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. 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, 1864. The process was originally conceived as a way of preventing wine and beer from souring.[1]
Pasteurization is not intended to destroy 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 pasteurization product is stored as indicated and consumed before its expiration date). Commercial-scale sterilisation of food is not common because it adversely affects the taste and quality of the product. Certain food products are processed to achieve the state of commercial sterility.[2]
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Pasteurization is typically associated with milk, first suggested by Franz von Soxhlet in 1886. It is the main reason for milk's extended shelf life. High Temperature Short Time (HTST) pasteurised milk typically has a refrigerated shelf life of two to three weeks, whereas ultra pasteurised milk can last much longer when refrigerated, 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 3–4 months. [3] Ordinary pasteurized milk needs constant refrigeration and should have a shelf life of 10 to 16 days unopened.
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 fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST.[4] Milk simply labeled "pasteurised" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurised" or simply "UHT" has been treated with the UHT method.
Pasteurization methods are usually standardised 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 pasteurised 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 a least 72 degrees Celsius for at least 16 seconds 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, mold, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic heat-resistant organisms (though, not 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.
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).[5] One of these pathogens, Mycobacterium avium subsp. paratuberculosis (MAP), is linked to Crohn's Disease.[6] Researchers have developed more sensitive diagnostics, such as real-time PCR and improved culture methods that have enabled them to identify pathogens in pasteurised milk.
Some of the diseases that pasteurization can prevent are diphtheria, salmonellosis, strep throat, scarlet fever, listeriosis, brucellosis and typhoid fever.