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Physical Agents

©2000 Kenneth Todar, University of Wisconsin-Madison

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.

  1. Sterilization (boiling, autoclaving, hot air oven) kills all microorganisms with heat; commonly employed in canning, bottling, and other sterile packaging procedures.

  2. 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. For pasteurzation of milk: batch method: 63°C for 30 minutes; flash method: 71 °C for 15 seconds.

Low temperature (refrigeration and freezing): Most organisms grow very little or not at all at 0°C. Store perishable foods at low temperatues to slow rate of growth and consequent spoilage (e.g. milk). 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.

Drying (removal of H2O): Most microorganisms cannot grow at reduced water activity (Aw < 0.90). Often used to preserve foods (e.g. fruits, grains, etc.). Methods involve removal of water from product by heat, evaporation, freeze-drying, addition of salt or sugar.

Irradiation (microwave, UV, x-ray): destroys microorganisms as described under sterilization. Many spoilage organisms are easily killed by irradiation. In some parts of Europe, fruits and vegetables are irradiated to increase their shelf life up to 500 percent. The practice has not been accepted in the U.S. Although the FDA has approved its use for meat. Public attitudes are starting to change with the recent food borne outbreaks and positive articles about irradiation from reputable sources.

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