Pasteurization is the use of heat to reduce the number of bacteria in a liquid. Developed by Louis Pasteur in 1864 as a technique to improve wine production, the process is now the primary method used to reduce bacterial pathogens in milk to levels that do not constitute a risk to human health. In the United States there are two standard time/temperature combinations used for milk pasteurization. The first is called vat or low-temperature long-time (LTLT) pasteurization which requires holding milk at 62.8 °C (145 °F) for 30 minutes. The second is called high-temperature short-time (HTST) pasteurization which requires keeping milk at 71.7 °C (161 °F) for 15 seconds. HTST pasteurization is done on a continuous flow basis and is favored by commercial manufacturers of fluid milk and cream. (Another continuous flow process called ultra-heat-treated (UHT) pasteurization is in use in Europe and Asia. UHT milk is heated to at least 135°C (275°F) for 1.0 seconds.)
These definitions were adapted from Encyclopedia of Dairy Sciences, Roginski, Fuquay & Fox, editors, Academic Press, 2003.