The lethal temperature varies in microorganisms. The time required to kill depends on the number of organisms, species, nature of the product being heated, pH, and temperature. Whenever heat is used to control microbial growth inevitably both time and temperature are considered .
Pasteurization is the use of mild heat to reduce the number of microorganisms in a product or food.
In the case of pasteurization of milk the time and temperature depend on killing potential pathogens that are transmitted in milk, i.e., staphylococci, streptococci, Brucella abortus and Mycobacterium tuberculosis . Low temperatures are not bactericidal. Psychrotrophs, rather than true psychrophiles, are the usual cause of food spoilage in refrigerated foods. Lysteria moncytogenes is of great concern in refrigerated foods and has been the topic of recent news articles and FDA action. Many spoilage organisms are easily killed by irradiation.