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A keg is a small cask used for storing liquids. Wooden kegs made by a cooper were used to transport nails, powder keg, and a variety of liquids. Nowadays a keg is normally constructed of stainless steel, although aluminium can be used if it is coated with plastic on the inside. It is commonly used to store, transport, and serve beer. Other alcoholic or non-alcoholic drinks, carbonated or non-carbonated, may be housed in a keg as well. Carbonated drinks are generally kept under pressure in order to maintain carbon dioxide in solution, preventing the beverage from becoming flat.
Since keg sizes are not standardized, the keg cannot be used as a standard unit of measure for liquid : despite this, a number of people still refer to kegs as if they were a unit of measure. This size standard varies from country to country and brewery to brewery, with most countries using the metric system rather than U.S. gallons.
In common parlance, the term keg refers to a half-barrel (15.5 U.S. gallon) vessel as this is the most common size used in restaurants, bars and limited home use. A quarter-barrel has a volume of 7.75 U.S. gallons. Generally, a keg is a vessel smaller than a barrel; thus, it is 30 gallons or smaller.
In the U.S. the terms half-barrel and quarter-barrel are derived from the U.S. beer barrel, legally defined as being equal to 31 U.S. gallons27 CFR § 25.11. (this is not the same volume as some other units also known as barrels).
A 15.5 U.S. gallon keg is also equal to:
However, beer kegs can come in many sizes:
| Mini Keg / Bubba (single-use/recyclable) |
| Eighth Barrel |
| Soda syrup / Cornelius kegs / Home Brew |
| Sixth Barrel / Torpedo Keg / Sixtel / Log |
| "Half Barrel" (Europe) |
| Quarter Barrel / Pony Keg |
| Import Keg (standard European "Barrel") |
| Half Barrel / Full Keg |
| 23.3 inches |
| 16.1–17.15627 inches |
| 1984 U.S. fluid ounces |
| 15.5 U.S. gallons |
| 12.90645 imp. gallons |
| 58.673882652 liters |
| 6.875 cases |
| 165 |
| 124 |
The German DIN 6647-1 and DIN 6647-2 have also defined kegs in sizes of 20 and 30 liters.
| 310 mm (high) | 363 mm (ø steel keg) | 216 mm (high) | 395 mm (ø steel keg) | |
| - | - | 327 mm (high) | 395 mm (ø steel keg) | |
| 400 mm (high) | 381 mm (ø steel keg) | 365 mm (high) | 408 mm (ø steel keg) | |
| 600 mm (high) | 381 mm (ø steel keg) | 532 mm (high) | 408 mm (ø steel keg) |
To serve the beverage, a keg must be tapped to breach the container so that pressurized gas can be added and the liquid can be dispensed. Originally, this was done by hammering a tapping rod through a cork bung, similar to how a keystone is still used today to tap unpressurized cask ales. Tapping a keg this way would often waste a bit of beer, which would be forced out under pressure until the tap was secured. By the 1950s and 60s when metal kegs had replaced wooden ones, common tap systems included Golden Gate, Hoff–Stevens, and Peerless taps, which all had one or two couplers for pressurizing and dispensing the beer but retained a separate bunghole for cleaning and filling the keg which was sealed with a wooden bung. These made it easier to tap the keg, but still had sanitation problems (from the wooden bung and attachments that penetrated the keg, and from ports that were at the bottom of the keg next to the floor) and tended to leave some beer inaccessible at the bottom of the keg.
In the 1960s and 70s, several similar styles of tap were created in Europe and America which used a single hole for cleaning, filling, pressurizing, and dispensing. A single bunghole at the top of the keg is used to clean and fill the keg, after which it is sealed with a metal assembly containing a ball bearing which acts as a stopper, held in place by the gas pressure inside the keg. The tap is twisted or slid into place atop the keg, and a lever provides the mechanical force needed to push the ball bearing down, providing access to the keg's contents. These taps, or "couplers", are more sanitary and easy to operate, and were adopted by major U.S. breweries like Anheuser-Busch in the 1970s and quickly displaced other taps to become the industry standard. One such system was referred to as Sankey after its designer (GKN, named for founder Joseph Sankey). The term Sankey, often misspelled "Sanke", has become a generic name for all of the similar industry standard couplers.
