Cask Ale - Cask Breather

Cask Breather

When a cask has been tapped, the beer starts to come into contact with oxygen—and a beer in contact with oxygen has a limited life. Stronger beers last longer, but for most ales with an ABV in the low 4% region, three days is typical. If the pub doesn't have a high turnover, or if a beer is not popular, such as dark milds, three days will not be enough to sell all the beer in the cask. A cask breather allows a small amount of CO₂ to replace the oxygen in the cask. Not enough CO₂ to enter the beer or push it up to the bar—that's "top pressure"—but enough "blanket pressure" to keep the beer fresh tasting for longer by replacing some or most of the oxygen that has made it into the cask with CO₂, an inert gas.

The use of cask breathers is considered "extraneous carbon dioxide", so CAMRA does not endorse this method.