Pasteurization
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Pasteurization (or pasteurisation) 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 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 (pathogenic) 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.
Pasteurisation is typically associated with milk, first suggested by Franz von Soxhlet in 1886. Pasteurisation differs from sterilization in that Pasteurisation typically involves the selective removal of a subset of microbes whereas sterilization is the complete removal of all life forms. Pasteurisation also typically uses temperatures below boiling since at temperatures above the boiling point for milk casein micelles will irreversibly aggregate (or "curdle"). There are two types of pasteurisation used today: high temperature/short time (HTST) and ultra-high temperature (UHT). There are basically two methods for the HTST type of pasteurisation in use- batch and continuous flow. In the batch process, a large quantity of milk is held in a heated vat at 63°C (145°F) for 30 minutes, followed by quick cooling to about 4°C (39°F). In the continuous flow process 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 "pasteurised" 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.
Pasteurisation 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 "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 pasteurizing 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.
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 pasteurisation is combined with sterile handling and container technology, it can even be stored unrefrigerated for long periods of time.
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[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.
In recent years, there has been some consumer interest in raw milk products, due to perceived health benefits. Advocates of raw milk maintain that some components survive in milk that has not been pasteurized. 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.[citation needed]
Commercial distribution of packaged raw milk is not allowed in most US states.
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 food-borne illness every year related to the consumption of raw milk." [1]
More than 300 people in the United States got sick from drinking raw milk or eating cheese made from raw milk in 2001, and nearly 200 became ill from these products in 2002, according to the Centers for Disease Control and Prevention. [2]
Raw milk may harbor a host of disease-causing organisms (pathogens), such as the bacteria campylobacter, escherichia, listeria, salmonella, yersinia, and brucella. Common symptoms of food-borne illness from many of these types of bacteria include diarrhea, stomach cramps, fever, headache, vomiting, and exhaustion.
Most healthy people recover from food-borne illness within a short period of time, but others may have symptoms that are chronic, severe, or life-threatening.
People with weakened immune systems, such as elderly people, children, and those with certain diseases or conditions, are most at risk for severe infections from pathogens that may be present in raw milk. In pregnant women, Listeria monocytogenes-caused illness can result in miscarriage, fetal death, or illness or death of a newborn infant, and Escherichia coli infection has been linked to hemolytic uremic syndrome, a condition that can cause kidney failure and death.
Some of the diseases that pasteurization can prevent are tuberculosis, diphtheria, polio, salmonellosis, strep throat, scarlet fever, and typhoid fever.
In fact, some doctors suggest that babies and breast-feeding mothers avoid all but UHT pasteurized dairy products.[citation needed]
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.[citation needed]
[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). 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.
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 pasteurization 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-pasteurization 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 pasteurization involves shorter exposure to higher temperatures, and is claimed to be better for preserving color and taste in some products.
The term 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".
[edit] Pasteurized products
Products that can be pasteurized :
[edit] See also
[edit] External links
- 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
[edit] References
- Rosenau, M.J., The Milk Question, Houghton Mifflin Company, Boston, 1913.
