Pasteurization is a process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food.
Unlike sterilization, pasteurization is not intended to kill all micro-organisms in the food. Instead pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated 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. Certain food products, like dairy products, are superheated to ensure pathogenic microbes are destroyed.[1]
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Pasteurization is typically associated with milk. Pasteurization [i.e., scalding and straining] of cream to increase the keeping qualities of butter was practiced in England before 1773 and had been introduced to Boston, New England, by 1773,[2] although it was not widely practiced in the US for the next twenty years. It was still being written of as a new process in American newspapers as late as 1802.[3]
Pasteurization of milk was suggested by Franz von Soxhlet in 1886.[4] 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 6–9 months
Pasteurization typically uses temperatures below boiling since at very high temperatures 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 135 °C (275 °F) for a minimum of one second. ESL milk has a microbial-filtration step and lower temperatures than UHT milk.[5] 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.[6]
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.[7] 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. 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. [8]
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 "pasteurization" label. There are different standards for different dairy products, 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 degrees Celsius for at least 16 seconds[9] and then cooling it to 4 degrees Celsius. This ensures that any harmful bacteria are destroyed.
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, 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 that 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.[6]
Milk pasteurization has been scientifically proven to be at least 90% effective in eliminating harmful bacteria in milk. While there are some few pathogens which 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.[10]
Non-pasteurized, 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. Raw milk is responsible for nearly three times more hospitalizations than any other foodborne disease outbreak.[11]
Diseases that 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 157 [12][13] among others.
Fans of raw milk (meaning milk that hasn't been pasteurized or homogenized) credit it with having more beneficial bacteria and enzymes than its processed counterpart. However, raw milk cannot be preserved for a long time and its disadvantages may exceed its benefits. In fact, raw milk is far more likely to contain harmful microbial contaminants, and pasteurization is the only effective way of killing most pathogen bacteria — which can include a.o., listeria, salmonella, and E. coli.[14] On the other hand raw milk does contain antimicrobial properties [15] which are destroyed with the heat of pasturization[16], along with many of the vitamins within the milk itself. [17] Raw milk consumption has also been shown to positively influence the immune system's resistance to the development of asthma, hay fever, and atopic sensitization, although the mechanism is not entirely understood. [18]
The process of heating wine for preservation purposes has been known in China since 1117,[19] and is documented in Japan in 1568 in the diary Tamonin-nikki, but the modern version involving immediate cooling was created by the French chemist and microbiologist Louis Pasteur, after whom it is named. The first pasteurization test was completed by Louis Pasteur and Claude Bernard in April 1862.[20] The process was originally conceived as a way of preventing wine and beer from souring.[21] The Talmud Bavli (Holin 84b) cautions against drinking water that was only lukewarm and suggests it should be boiled first to avoid harm.