Today there are six industry standard couplers:
There are two different types of tapping equipment that are available for kegs. A "party tap" or "picnic tap" is a hand-operated pump that utilizes outside air, thus introducing oxygen and bacteria into the keg. This causes the beer to oxidize, affecting the taste; the partial pressure of CO2 will also decrease, causing the beer to go flat. Kegs dispensed with a party pump should be used within 18–24 hours so that the beer does not become unpalatable. Commercial installations, as well as some home users, use pure pressurized gas; these can preserve a keg up to 120 days with proper refrigeration. In simpler installations only CO2 is used to pressurize and dispense the beer, but in installations with very long lines between the keg and dispensing location (bars with customer-operated faucets at each table being an extreme example), the pressure needed to pump the beer for dispensing would over-carbonate the beer. In these situations, "beer gas" or "mixed gas" is used which combines CO2 with another gas, usually nitrogen. Nitrogen is 80 times less soluble in water than CO2, so it can provide additional pressure without noticeably affecting flavor. Typical beer gas is 70-75% nitrogen and 25-30% CO2, but the ideal ratio depends on the beer being served and the installation; more advanced installations blend the gas on site so it can be adjusted for each beer. A few beers like Guinness are required to be pressurized and dispensed with mixed gas; they usually also require the use of a special faucet that deliberately creates additional friction to force the nitrogen out of solution, creating a thick frothy head.
As with any pressurized container, a keg can cause injury, even at normal operating pressure, whether with Pneumatics or carbon dioxide:
Commercially, kegs are usually kept in a refrigerated room for storage and dispensing, and liquid lines run from this room to all of the faucets in the restaurant. Kegs are too large to fit in a typical home refrigerator. A kegerator (specially designed for kegs, or converted from a suitable small refrigerator) can be used, but as these are somewhat specialized they are cost-prohibitive for the average consumer who has only occasional use for one, and are obviously impractical to bring to a beach or campsite. Instead, in the US and Australia, kegs are usually kept in a bucket of ice and/or water to keep the beer cool. Alternately, the keg can be kept at ambient temperature and served using a "jockey box", consisting of a cooler with beer coils ( metal dispensing lines arranged in a coil) and filled with ice, which acts as a heat exchanger to cool the beer to serving temperature by the time it reaches the faucet. European consumers, when not using a party pump, mainly use inline beer chillers, essentially the same concept as a jockey box but using a refrigeration unit instead of ice. Those chillers usually also have their own air compressor for keg pressurization.
Cornelius kegs were originally made by Cornelius, Inc., and were used for premix soft drinks. Since the arrival of newer technology such as bag-in-box packages, Cornelius kegs have become largely obsolete in the soft drink industry, and refurbished ones are readily available to hobbyists.
Cornelius kegs use two unique types of tap, both very similar to each other, known as ball locks and pin locks.
In Canada, Molson brewery dubbed the mini keg "Bubba". This name has now been genericized to generally apply to all 5-liter mini kegs in Canada . This might cause confusion, as a company called Bubba Keg is established in the U.S., and appears to not be associated with Molson. Today, mini kegs are widely available in liquor stores worldwide, with Heineken, Grolsch and Bitburger all adopting the design.
Recently in Austria, beer is available in mini kegs, either by a one-way type keg made of steel or out of a reinforced thermoplastic for about a €4 deposit.
A number of manufacturers also produce 18 imperial gallon (81.82 litres or 144 imperial pints) and 22 imperial gallon (100 litres or 176 imperial pints) kegs: however owing to their size they are not as popular, as manual handling is seen by some to be difficult, and as a result these tend to be used only for large-scale events and bars with a high output.
For bars that sell a large volume of beer, there is a 36 imperial gallon (163.64 litres or 288 imperial pints) keg. However, due to its very large size, few people can move it without assistance, making it an impractical choice otherwise.
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