How Is Milk Pasteurized?
Milk is pasteurized by heating it to a temperature of about 62°Celsius (about 143°Fahrenheit) and then maintaining that temperature for about half an hour. The treatment kills most of the bacteria which cause milk to go sour quickly or may produce disease in human beings. This temperature is chosen because it is the lowest temperature that will kill the micro bacterium tuberculosis, the carrier of tuberculosis which used to kill people of all ages.
At the same time it is low enough not to alter the taste too much. Pasteurized milk is therefore much safer than untreated milk, especially for babies, and in many countries is the only milk allowed to be sold. The process is named after the French scientist Louis Pasteur (1822—1895) whose discoveries revolutionized knowledge about the effects of bacteria.
In 1864 Pasteur discovered that heating beer and wine was enough to kill most of the bacteria that caused spoilage, preventing these beverages from turning sour. The process achieves this by eliminating pathogenic microbes and lowering microbial numbers to prolong the quality of the beverage. Today, pasteurization is used widely in the dairy industry and other food processing industries to achieve food preservation and food safety.
Unlike sterilization, pasteurization is not intended to kill all microorganisms in the food. Instead, it aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated and is consumed before its expiration date). Commercial-scale sterilization of food is not common because it adversely affects the taste and quality of the product. Certain foods, such as dairy products, may be superheated to ensure pathogenic microbes are destroyed.
Milk is an excellent medium for microbial growth, and when stored at ambient temperature bacteria and other pathogens soon proliferate. The US Centers for Disease Control (CDC) says improperly handled raw milk is responsible for nearly three times more hospitalizations than any other food-borne disease source, making it one of the world’s most dangerous food products. Diseases prevented by pasteurization can include tuberculosis, brucellosis, diphtheria, scarlet fever, and Q-fever; it also kills the harmful bacteria Salmonella, Listeria, Yersinia, Campylobacter, Staphylococcus aureus, and Escherichia coli, among others.
Pasteurization is the reason for milk’s extended shelf life. High-temperature, short-time (HTST) pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra-pasteurized milk can last much longer, sometimes two to three months. When ultra-heat treatment (UHT) is combined with sterile handling and container technology (such as aseptic packaging), it can even be stored un-refrigerated for up to 9 months.
Before the widespread urban growth caused by industrialization, people kept dairy cows even in urban areas and the short time period between production and consumption minimized the disease risk of drinking raw milk. As urban densities increased and supply chains lengthened to the distance from country to city, raw milk (often days old) became recognized as a source of disease. For example, between 1912 and 1937 some 65,000 people died of tuberculosis contracted from consuming milk in England and Wales alone. In the early 1900s, in Arizona, Jane H. Rider “publicized the link between infant mortality and contaminated milk, and finally convinced the dairy industry to pasteurize milk.”
Developed countries adopted milk pasteurization to prevent such disease and loss of life, and as a result milk is now widely considered one of the safest foods. A traditional form of pasteurization by scalding and straining of cream to increase the keeping qualities of butter was practiced in Great Britain before 1773 and was introduced to Boston in the British Colonies by 1773, although it was not widely practiced in the United States for the next 20 years. It was still being referred to as a “new” process in American newspapers as late as 1802.
Pasteurization of milk was suggested by Franz von Soxhlet in 1886. In the early 20th century, Milton Joseph Rosenau established the standards (i.e. low temperature, slow heating at 60 °C (140 °F) for 20 minutes) for the pasteurization of milk while at the United States Marine Hospital Service, notably in his publication of The Milk Question (1912). States in the U.S.A. began enacting mandatory dairy pasteurization laws with the first in 1947, and in 1973 the U.S. Federal Government required pasteurization of milk used in any interstate commerce.
Older pasteurization methods used temperatures below boiling, since at very high temperatures, micelles of the milk protein casein irreversibly aggregate, or curdle. Newer methods use higher temperature, but shorten the time. Among the pasteurization methods listed below, the two main types of pasteurization used today are high-temperature, short-time (HTST, also known as “flash”) and extended shelf life (ESL):
- HTST milk is forced between metal plates or through pipes heated on the outside by hot water, and the milk is heated to 72 °C (161 °F) for 15 seconds. Milk simply labeled “pasteurized” is usually treated with the HTST method.
- UHT, also known as ultra-heat-treating, processing holds the milk at a temperature of 140 °C (284 °F) for four seconds. During UHT processing milk is sterilized and not pasteurized. This process lets consumers store milk or juice for several months without refrigeration. The process is achieved by spraying the milk or juice through a nozzle into a chamber filled with high-temperature steam under pressure. After the temperature reaches 140 °C the fluid is cooled instantly in a vacuum chamber, and packed in a pre-sterilized airtight container. Milk labeled “ultra-pasteurized” or simply “UHT” has been treated with the UHT method.
- ESL milk has a microbial filtration step and lower temperatures than UHT milk. Since 2007, it is no longer a legal requirement in European countries (for example in Germany) to declare ESL milk as ultra-heated; consequently, it is now often labeled as “fresh milk” and just advertised as having an “extended shelf life,” making it increasingly difficult to distinguish ESL milk from traditionally pasteurized fresh milk.
- A less conventional, but US FDA-legal, alternative (typically for home pasteurization) is to heat milk at 63 °C (145 °F) for 30 minutes.
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 that milk be HTST pasteurized to qualify for the pasteurized label. Dairy product standards differ, depending on fat content and intended usage. For example, pasteurization standards for cream differ from standards for fluid milk, and standards for pasteurizing cheese are designed to preserve the enzyme phosphatase, which aids cutting.
In Canada, all milk produced at a processor and intended for consumption must be pasteurized, which legally requires that it be heated to at least 72 °C for at least 16 seconds, then cooling it to 4 °C to ensure any harmful bacteria are destroyed. The UK Dairy Products Hygiene Regulations 1995 requires that milk be heat treated for 15 seconds at 71.7 °C or other effective time/temperature combination.
Some older references point to one or multiple cycles of heating and cooling (to ambient temperature or below) as either a definition of pasteurization or a general method thereof.
A process similar to pasteurization is thermization, which uses lower temperatures to kill bacteria in milk. It allows a milk product, such as cheese, to retain more of the original taste, but thermized foods are not considered pasteurized by food regulators.
Microwave volumetric heating (MVH) is the newest available pasteurization technology. It uses microwaves to heat liquids, suspensions, or semi-solids in a continuous flow. Because MVH delivers energy evenly and deeply into the whole body of a flowing product, it allows for gentler and shorter heating, so that almost all heat-sensitive substances in the milk are preserved.