Louis Pasteur found a practical method of preventing the spoilage of beer and wine. Pasteur used mild heating, which was sufficient to kill the organisms that caused the particular spoilage problem without seriously damaging the taste of the product. The same principle was later applied to milk to produce what we now call pasteurized milk.
Purpose of Pasteurization:
Pasteurization is not the same as sterilization. Its purpose is to reduce the bacterial population of a liquid such as milk and to destroy organisms that may cause spoilage and human disease. Spores are not affected by pasteurization. The intent of pasteurization of milk is to eliminate pathogenic microbes. It also lowers microbial numbers, which prolongs milk's good quality under refrigeration. Many relatively heat-resistant (thermoduric) bacteria survive pasteurization, but these are unlikely to cause disease or cause refrigerated milk to spoil.
In the classic pasteurization treatment of milk, the milk was exposed to a temperature of about 63oC for 30 minutes, called the holding method. Most milk pasteurization today uses higher temperatures, at least 72oC, but for only 15 seconds. This treatment, known as high-temperature short-time (HTST) pasteurization, is applied as the milk flows continuously past a heat exchanger. In addition to killing pathogens, HTST pasteurization lowers total bacterial counts, so the milk keeps well under refrigeration.
Milk can also be sterilized – something quite different from pasteurization --- ,Ultrahigh temperature (UHT) processing raises the temperature from 74 ° C to 140 ° C and then drops it back to 74 ° C in less than 5 seconds. In the United States, sterilization is sometimes used on the small containers of coffee creamers found in restaurants. To avoid giving the milk a cooked taste, a UHT system is used in which the liquid milk never touches a surface hotter than the milk itself while being heated by steam. The milk falls in a thin film through a chamber of superheated steam and reaches 140oC and drops back to 74oC .
Aims of pasteurization:
For decades, pasteurization has been aimed at destroying mycobacterium tuberculosis, long considered the most heat resistant bacterium. More recently, however, attention has shifted to destruction of Coxiella burnetii, the agent of Q fever, because these organisms have a higher resistance to heat. Since both organisms are eliminated by pasteurization, dairy microbiologists assume that other pathogenic bacteria are also destroyed.
Factors of Pasteurization:
Products other than milk, such as ice cream, yogurt, and beer, all have their own pasteurization times and temperatures, which often differ considerably. There are several reasons for these variations. For example, heating is less efficient in foods that are more viscous, and fats in food can have a protective effect on microorganisms.
Perfect Pasteurization Indicators:
The dairy industry routinely uses to test to determine whether products have been pasteurized: the phosphatase test (phosphatase is an enzyme naturally present in milk). If the product has been pasteurized, phosphatase will have been inactivated.
Concept of equivalent Treatment:
The heat treatments we have just discussed illustrate the concept of equivalent treatments: as the temperature is increased, much less time is needed to kill the same number of microbes. For example, the destruction of highly resistant endospores might take 70 minutes at 115oC, whereas only 7 minutes might be needed at 125oC. Both treatments yield the same result. The concept of equivalent treatments also explains why classic pasteurization at 63oC for 30 minutes, HTST treatment at 72oC for 15 seconds, and UHT treatment at 140oC for less than a second can have similar effects.
Significance of Pasteurization:
Some years ago certain strains of bacteria of the genus Listeria were found in pasteurized milk and cheeses. This pathogen causes diarrhea and encephalitis and can lead to death in pregnant women. A few such infections have prompted questions about the need to revise standard procedures for pasteurization. However, finding these pathogens in pasteurized milk has not become a persistent problem, and no action has been taken.
Although most milk for sale in the United States is pasteurized fresh milk, sterile milk also is available. All evaporated or condensed canned milk is sterile, and some milk packaged in cardboard containers also is sterile. The canned milk is subjected to steam under pressure and has a "cooked" flavor. Sterilized milk in cardboard containers is widely available in Europe and can be found in some stores in the United States. It is subjected to a process that is similar to pasteurization but uses higher temperatures. It too has a "cooked" flavor but can be kept unrefrigerated as long as the container remains sealed. Such milk is often flavored with vanilla, strawberry, or chocolate.