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Describe pasteurization.

Pasteurization is the process of heat processing a liquid or a food to kill pathogenic bacteria to make the food safe to eat. The use of pasteurization to kill pathogenic bacteria has helped reduce the transmission of diseases, such as typhoid fever, tuberculosis, scarlet fever, polio, and dysentery.  

It is important to note that foods can become contaminated even after they have been pasteurized. For example, all pasteurized foods must be refrigerated. If the pasteurized food is temperature-abused (e.g., if milk or eggs are not kept refrigerated), it could become contaminated. Therefore, it is important to always handle food properly by handling it with clean hands, preventing it from becoming contaminated, and keeping it at a safe temperature.

How Pasteurization Works

Foods are heat-processed to kill pathogenic bacteria. Foods can also be pasteurized using gamma irradiation. Such treatments do not make the foods radioactive. The pasteurization process is based on the use of one of following time and temperature relationships.

High-Temperature-Short-Time Treatment (HTST) -- this process uses higher heat for less time to kill pathogenic bacteria. For example, milk is pasteurized at 161°F (72°C) for 15 seconds. 

Low-Temperature-Long-Time Treatment (LTLT) -- this process uses lower heat for a longer time to kill pathogenic bacteria. For example, milk is pasteurized at 145°F (63°C) for 30 minutes. 

It is important to remember that the times and temperatures depend on: (1) the type of food and (2) the final result one wants to achieve, such as retaining a food’s nutrients, color, texture, and flavor.

Processes Used to Pasteurized Foods

Flash Pasteurization - Involves a high-temperature, short-time treatment in which pourable products, such as juices, are heated for 3 to 15 seconds to a temperature that destroys harmful micro-organisms. After heating, the product is cooled and packaged. Most drink boxes and pouches use this pasteurization method as it allows extended unrefrigerated storage while providing a safe product.

Steam Pasteurization - This technology uses heat to control or reduce harmful microorganisms in beef. This system passes freshly-slaughtered beef carcasses that are already inspected, washed, and trimmed, through a chamber that exposes the beef to pressurized steam for approximately 6 to 8 seconds. The steam raises the surface temperature of the carcasses to 190° to 200° F (88° to 93°C). The carcasses are then cooled with a cold-water spray. This process has proven to be successful in reducing pathogenic bacteria, such as E. coli O157:H7, Salmonella, and Listeria, without the use of any chemicals. Steam pasteurization is used on nearly 50% of U.S. beef. 

Irradiation Pasteurization - Foods, such as poultry, red meat, spices, and fruits and vegetables, are subjected to small amounts of gamma rays. This process effectively controls vegetative bacteria and parasitic foodborne pathogens and increases the storage time of foods.

The Effect of Pasteurization on Nutrients and Flavor 

Pasteurization can affect the nutrient composition and flavor of foods. In the case of milk, for example, the high-temperature,short-time treatments (HTST) cause less damage to the nutrient composition and sensory characteristics of foods than do the low-temperature, long-time treatments (LTLT).

Examples of Foods that Are Commonly Pasteurized

Whole Eggs Removed from Shells and Sold As a Liquid - Large quantities of eggs are sold to restaurants and institutions out of the shell. The yolk and whole-egg products are pasteurized in their raw form. The egg white is pasteurized in its raw form if it is sold as a liquid or frozen product. 

Dried Eggs - If eggs are sold dried, the egg white with the glucose removed is normally heat-treated in the container by holding it for 7 days in a hot room at a minimum temperature of 130°F (54°C). 

Whole Eggs Pasteurized in the Shell - Traditionally, eggs sold to customers in the shell have not been pasteurized. However, new time/temperature pasteurization methods are making this possible. Egg whites coagulate at 140°F (60°C). Therefore, heating an egg above 140°F would cook the egg, so processors pasteurize the egg in the shell at a low temperature, 130°F (54°C), for a long time, 45 minutes. This new process is being used by some manufacturers, but it is not yet widely available. Pasteurizing eggs reduces the risk of contamination from pathogenic bacteria, such as Salmonella, which can cause severe illness and even death. Pasteurized eggs in the shell may be used in recipes calling for raw eggs, such as Caesar salad, hollandaise or bé arnaise sauces, mayonnaise, egg nog, ice cream, and egg-fortified beverages that are not thoroughly cooked.

Milk - Pasteurization improves the quality of milk and milk products and gives them a longer shelf life by destroying undesirable enzymes and spoilage bacteria. For example, the liquid is heated to 145°F (63°C) for at least 30 minutes or at least 161°F (72°C) for 15 seconds.

Today, many foods, such as eggs, milk, juices, spices and ice cream,are pasteurized. Sometimes higher temperatures are applied for a shorter period of time. The temperatures and times are determined by what is necessary to destroy pathogenic bacteria and other more heat-resistant disease-causing microorganisms that may be found in milk. The liquid is then quickly cooled to 40°F (4°C).  Other liquids, such as juices, are heat-processed in a similar manner. Temperatures and times vary, depending on the product and the target organism.

Other types of milk pasteurization

Ultrapasteurization - This involves the heating of milk and cream to at least 280° F (138° C) for at least 2 seconds, but because of less stringent packaging, they must be refrigerated. The shelf life of milk is extended 60 to 90 days. After opening, spoilage times for ultrapasteurized products are similar to those of conventionally pasteurized products. 

Ultra-High-Temperature (UHT) Pasteurization - typically involves heating milk or cream to 280° to 302°F (138° to 150°C) for 1 or 2 seconds. The milk is then packaged in sterile, hermetically-sealed (airtight) containers and can be stored without refrigeration for up to 90 days. After opening, spoilage times for UHT products are similar to those of conventionally pasteurized products. 

SOURCE:  Adapted from the A to Z Guide for Food Safety. The complete publication is available on-line at: