Pasteurization
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Pasteurization (or pasteurisation) 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 pasteurization test was completed by Pasteur and Claude Bernard on April 20, 1862.
Unlike sterilization, pasteurization is not intended to kill all micro-organisms (pathenogenic) in the food. Instead, pasteurization aims to achieve a "log reduction" in the number of viable organisms, reducing their number so they are unlikely to cause disease (assuming the pasteurized product is refrigerated and consumed before its expiration date). Commercial scale sterilization of food is not common, because it adversely affects the taste and quality of the product.
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[edit] Milk pasteurization
Pasteurization is typically associated with milk, first suggested by Franz von Soxhlet in 1886. Pasteurization differs from sterilization in that Pasteurization typically involves the selective removal of a subset of microbes whereas sterilization is the complete removal of all life forms. Pasteurization also typically uses temperatures below boiling since at temperatures above the boiling point for milk casein micelles will irreversibly aggregate. There are two types of pasteurization used today: high temperature/short time (HTST) and ultra-high temperature (UHT). There are basically two methods for the HTST type of pasteurization in use- batch and continuous flow. In the batch process, a large quantity of milk is held in a heated vat at 149°F./65°C. for 30 seconds, followed by quick cooling to about 39°F./4°C. In the continuous flow process (continuous flow pasteurizer, below, right)-also known as HTST, for high temperature, short time, milk is forced between metal plates or through pipes heated on the outside by hot water. HTST is by far the most common method (except in Europe where UHT is more common). Milk simply labeled "pasteurized" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurized" or simply "UHT" must be treated with the UHT method. UHT involves holding the milk at a temperature of 138 °C (280 °F) for at least two seconds.
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 milk to be HTST pasteurized in order to qualify for the "pasteurized" 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.
The HTST pasteurization 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 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.
HTST pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra pasteurized milk can last much longer when refrigerated, sometimes two to three months. When UHT pasteurization is combined with sterile handling and container technology, it can even be stored unrefrigerated for long periods of time.
[edit] Alternative milk pasteurization standards
In addition to the standard HTST and UHT pasteurization standards, there are other lesser-known pasteurization techniques. The first technique, called "batch pasteurization", 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. Pasteurization causes some irreversible and some temporary denaturization of the proteins in milk.
In most legislations, double pasteurization is not considered pasteurized. 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, "unpasteurized milk". It cannot be called "pasteurized", even though a significant number of pathogens are destroyed during the process.
[edit] Raw milk
In recent years, there has been some consumer interest in raw milk products, due to perceived health benefits. Advocates of raw milk maintain that pasteurization destroys enzymes, diminishes vitamin content, denatures fragile milk proteins, destroys vitamins C, B12 and B6, kills beneficial bacteria, promotes pathogens and is associated with allergies, increased tooth decay, colic in infants, growth problems in children, osteoporosis, arthritis, heart disease and cancer. Calves fed pasteurized milk do poorly and many die before maturity. Raw milk sours naturally but pasteurized milk turns putrid; processors must remove slime and pus from pasteurized milk by a process of centrifugal clarification. Inspection of dairy herds for disease is not required for pasteurized milk. Pasteurization was instituted in the 1920s to combat TB, infant diarrhea, undulant fever and other diseases caused by poor animal nutrition and dirty production methods. But times have changed and modern stainless steel tanks, milking machines, refrigerated trucks and inspection methods make pasteurization absolutely unnecessary for public protection. And pasteurization does not always kill the bacteria for Johne’s disease suspected of causing Crohn's disease in humans with which most confinement cows are infected. Much commercial milk is now ultra-pasteurized to get rid of heat-resistant bacteria and give it a longer shelf life. Ultra-pasteurization is a violent process that takes milk from a chilled temperature to above the boiling point in less than two seconds. Clean raw milk from certified healthy cows is available commercially in several states and may be bought directly from the farm in many more.
Commercial distribution of packaged raw milk is not allowed in most US states.
According to the FDA, drinking raw (untreated) milk or eating raw milk products is "like playing Russian roulette with your health," says John Sheehan, director of the Food and Drug Administration's Division of Dairy and Egg Safety. "We see a number of cases of foodborne illness every year related to the consumption of raw milk." [1]
[edit] Are current milk pasteurization 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).
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 pasteurized 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.
[edit] Pasteurised products
Products that can be pasteurized :
[edit] See also
- Dairy product
- Raw milk, milk which has not been pasteurized
- Flash pasteurization
- Cold pasteurization
- Viral processing
[edit] External links
- Water Pasteurization FAQ
- A large collection of photographs from Dr. Robert Metcalf's solar water pasteurization projects in Africa
[edit] References
- Rosenau, M.J., The Milk Question, Houghton Mifflin Company, Boston, 1913.
