Pasteurisation (or pasteurization) is the process of heating liquids for the purpose of destroying viruses and harmful organisms such as bacteria, protozoa, molds, and yeasts. The process was named after its inventor, French chemist and microbiologist Louis Pasteur. The first pasteurisation test was completed by Pasteur and Claude Bernard on April 20, 1862.
Unlike sterilisation, pasteurisation is not intended to kill all micro-organisms (pathogenic) in the food. Instead, pasteurisation aims to achieve a "logarithmic reduction" in the number of viable organisms, reducing their number so they are unlikely to cause disease (assuming the pasteurised product is refrigerated 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.
Pasteurisation typically uses temperatures below boiling since at temperatures above the boiling point for milk, casein micelles will irreversibly aggregate (or "curdle"). There are two main types of pasteurisation used today: high temperature/short time (HTST) and Extended Shelf Life (ESL) treatment. Ultra-high temperature (UHT, Also known as 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 138 °C (250 °F) for a fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST. Milk simply labeled "pasteurised" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurized" or simply "UHT" must be treated with the UHT method.
Pasteurisation 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 "pasteurised" label. There are different standards for different dairy products, depending on the fat content and the intended usage. For example, the pasteurisation standards for cream differ from the standards for fluid milk, and the standards for pasteurising cheese are designed to preserve the phosphatase enzyme, which aids in cutting.
The HTST pasteurisation standard was designed to achieve a 5-log reduction (0.00001 times the original) in the number of viable microorganisms 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 (including particularly Mycobacterium tuberculosis, which causes tuberculosis and Coxiella burnetii, which causes Q fever). HTST pasteurisation 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 newer method called flash pasteurisation involves shorter exposure to higher temperatures, and is claimed to be better for preserving color and taste in some products.
The term cold pasteurisation is used sometimes for the use of ionizing radiation (see Food irradiation) or other means (e.g. chemical) to kill bacteria in food. Food irradiation is also sometimes called "electronic pasteurisation".
Products that can be pasteurised
Pasteurisation of Milk
Pasteurisation is typically associated with milk, first suggested by Franz von Soxhlet in 1886. 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 UHT treatment is combined with sterile handling and container technology (such as aseptic packaging), it can even be stored unrefrigerated for 3-4 months.
Alternative milk pasteurisation standards
In addition to the standard HTST and UHT standards, there are other lesser-known pasteurisation techniques. The first technique, called "batch pasteurisation", involves heating large batches of milk to a lower temperature, typically 68 °C (145 °F) for 30 minutes, followed by quick cooling to about 4 °C (39 °F). The other technique is called higher-heat/shorter time (HHST), and it lies somewhere between HTST and UHT in terms of time and temperature. Pasteurisation causes some irreversible and some temporary denaturation of the proteins in milk.
In 2001, the Animal and Plant Health Inspection Service of the USDA considered new rules requiring double pasteurisation, which would have entailed holding milk at 72 °C (161 °F) for two separate 15-second periods, instead of one 30-second period as was the current standard. In most jurisdictions, milk treated by double pasteurisation might not be considered pasteurised, depending on the temperature and duration of the heat treatment. Heat treatment might also be performed at a lower temperature or for a shorter time. Such milk could possibly be called "raw milk" or, confusingly, "unpasteurised milk". It cannot be called "pasteurised", even though a significant number of pathogens are destroyed during the process.
In fact, some doctors suggest that babies and breast-feeding mothers avoid all but UHT dairy products.Template:Weasel-inline
In regions including Africa and South Asian countries, it is common to boil milk to sterilize it after it is harvested. This intense heating greatly changes the flavor of milk, which the respective people are accustomed to.
Are current milk pasteurisation standards adequate?
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).Researchers have developed more sensitive diagnostics, such as real-time PCR and improved culture methods, that have enabled them to identify pathogens in pasteurized milk.
- Cold pasteurization
- Dairy product
- Flash pasteurization
- Solar water disinfection (also known as "solar water pasteurization")
- Water pasteurization article in the Solar Cooking Wiki
- A large collection of photographs from Dr. Robert Metcalf's solar water pasteurization projects in Africa
- Niro and GEA Liquid Processing information about pasteurization
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