Why isn't all beer real?
Real ale is a natural, living product. By its nature this means it has a limited shelf life and needs to be looked after with care in the pub cellar and kept at a certain temperature to enable it to mature and bring out its full flavours for the drinker to enjoy.
Brewery-conditioned, or keg, beer has a longer shelf life as it is not a living product. Basically, after the beer has finished fermentation in the brewery and has been conditioned, it is chilled and filtered to remove all the yeast and then it is pasteurised to make it sterile. This is then put in a sealed container, called a keg, ready to be sent to the pub.
The problem is that removing the yeast and ‘killing off' the product through pasteurisation also removes a great deal of the taste and aroma associated with real ale. Because there is no secondary fermentation occurring in the container (i.e keg) in which is held, there is no natural carbonation of the beer so gas either carbon dioxide or a mixture of carbon dioxide and nitrogen has to be added to "fizz up" the beer. This creates an unnaturally fizzy beer rather than the gentle carbonation produced by the slow secondary fermentation in a cask of real ale.