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In food processing, pasteurization (also pasteurisation) is a process of food preservation in which packaged foods (e.g., milk and ) are treated with mild heat, usually to less than , to eliminate and extend shelf life. Pasteurization either destroys or deactivates and that contribute to food spoilage or the risk of disease, including vegetative bacteria, but most Endospore survive the process.P. J. Fellows (2025). 9780081019078, Woodhead Publishing Series in Food Science, Technology and Nutrition. ISBN 9780081019078
Pasteurization is named after the French microbiologist Louis Pasteur, whose research in the 1860s demonstrated that thermal processing would deactivate unwanted microorganisms in wine. Spoilage enzymes are also inactivated during pasteurization. Today, pasteurization is used widely in the dairy industry and other food processing industries for food preservation and food safety.
By the year 1999, most liquid products were heat treated in a continuous system where heat was applied using a heat exchanger or the direct or indirect use of hot water and steam. Due to the mild heat, there are minor changes to the nutritional quality and sensory characteristics of the treated foods. Pascalization or high-pressure processing (HPP) and pulsed electric field (PEF) are non-thermal processes that are also used to pasteurize foods.
In 1768, research performed by the Italian priest and scientist Lazzaro Spallanzani proved that a product could be made "sterile" after thermal processing. Spallanzani boiled meat broth for one hour, sealed the container immediately after boiling, and noticed that the broth did not spoil and was free from microorganisms. 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 French franc 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 (" The Art of Preserving Animal and Vegetable Substances"). This was the first cookbook on modern food preservation methods.
La Maison Appert , in the town of Massy, near Paris, became the first food-bottling factory in the world, preserving a variety of foods in sealed bottles. Appert filled thick, large-mouthed glass bottles with produce of every description, ranging from beef and fowl to eggs, milk, and prepared dishes. 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 appertisation in his honor.
Appert's method was so simple and workable that it quickly became widespread. In 1810, the British inventor and merchant Peter Durand, also of French origin, patented his method, but this time in a tin can, so creating the modern-day process of canning foods. In 1812, the Englishmen Bryan Donkin and John Hall purchased both patents and began producing preserves. A decade later, Appert's canning method had come to America. Tin can production was 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 Robert Yeates in 1855.
A less aggressive method was developed by French chemist Louis Pasteur during an 1864 summer holiday in Arbois. To remedy the frequent acidity of the local aged , he found out experimentally that it is sufficient to heat a young wine to only about for a short time to kill the microbes, and that the wine could subsequently be aged without sacrificing the final quality. In honor of Pasteur, this process is known as pasteurization. Pasteurization was originally used as a way of preventing wine and beer from souring,Carlisle, Rodney (2004). Scientific American Inventions and Discoveries, p. 357. John Wiley & Songs, Inc., New Jersey. . and it would be many years before milk was pasteurized. In the United States in the 1870s, before milk was regulated, it was common for milk to contain substances intended to mask spoilage.
Before industrialization, dairy cows were kept in urban areas to limit the time between milk production and consumption, hence the risk of disease transmission via raw milk was reduced. As urban densities increased and supply chains lengthened to the distance from country to city, raw milk (often days old) became 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. Because tuberculosis has a long incubation period in humans, it was difficult to link unpasteurized milk consumption with the disease. In 1892, chemist Ernst Lederle experimentally inoculated milk from tuberculosis-diseased cows into guinea pigs, which caused them to develop the disease. In 1910, Lederle, then in the role of Commissioner of Health, introduced mandatory pasteurization of milk in New York City.
Developed countries adopted milk pasteurization to prevent such disease and loss of life, and as a result, milk is now considered a safer food. A traditional form of pasteurization by scalding and straining of cream to increase the keeping qualities of butter was practiced in Great Britain in the 18th century and was introduced to Boston in the British Colonies by 1773,Kaden H. 2017. Food Preservation tools and techniques: In Food Industry process and technologies. Library press. pages 129–178 although it was not widely practiced in the United States for the next 20 years. Pasteurization of milk was suggested by Franz von Soxhlet in 1886.Franz Soxhlet (1886) "Über Kindermilch und Säuglings-Ernährung" (On milk for babies and infant nutrition), Münchener medizinische Wochenschrift (Munich Medical Weekly), vol. 33, pp. 253, 276. In the early 20th century, Milton Joseph Rosenau established standards – i.e. low-temperature, slow heating at for 20 minutes – for the pasteurization of milk while at the United States Marine Hospital Service, notably in his publication of The Milk Question (1912). States in the U.S. soon began enacting mandatory dairy pasteurization laws, with the first in 1947, and in 1973 the U.S. federal government required pasteurization of milk used in any interstate commerce.
The shelf life of refrigerated pasteurized milk is greater than that of raw milk. For example, high-temperature, short-time (HTST) pasteurized milk typically has a Refrigeration 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 non-refrigerated for up to 9 months.
According to the Centers for Disease Control, between 1998 and 2011, 79% of dairy-related disease outbreaks in the United States were due to raw milk or cheese products. They report 148 outbreaks and 2,384 illnesses (with 284 requiring hospitalization), as well as two deaths due to raw milk or cheese products during the same period.
