The Oxford Companion to Beer definition of
A cask breather,
A Cask Breather, sometimes called an “aspirator,” is a demand valve used in conjunction with a beer engine and a carbon dioxide tank for the dispense of cask-conditioned beers. It allows beer drawn from the cask to be replaced with the equivalent amount of sterile gas at atmospheric pressure. See beer engine. This is a nontraditional and thus controversial method of dispensing cask-conditioned ale because some purists, including the British Campaign for Real Ale (CAMRA), take the position that the only proper way to pour cask ale is to allow ambient air, not gas, to enter the cask as the beer engine empties it. Such air, of course, not only contains oxygen, which can cause the beer to become stale fairly quickly, but also harbors airborne bacteria, such as acetobacter and lactobacillus, which may quickly have deleterious effects, especially in the presence of oxygen. CAMRA argues that oxygen can actually improve a cask ale’s flavors over the very limited number of hours that it is servable after the cask has been broached. They also argue that the use of the cask breather allows ingress of CO2 into the beer, changing flavor and texture, and that the device is the “thin edge of the wedge,” a “crutch” that will ease the way for various other changes to cask beer service. Extensive taste testing has failed to demonstrate that the cask breather has any effect other than to extend the service life of a broached cask by an extra day or two, which is sometimes critical to the trade of smaller, quieter pubs, especially in the countryside. The debate rages on, but CAMRA continues its policy of delisting from its influential yearly Good Beer Guide any pub where the cask breather is in use.