimethyl sulfide, which gives beer vegetal notes; and hop-derived mercaptan, which is partially responsible for skunk-like off-aromas, especially in beer exposed to light.

See also acetaldehyde, diacetyl, dimethyl sulfide (dms), maltose, maltotriose, maturation, oxidation, and vicinal diketones.