Pasteurization

Pasteurisation (or Pasteurization) is the process of heating food for the purpose of killing harmful organisms such as bacteria, viruses, protozoa, molds, and yeasts. The process was named after its inventor, French scientist 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 in the food. Instead, pasteurisation aims to achieve a "log 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.

Milk pasteurisation

Pasteurisation is typically associated with milk, first suggested by Franz von Soxhlet in 1886. There are two widely used methods to pasteurise milk: high temperature/short time (HTST), and ultra-high temperature (UHT). HTST is by far the most common method. Milk simply labeled "pasteurised" is usually treated with the HTST method, whereas milk labelled "ultra-pasteurised" or simply "UHT" must be treated with the UHT method. HTST involves holding the milk at a temperature of 72°C (161.5°F) for at least 15 seconds. UHT involves holding the milk at a temperature of 138°C (280°F) for at least two seconds.

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 curing the cheese.

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.

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 pasteurisation is combined with sterile handling and container technology, it can even be stored unrefrigerated for long periods of time.

Alternative pasteurisation standards and raw milk

In addition to the standard HTST and UHT pasteurisation 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 (155°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 denaturisation of the proteins in milk.

In most legislations, double pasteurisation is not allowed. A heat treatment at a lower temperature or for a shorter time is sometimes performed. Possibly, such milk could 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 recent years, there has been some consumer interest in raw milk products, due to perceived health benefits. Advocates of raw milk maintain, correctly, that some components survive in milk that has not been pasteurised. Specifically, raw milk contains immunoglobulins and the enzymes lipase and phosphatase, which are inactivated by heat. Raw milk also contains vitamin B6 of which up to 20% may be lost on heat treatment. It is also claimed to contain beneficial bacteria which aid digestion and boost immunity.

Commercial distribution of packaged raw milk is not allowed in most US states.

Some doctors (and some raw milk advocates) acknowledge that certain people should not drink raw milk, including pregnant or breast-feeding mothers, those undergoing immunosuppression treatment for cancer, organ transplant or autoimmune diseases, and those who are immunocompromised due to diseases like AIDS.

In fact, some doctors suggest that babies and breast-feeding mothers avoid all but UHT pasteurised dairy products.

In regions including Africa and South Asian countries, it is common to boil milk after it is harvested. This intense heating greatly changes the flavor of milk, which the respective people are accustomed to.

Are current standards adequate?

Milk pasteurisation 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 pasteurisation 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 pasteurised milk.

Note: The following paragraphs in this section discuss controversial, ongoing research.

One bacterium in particular, the organism Mycobacterium avium subspecies paratuberculosis (MAP), which causes Johne's disease in cattle and is suspected of causing at least some Crohn's disease in humans, has been found to survive pasteurisation in retail milk in the U.S., the UK, Greece, and the Czech Republic. The food safety authorities in the UK have decided to re-evaluate pasteurisation standards in light of the MAP results and other evidence of harmful, pasteurisation-resistant pathogens.

The USDA (which is responsible for setting pasteurisation standards in the U.S.) has not re-evaluated their position on pasteurisation adequacy. They do not dispute the studies, which are at this point accepted by the scientific community, but maintain that the presence of MAP in retail pasteurised milk must be due to post-pasteurisation contamination. However, some researchers within the Food and Drug Administration, which is responsible for food safety in the U.S., have begun pushing for a re-evaluation of these results. There is a small but growing body of criticism directed at these agencies by Crohn's disease sufferers, scientists, and doctors. Some have suggested that the U.S. dairy industry has been successful in suppressing the agencies' response to a potential health crisis, for fear of consumer panic which would lead to a decrease in milk consumption. It is worth noting that while MAP has not been definitely proven to be harmful in humans, all other mycobacteria are pathogenic, and it has been definitively shown to cause disease in cattle and other ruminants.

A newer method called flash pasteurisation flash pasteurization involves shorter exposure to higher temperatures, and is claimed to be better for preserving color and taste in some products.

The term cold pasteurisation cold pasteurization 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".

Pasteurised products

Products that can be pasteurised :

See also: dairy products, Unpasteurised milk.

References

Rosenau, M.J., The Milk Question, Haughton Mifflin Company, Boston, 1913.

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Gourt Home::Gourt :: Pasteurization

Milk pasteurisation

Alternative pasteurisation standards and raw milk

Are current standards adequate?

Pasteurised products

References