Oxidation is the biggest threat to cask beer. As beer is drawn out of the cask air fills the space that remains. Air has an oxygen content of about 21% its effect on the flavour of beer is rapid and not very pretty. The presence of oxygen also encourages the growth of various non-beneficial bacteria that can lead to off-flavours. Heat speeds up both of these detrimental processes. For this reason, you have about three days to consume a refrigerated cask ale and as little as a day for one stored at room temperature.
However science can help even the most natural of products. To the rescue is the cask “breather” a device that will allow an inert gas to enter the cask instead of air. The inert gas of choice is nitrogen, which is about 78% of the air we breathe but does not interfere with the chemistry of the beer. The use of a breather should not be confused with kegged beer that is pushed in the beer lines under pressure. The cask breather does not pressurize the keg or introduce carbon dioxide into the beer.