Cask Conditioning is the process in which a draught beer retains yeast to enable a secondary fermentation to take place in a cask in the pub cellar. Cask conditioned beer is the traditional drink of the British pub, and served properly, it can be among the most subtle and beguiling of beer types. Beer that is cask conditioned is neither filtered nor pasteurized and is often called cask ale, real ale, or in Britain, traditional ale.

Conditioning is a complex business and the term covers the chemical, biological, and physical changes occurring from when the beer leaves the fermenting vessel in the brewery and is racked into a cask, up to the time that it is dispensed by one of several methods from the cask into a glass ready to be served to the drinker. Achieving the correct “condition” for cask conditioned beers is a skilled job usually performed by the publican or the cellarman. They are often the same person. See cellarmanship, art of.

Most of this activity takes place in the pub cellar and while many breweries transfer beer from fermenting vessels into racking tanks and from there into casks, some also use maturation tanks where the gentle process of secondary fermentation commences. Regardless of which method is used, maturation continues in the cellar. Some beers have a small amount of sugar solution, called primings, added to produce natural carbonation and help create the beer’s foam. The residual yeast that is left in the beer after it is transferred to the cask will continue changing the sugars into alcohol and produce carbon dioxide. The process usually reduces the final gravity and this causes a slight increase, say 0.1% ABV, in alcoholic strength. The cask must be vented during conditioning by using a soft spile that allows excess gas to escape. Contrary to much foreign opinion, cask conditioned beer should never be flat. A certain amount of carbon dioxide must be retained in the beer to give it liveliness on the palate; this is the “condition” sought by the cellarman, and without it the beer can become flat and lifeless. The flavor of the beer is materially affected by the secondary fermentation.

The period of conditioning will vary according to the style, strength, and brand of the beer and it is usual for breweries to advise publicans as to what conditioning period is recommended and at what temperature. It can vary widely, between 24 hours and 16 days, and the usual temperature should be 13°C–14°C (55°F–57°F). The beer goes through a stage of “dropping bright,” the settling of the yeast and proteins to the bottom of the cask, leaving the beer perfectly clear. This task is accomplished by adding finings to the beer that bind the sediment and carry it to the bottom of the cask. Finings are made, improbably, from isinglass, the swim bladders of sturgeon. Isinglass has been processed so as to do its job without adding any flavors to the beer.

Other changes take place during the maturation period in the cask. The beer can attain flavors from dry hops that may be added at the cask-filling stage mainly for aroma. Strong beers require long periods of maturation and the casks are often stored in the brewery for weeks or months before being released into trade.

The method of dispensing beer from the cask into the glass can vary and sometimes reflects regional tastes. The most common way to serve cask conditioned beer is by the use of a beer engine, a hand-operated hydraulic pump, that draws beer from a cask in the cellar up to the bar. It is commonly called a hand-pull or a hand-pump. The requirement for a head on beer is certain, but the size or density of the foam desired depends on geography. In Yorkshire a tight creamy head is required; in Teeside and the northeast it should be large and loose; and in most of the rest of England and Wales a narrow “string of beads” head satisfies. Electric metered and free-flow systems have lost their popularity and never produced the Yorkshire head. In Scotland air pressure dispense was once predominant but is gradually being taken over by the beer engine. This leaves the age old system of “gravity dispense” where the beer runs from the cask through a tap directly into either a glass or a jug. See gravity dispense. The inconvenience of this system is obvious but some publicans prefer its traditional aspect; the cask can literally be set directly on the bar, kept cool by a refrigerant blanket.