Secondary Fermentation is a somewhat catch-all term referring to any phase of fermentation following the very active “primary” fermentation, but before complete removal of the yeast. In lager beers, secondary fermentation can refer to the period of maturation and lagering, during which important flavor changes occur, particularly reabsorption of diacetyl by yeast. In traditional German lager brewing, the term can also refer to the krausening process, where actively fermenting wort is added to beer that has finished its primary fermentation. This addition restarts fermentation in the combined batch, reducing diacetyl and, if the reactivated fermentation is in a closed tank, building natural carbonation.

In Britain, secondary fermentation refers to an important part of the traditional ale-brewing process whereby condition (dissolved carbon dioxide) is built up as residual sugar and is slowly taken up by the yeast. This gives sparkle and mouthfeel to the finished beer. Secondary fermentation may also remove unwanted flavor compounds, such as sulfurous character in the beer, giving it a “cleaner,” more pleasant palate. Secondary fermentation can take place in the brewery in conditioning tanks or, in the case of traditional beer in the UK, in casks. Secondary fermentation in conditioning tanks can be prompted by the addition of wort or sugar solution to the tank.

When a secondary fermentation takes place in a bottle, giving the beer a natural and often lively carbonation, this is referred to as “bottle conditioning.” See bottle conditioning. In the case of bottle-conditioned beer, the wort or sugar solution is added to the beer, which either still has yeast in it or has yeast added before bottling, and the secondary fermentation takes place in the bottle over a period of time. In the case of traditional cask beer in the UK, the beer either has a small amount of fermentable sugar left after primary fermentation or a sugar solution (called priming sugar) is added to the cask, which contains unfiltered beer and clarifying agents (finings). The sugar is fermented by the yeast in the cask, which builds up condition over several days. The cask is ready to serve after the secondary fermentation is finished and the beer has been clarified by the finings.

Amateur brewers often use the term “secondary fermentation” to refer to an aging period after the primary fermentation; this usually involves transferring the young beer to another fermentation vessel to remove it from dormant yeast.

See also cask conditioning, conditioning, and priming sugar.