Draught beer
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Draught beer (also called draft beer or tap beer) is beer that has been served from, or has been conditioned in, a cask.
The term draught beer is used because, originally, beer was pulled from casks with a hand pump. The word "draught" comes from the verb "to draw" (in the sense of "to pull"), and thus means beer that is drawn from a barrel. This is still widespread for real ale. In modern commercial beer dispense, the metal keg barrel is pressurized with carbon dioxide (CO2) gas. Pressure in the keg drives the beer to the dispensing tap, or faucet.
The pressure of the CO2 in the keg is intended to maintain the dissolved CO2 in the beer. The CO2 pressure varies depending on the amount of CO2 the brewer crafted into the beer and the keg storage temperature. Occasionally the CO2 gas is blended with nitrogen (N2) gas. Blending with nitrogen gas allows the keg to be pressurized above the ideal set point for 100% CO2. Nitrogen is used because it is 80 times less soluable in water than CO2 (it is much more difficult to 'nitrogenate' than to 'carbonate').
Draught beer is usually unpasteurised and therefore suffers no loss of taste due to heating of pasteurisation. It should be consumed quickly after being "tapped", and is generally truer to the flavours of the ingredients as pasteurisation exposes the beer to heat and changes the taste profile. Draught beer should be kept refrigerated between 2°C (35°F) and 4°C (40°F). Above 6°C (44°F), a beer may become wild, turn sour and cloudy in a day or two. Below 6°C (44°F), a keg of draft beer should last 20-30 days before it loses its fresh brewery taste and aroma.
Recently, the term has been used misleadingly to describe unpasteurised canned or bottled beers, implying that these taste and appear as cask ales. Some types of canned 'draught' beer use widgets to simulate the turbulence caused when draught beer is forced or pulled through a sparkler draught faucet (also know as a swan neck).de:Fassbier