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Draught beer, also spelt draft, is beer served from a cask or keg rather than from a bottle or can. Draught beer served from a pressurised keg is also known as
Canned draught is beer served from a pressurised container featuring a widget. Smooth flow (also known as cream flow, nitrokeg, or smooth) is the name brewers give to draught beers pressurised with a partial nitrogen gas blend.
In the early 20th century, draught beer started to be served from pressurised containers. Artificial carbonation was introduced in the United Kingdom in 1936, with James Watney's experimental Pasteurization beer Red Barrel. Though this method of serving beer did not take hold in the UK until the late 1950s, it did become the favoured method in the rest of Europe, where it is known by such terms as en pression. The carbonation method of serving beer subsequently spread to the rest of the world; by the early 1970s the term "draught beer" almost exclusively referred to beer served under pressure as opposed to the traditional cask or barrel beer.
In Britain, the Campaign for Real Ale (CAMRA) was founded in 1971 to protect traditional—unpressurised—beer and brewing methods. The group devised the term real ale to differentiate between beer served from the cask and beer served under pressure. The term real ale has since been expanded to include bottle-conditioned beer.
In brewing parlance, a keg is different from a cask. A cask has a tap hole near the edge of the top, and a spile hole on the side used for conditioning the unfiltered and unpasteurised beer. A keg has a single opening in the centre of the top to which a flow pipe is attached. Kegs are artificially pressurised after fermentation with carbon dioxide or a mixture of carbon dioxide and nitrogen gas or especially in Czech Republic solely compressed air.
Keg has become a term of contempt used by some, particularly in the UK, since the 1960s when pasteurised draught beers started replacing traditional cask beers.
Keg beer was replacing traditional cask ale in all parts of the UK, primarily because it requires less care to handle. Since 1971, CAMRA has conducted a consumer campaign on behalf of those who prefer traditional cask beer. CAMRA has lobbied the British Parliament to ensure support for cask ale and microbrewery have sprung up to serve those consumers who prefer traditional cask beer.
Pressurised CO2 in the keg's headspace maintains carbonation in the beer. The CO2 pressure varies depending on the amount of CO2 already in the beer and the keg storage temperature. Occasionally the CO2 gas is blended with nitrogen gas. CO2 / nitrogen blends are used to allow a higher operating pressure in complex dispensing systems.
Nitrogen is used under high pressure when dispensing dry Stout beer (such as Guinness) and other creamy beers because it displaces CO2 to (artificially) form a rich tight head and a less carbonated taste. This makes the beer feel smooth on the palate and gives a foamy appearance. Premixed bottled gas for creamy beers is usually 75% nitrogen and 25% CO2. This premixed gas, which only works well with creamy beers, is often referred to as Guinness Gas, Beer Gas, or Aligal (an Air Liquide brand name). Using "Beer Gas" with other can cause the last 5% to 10% of the beer in each keg to taste very flat and lifeless. In the UK, the term keg beer would imply the beer is pasteurised, in contrast to unpasteurised cask ale. Some of the newer microbreweries may offer a nitro keg stout which is filtered but not pasteurised.
In countries, such as China and Japan, the term "draft beer" () applied to canned or bottled beer indicates that the beer is not pasteurised (though it may be filtered), giving it a fresher taste but shorter shelf-life than conventional packaged beers.
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