Pasteurization is a process which retards microbial growth in foods. The process was named after its creator, French chemist and microbiologist Louis Pasteur. The first pasteurization test was completed by Louis Pasteur and Claude Bernard on April 20, 1862.
Unlike sterilization, pasteurization is not intended to kill all pathogenic micro-organisms in the food or liquid. Instead, pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurisation product is refrigerated and consumed before its expiration date). Commercial-scale sterilization of food is not common because it adversely affects the taste and quality of the product.
Pasteurization typically uses temperatures below boiling since at temperatures above the boiling point for milk, casein micelles will irreversibly aggregate (or "curdle"). There are two main types of pasteurization used today: High Temperature/Short Time (HTST) and Extended Shelf Life (ESL) treatment. Ultra-high temperature (UHT or ultra-heat treated) is also used for milk treatment. In the HTST process, milk is forced between metal plates or through pipes heated on the outside by hot water, and is heated to 71.7 °C (161 °F) for 15-20 seconds. UHT processing holds the milk at a temperature of 138 °C (250 °F) for a fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST.[1] Milk simply labeled "pasteurisation " is usually treated with the HTST method, whereas milk labeled "ultra-pasteurization " or simply "UHT" has been treated with the UHT method.
Pasteurization methods are usually standardized and controlled by national food safety agencies (such as the USDA in the United States and the Food Standards Agency in the United Kingdom). These agencies require milk to be HTST pasteurized in order to qualify for the "pasteurisation " label. There are different standards for different dairy products, depending on the fat content and the intended usage. For example, the pasteurisation standards for cream differ from the standards for fluid milk, and the standards for pasteurizing cheese are designed to preserve the phosphatase enzyme, which aids in cutting.
The HTST pasteurization standard was designed to achieve a 5-log reduction, killing 99.999% of the number of viable micro-organisms in milk. This is considered adequate for destroying almost all yeasts, mold, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic heat-resistant organisms (including Mycobacterium tuberculosis, which causes tuberculosis and Coxiella burnetii, which causes Q fever). HTST pasteurization processes must be designed so that the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature.
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A newer method called flash pasteurization involves shorter exposure to higher temperatures, and is claimed to be better for preserving color and taste in some products.
The term cold pasteurization is used sometimes for the use of ionizing radiation (see Food irradiation) or other means (e.g. chemical) to kill bacteria in food. Food irradiation is also sometimes called "electronic pasteurization".
Pasteurization is typically associated with milk, first suggested by Franz von Soxhlet in 1886. HTST pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra pasteurized milk can last much longer when refrigerated, sometimes two to three months. When UHT treatment is combined with sterile handling and container technology (such as aseptic packaging), it can even be stored unrefrigerated for 3-4 months.
In addition to the standard HTST and UHT standards, there are other lesser-known pasteurization techniques. The first technique, called "batch pasteurization", involves heating large batches of milk to a lower temperature, typically 63 °C (145 °F) for 30 minutes, followed by quick cooling to about 4 °C (39 °F). The other technique is called higher-heat/shorter time (HHST), and it lies somewhere between HTST and UHT in terms of time and temperature. Pasteurization causes some irreversible and some temporary denaturation of the proteins in milk.
In 2001, the Animal and Plant Health Inspection Service of the USDA considered new rules requiring double pasteurization, which would have entailed holding milk at 72 °C (161 °F) for two separate 15-second periods, instead of one 30-second period as was the current standard. In most jurisdictions, milk treated by double pasteurization might not be considered pasteurized, depending on the temperature and duration of the heat treatment. Heat treatment might also be performed at a lower temperature or for a shorter time. Such milk could possibly be called "raw milk" or, confusingly, "unpasteurized milk". It cannot be called "pasteurized", even though a significant number of pathogens are destroyed during the process.
In regions including Africa and South Asian countries, it is common to boil milk to sterilize it after it is harvested. This intense heating greatly changes the flavor of milk, which the respective people are accustomed to.
Milk pasteurization has been subject to increasing scrutiny in recent years, due to the discovery of pathogens that are both widespread and heat resistant (able to survive pasteurization in significant numbers).[2] Researchers have developed more sensitive diagnostics, such as real-time PCR and improved culture methods that have enabled them to identify pathogens in pasteurized milk.
Some of the diseases that pasteurization can prevent are tuberculosis, diphtheria, polio, salmonella, strep throat, scarlet fever, and typhoid fever.