Cask ale or cask-conditioned beer is unfiltered and unpasteurized beer which is both conditioned in and served from a cask without additional nitrogen or carbon dioxide pressure. The brewing process is exactly the same – mash, boil, ferment. The only difference is when the beer finishes primary fermentation, it is placed in a cask with finings (a substance that causes particles suspended in fluid to drop out of suspension) to help clarify the beer, and sugar, which will also be added to the cask to aid with the secondary fermentation and self-carbonation. Often times, extra hops are also added to this conditioning phase. With cask conditioned beer there is always a small amount of yeast remaining in the beer that causes secondary fermentation, which carbonates the beer. Conditioning is the penultimate stage in the brewing process when the beer matures, clarifies and carbonates. The conditioning time depends on the beer style and can last between 24 hours and 16 days. Traditionally, the casks are conditioned at the pub by the publican, but can also be conditioned at the brewery and shipped out when ready. When the cask beer is ready, the yeast and other sediment settles to the bottom, the beer is carbonated and served directly from the cask. Cask ale is always unfiltered, unpasteurized and always best fresh.
Since cask conditioned ales are not filtered and not pasteurized, they contain live yeast that continues to add complexity, new flavors and new aromas to a beer. The exact differences vary from beer to beer. The texture of a cask conditioned beer on your palette is often more creamy and smooth than its non-cask counterpart. There are even some beers you can only get in the cask.
The cask is a barrel-shaped container that, in general, is longer than wide and has a bulge in the middle. Unlike a keg, a cask does not contain any valves or internal tubes; instead, it has two holes, one hole on the bulge on the side of the cask and another hole, called the Bunghole, on the circle face of the cask. The hole on the side of the cask has a plastic or wooden fitting called a Shive to regulate the flow of air into and CO2 out of the cask. The Bunghole is the opening from which the beer is dispensed. This hole is sealed with a fitting, called a Keytone, which is first thoroughly cleaned and then hammered out with a mallet to attach the tap. Up until the mid-20th century, most casks were made of wood, but now most are made from stainless steel, and a few are plastic. By far the most popular sized cask is called the Firkin, which holds 9 Imperial gallons, or 10.8 US gallons. Most other sizes are rarely ever seen.
Normal keg beer is filtered, carbonated to usually 2.4 volumes of CO2 and pushed out of the keg with CO2. Cask beer is dispensed traditionally (and preferably in my opinion) by being pumped out of the cask using what is called a beer engine (hand pump). Since there is no CO2 pushing the beer out of the vessel, air is allowed to enter the cask as the beer is dispensed. This means that once tapped, the beer must be consumed fairly quickly, (usually within 2-5 days) before it will start to go completely flat and spoil. Also, unlike keg beer, cask ale is best served at cellar temperatures of 50-55°F. This temperature allows all of the flavor nuances to emerge and permits appreciation of the unfiltered texture. Cask sizes range from whats called a “Pin” which is 5.4 gallons, a “Firkin” 10.8 gallons, or a “Kilderkin” which is 21.6 gallons.