Pasteurization

Pasteurization, heat-treatment process that destroys pathogenic microorganisms in certain foods and beverages. It is named for the French scientist Louis Pasteur, who in the 1860s demonstrated that abnormal fermentation of wine and beer could be prevented by heating the beverages to about 57 °C (135 °F) for a few minutes. Pasteurization of milk, widely practiced in several countries, notably the United States, requires temperatures of about 63 °C (145 °F) maintained for 30 minutes or, alternatively, heating to a higher temperature, 72 °C (162 °F), and holding for 15 seconds. The times and temperatures are those determined to be necessary to destroy Mycobacterium tuberculosis and other, more heat-resistant, non-spore-forming, disease-causing microorganisms found in milk. The treatment also destroys most of the microorganisms that cause spoilage and so prolongs the storage time of food.

Types of milk pasteurization

Milk can be pasteurized either through the batch (or vat) process or through continuous flow pasteurization. Batch pasteurization involves heating milk in large, double-jacketed vats at low temperatures for a long time, usually to 63 °C (145 °F) for 30 minutes. The first commercial milk pasteurization plant established in the United States in the early 20th century by the physician and public health activist Charles North used this low-temperature–long-time (LTLT) process. The process is still used in small processing units where the yield is typically less than 1,000 liters (264 gallons) per hour. More modern techniques involve continuously passing milk from a storage tank through a series of heat exchangers (thin metal plates), which typically heat the milk to 72 °C (162 °F) for 15 seconds, followed by rapid cooling to 4 °C (39 °F). This technique, used in continuous flow operations, is called high-temperature–short-time (HTST) pasteurization and is suitable for large-scale, commercial units that process more than 5,000 liters (1,320 gallons) of milk per hour on average.

Continuous flow pasteurization methods that involve still higher temperatures have also been developed. In higher-heat–shorter-time (HHST) pasteurization, milk is heated to 88.3 °C (190.94 °F) for one second or 90 °C (194 °F) for half a second, whereas in ultrahigh-temperature (UHT) pasteurization, milk or cream is heated to 138–150 °C (280–302 °F) for one or two seconds. Packaged in sterile, hermetically sealed containers, UHT milk may be stored without refrigeration for six to nine months, whereas refrigerated HTST milk can be stored only for two to three weeks. Ultrapasteurized (UP) milk and cream are heated to at least 138 °C (280.4 °F) for at least two seconds, but, because of less stringent packaging, they must be refrigerated. Shelf life is extended to 60–90 days. After opening, spoilage times for both UHT and UP products are similar to those of conventionally pasteurized products.

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Other types of pasteurization

Pasteurization of some solid and viscous foods involves a mild heat treatment, the exact definition of which depends on the food. In general, food and drinks that are acidic—that is, have a low intrinsic pH—require lower temperatures for pasteurization. Several nonthermal mechanisms of improving the shelf life of comestibles have been developed, including treatment with high pressure, electric pulse, chemicals, and radiation. Radiation pasteurization refers to the application of small amounts of beta or gamma rays to foods to increase their storage time. In high-pressure pasteurization (HPP), application of 200 and 300 megapascal (about 2,040 and 3,060 kg per square centimeter) pressure to milk at 30 °C (86 °F) results in similar shelf life as that achieved by pasteurization at 90 °C (194 °F) for 15 seconds.