Louis Pasteur and Claude Bernard initiated a test of the idea of pasteurization by heating blood and urine in sealed flasks, 1862.
Milk is pasteurized by heating, typically at 63 °C for 30 minutes or at 71 °C for 15 seconds, to kill bacteria and extend the milk’s usable life. The process kills pathogens but leaves relatively benign microorganisms that can sour improperly stored milk. It’s this second group of microorganisms that ultrapasteurization targets.
In ultrapasteurization, also known as ultrahigh-temperature (UHT) pasteurization, the milk is heated to temperatures on the order of 140 °C. The heating can be accomplished one of two ways–directly or indirectly, explains H. Douglas Goff, a professor who specializes in dairy products in the department of food science at the University of Guelph in Ontario.
In the direct method, the milk is brought into contact with steam at 140 °C for one or two seconds. The heating is instantaneous, and the milk must be cooled rapidly by evaporative cooling–exposure to a slight vacuum–to remove any water added to the milk by condensation of the steam.
“The key to high-quality products is in the heating and cooling process,” Goff says. “You want to hit 140 °C, and no hold time is required to destroy microorganisms at that temperature. You have to get up to that temperature and get it cooled back down instantaneously. Otherwise, the milk is too hot for too long and it tastes burned.”
Visit “What’s That Stuff” to read more about pasteurization.
Excerpted with permission, Chemical & Engineering News
Copyright © 2001 American Chemical Society