Pasteurization of Milk
Milk is an excellent growth medium for microorganisms such as bacteria, yeast, and molds. In the early 1900s, it was frequently the vehicle for carrying such serious foodborne illnesses as typhoid, diphtheria, scarlet fever, and tuberculosis. Pasteurization, named after Louis Pasteur (1822-1895), was originally used to treat wine and beer, but soon came into use to treat milk as well, when it was found that heating milk for a short time to below its boiling point killed microorganisms. Pasteurization destroys 100 percent of pathogenic bacteria, yeasts, and molds and 95 to 99 percent of other, nonpathogenic bacteria. The process of pasteurization also inactivates many of the enzymes that cause the off-flavors of rancidity. Almost all milk sold commercially in North America is first pasteurized. In some states, where allowed by law, there is a small niche for unpasteurized, or raw, milk.
To ensure that sufficient pasteurization has occurred, milk processors measure the activity of a specific enzyme found in milk, alkaline phosphatase. If this enzyme is no longer active, then the milk is safe for consumption. Pasteurization temperatures and times vary. Pasteurized milk, while no longer pathogenic, will still spoil, however, because the 1 to 5 percent nonpathogenic bacteria remaining convert lactose to lactic acid.
A process called ultra pasteurization uses higher temperatures than regular pasteurization temperatures to extend the shelf life of refrigerated milk products. If this same treatment is combined with sterile packaging techniques, it is called ultrahigh-temperature (UHT) processing. UHT processing destroys even more bacteria than standard pasteurization and increases the milk’s shelf life. This milk is then packaged aseptically in sterile containers and sealed so that it can be stored unrefrigerated for up to three months. Once the aseptic seal is broken, the milk must be refrigerated. Originally, this preparation method was used on less frequently purchased milk products such as whipping cream, half and half, and eggnog, but it is now used on a wider variety of products.
