Most milk goes through several processes before consumers purchase it.
The terms "ultra-pasteurized" and "homogenized" are usually associated with the production of milk. Milk is generally both pasteurized and homogenized; pasteurization is a process milk manufacturers use to ensure that the milk is safe to drink, while homogenization ensures that the milk is of uniform consistency.
Pasteurization
Pasteurization -- named for its inventor, Louis Pasteur -- is a was first used in 1862. During pasteurization, liquids are heated to below-boiling temperatures to destroy bacteria, protozoa, yeasts and molds. This process is well suited for sterilizing milk, as when milk curdles when boiled.
Ultra-Pasteurization
Ultra-pasteurization (also known as ultra-heat temperature or UHT) uses a process that forces milk between metal plates or pipes and held at 250 degrees F -- a much higher temperature than standard pasteurization -- for less than one second. The purpose for using this method is to extend the shelf life of the milk; pasteurized milk normally lasts for around two or three weeks; while ultra-pasteurized milk can last two or three months.
Homogenization
The homogenization process is used to obtain an even consistency in milk (or another liquid) and involves breaking apart cells in the liquid. In milk, homogenization is used to prevent the natural separation of the cream from the liquid in the milk; without homogenization, the cream naturally rises, leaving the rest of the liquids below it. To accomplish homogenization, milk is forced through small orifices at a very high pressure, causing the cells to break apart.
Details
Homogenization is an important process used also to develop different types of milk. For example, 2% milk is made by eliminating some of the fat content in the milk during homogenization; 1% and skim milk are also made this way. Some people prefer to drink milk that has not been pasteurized or homogenized; such milk is harder to find, but often there "raw" versions of milk are available at specialty or health food stores.
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