Pasteurization is the process of heat treating beer to inhibit the growth of potential beer spoilage microorganisms and prolong the shelf life of the beer. Named after the great French scientist Louis Pasteur, who was able to prolong the drinking quality of beer by holding the beer at 55°C–60°C (131°F–140°F) for a short time, pasteurization is used in the production of most of the draught and bottled/canned beers throughout the world. Pasteurization is often confused with sterilization. In the former, the beer is subject to sufficient heat processing to render the beer free from microbial spoilage during the course of its shelf-life. However, low levels of some microorganisms might still survive the heating, albeit without causing spoilage of beer. In sterilization, the heat treatment applied is of such intensity that it kills all microorganisms present.
Initially, based on a largely empirical observation, holding beer at a temperature of 60°C (140°F) for a few minutes was deemed sufficient to maintain microbiological integrity over its shelf-life of several months. The brewing industry uses this temperature of 60°C as a basis for quantifying the extent of the pasteurization process. For every minute the beer is held at 60°C it is said to be subject to one pasteurization unit (PU). Holding for 15 min at 60°C, therefore, is 15 PUs of treatment.
There are two major methods of beer pasteurization. Bottled and canned beers are pasteurized by passing the filled containers through a long, relatively narrow chamber in which hot water is sprayed over the containers for a fixed time before cooling. The spraying chamber is called a “tunnel” and the process is therefore called “tunnel pasteurization.” For large containers of beer such as kegs, heating the contents of the keg in a tunnel is impractical. Instead, beer is heat treated by being passed through a heat exchanger (hot water gives up its heat by exchanging with cold beer over a large surface area) where the beer will be heated to 70°C–72°C (158°F–162°F) for as little as 30 sec.
This is known as “high-temperature/short-time” processing or, more commonly, “flash pasteurization.” It is calculated that 72°C (162°F) for 30 sec is equivalent to 15 min at 60°C, or 15 PUs. Beer that has been flash pasteurized is then cooled rapidly before being filled into sterile containers. Depending on the perceived risk of microbiological spoilage of the beer, the number of PUs applied will vary but for most beers the range is 5–25 PUs, with 15 PUs the approximate industry standard. Beers with very low alcohol contents will tend to receive more PUs.
Although pasteurization is effective in preventing microbial spoilage in beer, it can adversely impact flavor by accelerating the “staling” or “aging” of beer. Staling is a natural phenomenon, the result of relatively slow chemical changes to the beer components with time, particularly if any quantity of oxygen is present in the beer at the time of filling the beer container. The application of heat during pasteurization increases the rate of the chemical changes. Brewers endeavor to minimize oxygen pickup during packaging and reduce PU application to a minimum to maintain the freshness of the beer for as long as possible. In recent years, instead of pasteurizing the beer, some brewers attempt to exclude beer spoilage microorganisms by a process of microfiltration known as “sterile filtration.”