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Pasteurization (or pasteurisation, see spelling differences) is a process of heating a food, which is usually a liquid, to a specific temperature for a predefined length of time and then immediately cooling it after it is removed from the heat. This process slows spoilage due to microbial growth in the food.
Unlike sterilization, pasteurization is not intended to kill all micro-organisms in the food. Instead, it aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated and is consumed before its expiration date). Commercial-scale sterilization of food is not common because it adversely affects the taste and quality of the product. Certain foods, such as dairy products, may be superheated to ensure pathogenic microbes are destroyed.
The process of heating wine for preservation purposes has been known in China since 1117, and was documented in Japan in 1568 in the diary Tamonin-nikki.
However, the modern version of pasteurization involving immediate cooling is much more recent. It was created by the renowned French chemist and microbiologist Louis Pasteur after whom it was named. The first pasteurization test was completed by Louis Pasteur and Claude Bernard in April 1862. The process was originally conceived as a way of preventing wine and beer from souring.
The process of pasteurization is applied to most milk today. Pasteurization [i.e., scalding and straining] of cream to increase the keeping qualities of butter was practiced in England before 1773 and was introduced to Boston in the USA by 1773, although it was not widely practiced in the United States for the next 20 years. It was still being referred to as a "new" process in American newspapers as late as 1802.
Pasteurization of milk was suggested by Franz von Soxhlet in 1886. It is the main reason for milk's extended shelf life. High-temperature, short-time (HTST) pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra-pasteurized milk can last much longer, sometimes two to three months. When ultra-heat treatment (UHT) is combined with sterile handling and container technology (such as aseptic packaging), it can even be stored unrefrigerated for 6 to 9 months.
Pasteurization typically uses temperatures below boiling, since at very high temperatures, casein micelles will irreversibly aggregate, or "curdle". The two main types of pasteurization used today are: 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 135°C (275°F) for a minimum of one second. ESL milk has a microbial filtration step and lower temperatures than UHT milk. Milk simply labeled "pasteurized" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurized" or simply "UHT" has been treated with the UHT method. Since 2007, however, it is no longer a legal requirement in European countries (such as Germany) to declare ESL milk as ultra-heated, consequently, it is now often labeled as "fresh milk" and just advertised as having an "extended shelf life", making it increasingly difficult to distinguish ESL milk from traditionally pasteurized fresh milk. A less conventional but US FDA-legal alternative (typically for home pasteurization) is to heat milk at 145 °F (63 °C) for 30 minutes.
Proponents of unpasteurized milk make the argument that if milk is obtained from humanely raised cows that are grass fed and handled hygienically, then there is little problem with disease. However, raw milk can become contaminated in a number of ways: by coming into contact with cow feces or bacteria living on the skin of cows, from an infection of the cow's udder, or from dirty equipment, among others. Improperly handled raw milk is responsible for nearly three times more hospitalizations than any other foodborne disease outbreak, making it one of the world's most dangerous food products.
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 to qualify for the "pasteurized" label. Standards for dairy products differ, depending on the fat content and the intended usage. For example, the pasteurization 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.
In Canada, all milk produced at a processor and intended for consumption must be pasteurized, legally requiring it to be heated to at least 72°C for at least 16 seconds, then cooling it to 4°C to ensure any harmful bacteria are destroyed.
The HTST pasteurization standard was designed to achieve a five-log reduction, killing 99.999% of the number of viable micro-organisms in milk. This is considered adequate for destroying almost all yeasts, molds, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic, heat-resistant organisms (including Mycobacterium tuberculosis, which causes tuberculosis, but not Coxiella burnetii, which causes Q fever). HTST pasteurization processes must be designed so the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature.
A process similar to pasteurization is thermization, which uses lower temperatures to kill bacteria in milk. It allows a milk product, such as cheese, to retain more of the original taste, but thermized foods are not considered pasteurized by food regulators.
Milk pasteurization has been scientifically proven to be at least 90% effective in eliminating harmful bacteria in milk. While some few pathogens are heat resistant, modern equipment is readily able to test and identify bacteria in milk being processed. Pasteurization is the only effective means of eliminating 90% or more of harmful organisms in milk.
Nonpasteurized, raw milk, according to the Centers for Disease Control (CDC), was responsible for 86 reported food poisoning outbreaks between 1998 and 2008, resulting in 1,676 illnesses, 191 hospitalizations, and two deaths. Improperly handled raw milk is responsible for nearly three times more hospitalizations than any other foodborne disease outbreak.
Diseases pasteurization can prevent include tuberculosis, brucellosis, diphtheria, scarlet fever, and Q-fever; it also kills the harmful bacteria Salmonella, Listeria, Yersinia, Campylobacter, Staphylococcus aureus, and Escherichia coli O157:H7, among others.
Proponents of non-pasteurized raw milk credit it with having more beneficial bacteria and enzymes than its processed counterpart; however, raw milk is far more likely to contain harmful microbial contaminants, and pasteurization is the only effective way of killing most pathogenic bacteria.. Raw milk does contain antimicrobial properties, which are destroyed with the heat of pasteurization, along with many of the vitamins within the milk itself.
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Raw milk expert testimony dated: April 25, 2008 Case: ORGANIC PASTURES DAIRY COMPANY, LLC, and CLARAVALE FARM, INC., Plaintiffs, vs. No. CU-07-00204 STATE OF CALIFORNIA and A.G. KAWAMURA, SECRETARY OF CALIFORNIA DEPARTMENT OF FOOD AND AGRICULTURE, Defendants. Expert Witnesses: Dr. Theodore Beals & Dr. Ronald Hull http://www.realmilk.com/documents/expert-testimony-0508.pdf
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