In acidic foods (with pH of 4.6 or less), such as fruit juice and beer, the heat treatments are designed to inactivate enzymes (pectin methylesterase and polygalacturonase in fruit juices) and destroy spoilage microbes (yeast and lactobacillus). Due to the low pH of acidic foods, pathogens cannot grow. The shelf-life is thereby extended by several weeks. In less acidic foods (with pH greater than 4.6), such as milk and liquid eggs, heat treatments are designed to destroy pathogens and spoilage organisms (yeast and molds). Not all spoilage organisms are destroyed under pasteurization parameters, so subsequent refrigeration is necessary.
High-temperature short-time (HTST) pasteurization, such as that used for milk ( for 15 seconds) ensures the safety of milk and provides a refrigerated shelf life of approximately two weeks. In ultra-high-temperature (UHT) pasteurization, milk is pasteurized at for 1–2 seconds. Along with special packaging this extends shelf life to three months without refrigeration.
Most liquid foods are pasteurized by using a continuous process that passes the food through a heating zone, a hold tube to keep it at the pasteurization temperature for the desired time, and a cooling zone, after which the product is filled into the package. Plate heat exchangers are often used for low-viscosity products such as animal milk, nut milk, and juices. A plate heat exchanger is composed of many thin vertical stainless steel plates that separate the liquid from the heating or cooling medium.
Shell and tube heat exchangers are often used for the pasteurization of foods that are non-Newtonian fluids, such as dairy products, tomato ketchup, and baby foods. A tube heat exchanger is made up of concentric stainless steel tubes. Food passes through the inner tube or tubes, while the heating/cooling medium is circulated through the outer tube.
Scraped-surface heat exchangers are a type of shell and tube that contain an inner rotating shaft having spring-loaded blades that serve to scrape away any highly viscous material that accumulates on the wall of the tube.
The benefits of using a heat exchanger to pasteurize foods before packaging, versus pasteurizing foods in containers are:
After being heated in a heat exchanger, the product flows through a hold tube for a set period to achieve the required treatment. If pasteurization temperature or time is not achieved, a flow diversion valve is used to divert the under-processed product back to the raw product tank. If the product is adequately processed, it is cooled in a heat exchanger, then filled.
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 pasteurization. Destruction of alkaline phosphatase ensures the destruction of common milk pathogens. Therefore, the presence of alkaline phosphatase is an ideal indicator of pasteurization efficacy. For Breaker eggs, the effectiveness of the heat treatment is measured by the residual activity of Alpha-amylase.
The mean log10 reductions and temperatures of inactivation of the major milk-borne pathogens during a 15-second treatment are:
The Codex Alimentarius Code of Hygienic Practice for Milk notes that milk pasteurization is designed to achieve at least a 5 log10 reduction of Coxiella burnetii. The Code also notes that: "The minimum pasteurization conditions are those having bactericidal effects equivalent to heating every particle of the milk to for 15 seconds (continuous flow pasteurization) or for 30 minutes (batch pasteurization)” and that "To ensure that each particle is sufficiently heated, the milk flow in heat exchangers should be turbulent, i.e. the Reynolds number should be sufficiently high". The point about turbulent flow is important because simplistic laboratory studies of heat inactivation that use test tubes, without flow, will have less bacterial inactivation than larger-scale experiments that seek to replicate conditions of commercial pasteurization.
As a precaution, modern HTST pasteurization processes must be designed with flow-rate restriction as well as divert valves which ensure that the milk is heated evenly and that no part of the milk is subject to a shorter time or a lower temperature. It is common for the temperatures to exceed by .
The acceptance of double pasteurization varies by jurisdiction. In places where it is allowed, milk is initially pasteurized when it is collected from the farm so it does not spoil before processing. Many countries prohibit the labelling of such milk as "pasteurized" but allow it to be marked "thermized", which refers to a lower-temperature process. Heat Exchangers , Tetrapak Dairy Processing Handbook
Regarding color, the pasteurization process does not have much effect on pigments such as , anthocyanins, and in plants and animal tissues. In fruit juices, polyphenol oxidase (PPO) is the main enzyme responsible for causing browning and color changes. This enzyme is deactivated in the deaeration step before pasteurization with the removal of oxygen.
In milk, the color difference between pasteurized and raw milk is related to the homogenization step that takes place before pasteurization. Before pasteurization milk is homogenized to emulsify its fat and water-soluble components, which results in the pasteurized milk having a whiter appearance compared to raw milk. For vegetable products, color degradation is dependent on the temperature conditions and the duration of heating.
Pasteurization may result in some textural loss as a result of enzymatic and non-enzymatic transformations in the structure of pectin if the processing temperatures are too high as a result. With mild heat treatment pasteurization, tissue softening in the vegetables that causes textural loss is not of concern as long as the temperature does not get above .
Microwave volumetric heating (MVH) is the newest available pasteurization technology. It uses to heat liquids, suspensions, or semi-solids in a continuous flow. Because MVH delivers energy evenly and deeply into the whole body of a flowing product, it allows for gentler and shorter heating, so that almost all heat-sensitive substances in the milk are preserved.
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