Cask

What is Cask? A cask is a closed container used to hold and serve cask-conditioned ale, a style of beer that undergoes secondary fermentation inside the very vessel from which it is poured. Unlike modern keg beer, cask ale is unfiltered, unpasteurised, and served without any added carbon dioxide pressure, relying instead on the live yeast inside to produce its own natural carbonation. The format is closely tied to British brewing tradition and was championed by the Campaign for Real Ale (CAMRA) in the 1970s, who coined the term real ale to distinguish it from filtered, pasteurised keg beer. Casks come in a doubling hierarchy of sizes, from the small pin up to the enormous tun, with the 9-gallon firkin being the workhorse of most British cellars.

How Cask Ale Is Conditioned and Served

Cask ale is alive in a literal sense. Because it is neither filtered nor pasteurised, the yeast continues to work after the beer leaves the brewery, slowly fermenting residual sugars and producing the gentle, natural carbonation that defines the style. CAMRA formally describes real ale as beer brewed from traditional ingredients, matured by secondary fermentation in the container from which it is dispensed, and served without the use of extraneous carbon dioxide.

Dispense is just as important as conditioning. Cask beer is drawn by gravity straight from the cask, pulled through a hand pump (the classic beer engine you see on British bar tops), or occasionally via electric pumps, but never pushed by added CO2. In 2018 CAMRA softened its rules slightly to allow the use of a cask breather, a device that protects the beer from oxidation with demand-led CO2, while still classifying the result as real ale. Service temperature sits around cellar level, roughly 11 to 13 C, warm enough for hop aroma, malt character, and that soft natural fizz to come through clearly. In 2019, cask-conditioned ale still accounted for around 420 million pints sold in the UK, about 13.5 percent of total pint sales.

Sizes, Construction and How a Cask Differs from a Keg

English brewery casks follow a neat doubling system. A pin holds 4.5 imperial gallons (20.457 L), a firkin 9 gallons (40.915 L), a kilderkin 18 gallons (81.830 L), a barrel 36 gallons (163.659 L), a hogshead 54 gallons, a butt 108 gallons, and the largest, a tun, a full 216 gallons. The imperial beer barrel was standardised at 36 imperial gallons when the UK adopted the imperial system in 1824. The firkin, equal to a quarter of a barrel, takes its name from the Middle English ferdekyn, itself from a Middle Dutch diminutive of vierde, meaning fourth. Note that a beer firkin (9 imp gal, around 41 litres) is quite distinct from the much larger 70-gallon wine firkin.

Traditional casks were made of oak; modern ones are almost always stainless steel, sometimes aluminium with a plastic lining. Structurally, a cask is not a keg. A cask has two or more openings, a shive on top and a keystone in the head, through which a tap and a porous peg control conditioning and dispense. A keg, by contrast, has a single bung with an internal spear or tube designed for pressurised service. That structural difference is the physical expression of two very different philosophies of beer: living and gravity-fed on one side, sealed and gas-driven on the other.