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Pasteurization or pasteurisation is a process of heating 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. It is named after microbiologist Louis Pasteur for his work on his pasteurization process. This process slows spoilage caused by 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 1900 in the diary Tamonin-nikki.
Much later, in 1768, an Italian priest and scientist Lazzaro Spallanzani proved experimentally that heat killed bacteria, and that they do not re-appear if the product is hermetically sealed. In 1795, a Parisian chef and confectioner named Nicolas Appert began experimenting with ways to preserve foodstuffs, succeeding with soups, vegetables, juices, dairy products, jellies, jams, and syrups. He placed the food in glass jars, sealed them with cork and sealing wax and placed them in boiling water. In that same year, the French military offered a cash prize of 12,000 francs for a new method to preserve food. After some 14 or 15 years of experimenting, Appert submitted his invention and won the prize in January 1810. Later that year, Appert published L'Art de conserver les substances animales et végétales (or The Art of Preserving Animal and Vegetable Substances). This was the first cookbook of its kind on modern food preservation methods.
La Maison Appert (English: The House of Appert), in the town of Massy, near Paris, became the first food-bottling factory in the world, preserving a variety of food in sealed bottles. Appert's method was to fill thick, large-mouthed glass bottles with produce of every description, ranging from beef and fowl to eggs, milk and prepared dishes. His greatest success for publicity was an entire sheep. He left air space at the top of the bottle, and the cork would then be sealed firmly in the jar by using a vise. The bottle was then wrapped in canvas to protect it, while it was dunked into boiling water and then boiled for as much time as Appert deemed appropriate for cooking the contents thoroughly. Appert patented his method, sometimes called in his honor "appertisation".
Appert's method was so simple and workable that it quickly became widespread. In 1810, British inventor and merchant Peter Durand, also of French origin, patented his own method, but this time in a tin can, so creating the modern-day process of canning foods. In 1812, Englishmen Bryan Donkin and John Hall purchased both patents and began producing preserves. Just a decade later, Appert's method of canning had made its way to America. Tin can production was however not common until the beginning of the 20th century, partly because a hammer and chisel were needed to open cans until the invention of a can opener by an Englishman named Yates in 1855.
Appert's preservation by boiling involved heating the food to an unnecessarily high temperature, and for an unnecessarily long time, which could destroy some of the flavour of the preserved food.
A more timid method was developed by the renowned French chemist and microbiologist Louis Pasteur during his 1864 summer vacation in Arbois, to remedy the frequent acidity of the local wines. He found out experimentally that it is sufficient to heat a young wine to only about 50–60 °C (122–140 °F) for a brief time to kill the microbes, and that the wine could be nevertheless properly aged without sacrificing the final quality. In honor of Pasteur, the process became known as "pasteurization". Pasteurization was originally used as a way of preventing wine and beer from souring, and it would be many years before milk was pasteurized. In the United States in the 1870s, it was common for milk to contain contaminants to mask spoilage before milk was regulated.
The Centers for Disease Control (CDC) says 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. 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.
Pasteurization 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.
Before the widespread urban growth caused by industrialization, people kept dairy cows even in urban areas, and the short time between milking and consumption minimised the disease risk of drinking raw milk. However, as urban densities increased and supply chains lengthened to the distance from country to city, the often days-old raw milk began to be recognized as a source of disease. For example, between 1912 and 1937 some 65,000 people died of tuberculosis contracted from consuming milk in England and Wales alone.
Developed countries adopted milk pasteurization to prevent such disease and loss of life, and as a result milk is now widely considered one of the safest foods. A traditional form of pasteurization by 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.
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, also known as "flash") and extended shelf life (ESL). Ultra-high temperature processing (UHT, also known as ultra-heat-treating) 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 72°C (161°F) for 15 seconds.:8 UHT processing holds the milk at a temperature of 138°C (280°F) for a minimum of two seconds.:90 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 (for example in 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.
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 that milk be HTST pasteurized to qualify for the pasteurized label. Dairy product standards differ, depending on fat content and intended usage. For example, pasteurization standards for cream differ from standards for fluid milk, and standards for pasteurizing cheese are designed to preserve the enzyme phosphatase, which aids cutting. In Canada, all milk produced at a processor and intended for consumption must be pasteurized, which legally requires that it 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 UK Dairy Products Hygiene Regulations 1995 requires that milk be heat treated for 15 seconds at 71.7 °C or other effective time/temperature combination.
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.
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). As a precaution, modern equipment tests and identifies bacteria in milk being processed. 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.
Even pasteurization without quality control can be effective, though this is generally not permitted for human consumption; a study of farms feeding calves on pasteurised waste milk using a mixture of pasteurization technologies (none of which were routinely monitored for performance) found the resulting pasteurized milk to meet safety requirements at least 92% of the time.
An effect of the heating of pasteurization is that some vitamin, mineral, and beneficial (or probiotic) bacteria is lost. Soluble calcium and phosphorus decrease by 5%, thiamin and vitamin B12 by 10%, and vitamin C by 20%. However, these losses are not significant nutritionally. 
Direct microbiological techniques are the ultimate measurement of pathogen contamination but these are costly and time consuming (24-48 hours), which means that products are able to spoil by the time pasteurisation is verified.
As a result of the unsuitability of microbiological techniques, milk pasteurization efficacy is typically monitored by checking for the presence of alkaline phosphatase, which is denatured by pasteurisation. B. tuberculosis, the bacterium requiring the highest temperature to be killed of all milk pathogens is killed at all ranges of temperature and time which also denatured alkaline phosphatase. As a result of this, presence of alkaline phosphatase was deemed to be an ideal diagnostic for pasteurization efficacy. Phosphatase denaturation was originally monitored using a phenol-phosphate substrate. When hydrolysed by the enzyme these compounds liberate phenols, which were then reacted with dibromoquinonechlorimide to give a colour change, which was measured by checking absorption at 610 nm (spectrophotometry). Some of the phenols used were inherently coloured (phenolpthalein, nitrophenol) and were simply assayed unreacted. Spectophotometric analysis is satisfactory but it is relatively low accuracy because many natural products are coloured. As a result of this, modern systems (since 1990) use fluorometry which is able to detect much lower levels of raw milk contamination.
According to the Centers for Disease Control between 1998 and 2011 79% of the dairy related outbreaks were due to raw milk or cheese products. They report 148 outbreaks, 2,384 illnesses (284 requiring hospitalizations) as well as 2 deaths due to raw milk or cheese products during the same time period.
As pasteurisation is a very old and traditional way of preservation, it is well known and accepted by consumers. Nearly every label of milk products contains the word "pasteurisation" and it is associated by consumers with good quality attributes and safety. In the consumer studies of Hightech Europe consumers mentioned more positive than negative associations for this technology showing that these products are well accepted.
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