Pasteurization (or pasteurisation) is a process of heat processing a liquid or a food to kill pathogenic bacteria to make the food safe to eat and to preserve longer. It involves heating the food to kill most harmful microorganisms. Producers pasteurize dairy and other foods to make them safe to eat. The process is named after Louis Pasteur. He was the first person to learn how to do it successfully. The first pasteurization was done by Louis Pasteur and Claude Bernard on 20 April 1862.
Unlike sterilization, pasteurization is not intended to kill all microorganisms in the food. Instead, pasteurization gets a "log reduction" in the number of microorganisms. It brings their number down so they are unlikely to cause disease if the product is kept in the refrigerator and consumed before its sell-by date. Commercial sterilization of food is not common, because it tends to destroy the flavour of the food.
Milk pasteurization
Pasteurization is usually done to milk, which was first suggested by Franz von Soxhlet in 1886. There are two popular ways to pasteurize milk. First, high temperature/short time (HTST), and second, ultra-high temperature (UHT). HTST is the most common method. Milk only labeled pasteurized (or past) is usually treated with the HTST method, whereas milk labelled ultra-pasteurized or UHT must be treated with the UHT method. HTST is when the milk is at a temperature of 72 degrees Celsius (or 161.5 degrees Fahrenheit) for at least 15 seconds. UHT is when the milk is at a temperature of 138 °C or 280 °F for at least two seconds.
Pasteurization rules are usually set by national food safety agencies (like the USDA in the United States and the Food Standards Agency in the United Kingdom). These agencies say milk has to be HTST pasteurized to have the "pasteurized" label. There are different rules for different dairy products, depending on the fat content and what they will be used for. For example, the pasteurization rules for cream are different than the rules for fluid milk, and the standards for pasteurizing cheese are designed to keep the phosphatase enzyme fresh, which helps in making the cheese.
HTST pasteurized milk normally has a refrigerated shelf life of two or three weeks, but ultra pasteurized milk can last much longer when refrigerated, sometimes two or three months. When UHT pasteurization is mixed with safe handling and container technology, it can even be stored unrefrigerated for even longer periods of time.
The pasteurization of milk allowed it to be kept and stored in homes. People began to purchase milk in order to drink it more often.
Alternative pasteurization standards and raw milk
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 . 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 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, "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, correctly, 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.
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 medical conditions such as AIDS.
In fact, some doctors suggest that babies and breast-feeding mothers avoid all but UHT pasteurized dairy products.
In Africa, it is common to boil milk whenever it is harvested. This intense heating greatly changes the flavor of milk, which the people in Africa are accustomed to.