When that second fermentation occurs in the cask, it's called cask conditioning, and the layer of yeast collects in the bottom of the cask. Normally, draft beer is dispensed by forcing CO2 into the keg and pushing it up through the draft lines. This method not only adds carbonation to the beer, but also protects the beer remaining in the keg from spoiling. In both gravity and hand pump dispensing, the displaced beer in the cask is replaced by air, which will break down the beer quicker than CO2 will in a conventional keg. Because the beer is undergoing a second fermentation as it's being poured, it creates its own carbonation.
Pasteurization is a process which keeps perishable liquids from spoiling quickly.
It can be done by several methods, the most common of which in the beer business is to pass a filled keg or bottle through a hot water bath to kill any microorganisms that might remain in the beer. The alcohol in beer will kill any harmful bacteria, which is why unpasteurized beer is not the health risk that unpasteurized milk is. The second problem with cask-conditioned ale is the special care it requires in order to be served at optimum flavor. If you are considering selling cask-conditioned ale, look to your distributor or brewery to supply this stuff. Special handling A cask needs to be put on its stillage about 24 hours before it's tapped so the yeast can settle to the bottom.