What Is Pasteurization?

Definition and Examples

Pasteurization involves applying low heat to kill pathogens and extend shelf life.
Pasteurization involves applying low heat to kill pathogens and extend shelf life. Witthaya Prasongsin / Getty Images

Pasteurization (or pasteurisation) is the process by which heat is applied to food and beverages to kill pathogens and extend shelf life. Typically, the heat is below the boiling point of water (100 °C or 212 °F). While pasteurization kills or inactivates many microorganisms, it is not a form of sterilization, because bacterial spores are not destroyed. Pasteurization extends shelf life via heat inactivation of enzymes that spoil food.

Key Takeaways: Pasteurization

  • Pasteurization is the process of applying low heat to kill pathogens and inactivate spoilage enzymes.
  • It does not kill bacterial spores, so pasteurization does not truly sterilize products.
  • Pasteurization is named for Louis Pasteur, who developed a method to kill microbes in 1864. However, the process has been in use since at least 1117 AD.

Commonly Pasteurized Products

Pasteurization may be applied to both packaged and unpackaged solids and liquids. Examples of commonly pasteurized products include:

  • Beer
  • Canned goods
  • Dairy products
  • Eggs
  • Fruit juices
  • Milk
  • Nuts
  • Syrup
  • Vinegar
  • Water
  • Wine

History of Pasteurization

Pasteurization is named in honor of French chemist Louis Pasteur. In 1864, Pasteur developed a technique to heat wine to 50–60 °C (122–140 °F) before aging it to kill microbes and reduce acidity.

However, the technique had been in use since at least 1117 AD in China to preserve wine. In 1768, Italian scientist Lazzaro Spallanzani demonstrated heating meat broth to boiling and immediately sealing the container kept the broth from spoiling. In 1795, French chef Nicolas Appert sealed foods in glass jars and immersed them in boiling water to preserve them (canning). In 1810, Peter Durand applied a similar method to preserve foods in tin cans. While Pasteur applied his process to wine and beer, it wasn't until 1886 that Franz von Soxhlet suggested pasteurization of milk.

So, why is the process called "pasteurization," when it had been in use before Pasteur? The most likely explanation is that Pasteur's experiments demonstrated particles in air, as opposed to pure air, caused food spoilage. Pasteur's research pointed toward microorganisms as the culprit for spoilage and disease, ultimately leading to the Germ Theory of Disease.

How Pasteurization Works

The basic premise behind pasteurization is that heat kills most pathogens and inactivates some proteins, including enzymes responsible for food spoilage. The exact process depends on the nature of the product.

For example, liquids are pasteurized while flowing through a pipe. Along one section, heat may be applied directly or using steam/hot water. Next, the liquid is cooled. The temperature and duration of the phases are carefully controlled.

Liquid pasteurization occurs in a closed system to avoid contamination during cooling.
Liquid pasteurization occurs in a closed system to avoid contamination during cooling. MiguelMalo / Getty Images

Food may be pasteurized after it has been packaged into a container. For glass containers, hot water is used to attain the desired temperature, to avoid shattering the glass. For plastic and metal containers, either steam or hot water may be applied.

Improving Food Safety

Early pasteurization of wine and beer was intended to improve flavor. Canning and present-day pasteurization of food primarily targets food safety. Pasteurization kills yeast, mold, and most spoilage and pathogenic bacteria. The effect on food safety has been dramatic, particularly regarding milk.

Milk is an excellent growth medium for numerous pathogens, including those known to cause tuberculosis, diphtheria, scarlet fever, brucellosis, Q-fever, and food poisoning from Salmonella, E. coli, and Listeria. Prior to pasteurization, raw milk caused many deaths. For example, approximately 65,000 people died between 1912 and 1937 in England and Wales from tuberculosis contracted from consuming raw milk. After pasteurization, milk-related illness dropped dramatically. According to the Centers for Disease Control, 79% of dairy-related disease outbreaks between 1998 and 2011 were due to consumption of raw milk or cheese.

How Pasteurization Affects Food

Pasteurization greatly reduces the risk of food poisoning and extends shelf life by days or weeks. However, it does affect the texture, flavor, and nutritional value of foods.

For example, pasteurization increases vitamin A concentration, decreases vitamin B2 concentration, and affects several other vitamins for which milk is not a major nutritional source. The color difference between pasteurized and unpasteurized milk isn't actually caused by pasteurization, but by the homogenization step prior to pasteurization.

Pasteurization of fruit juice does not have a significant impact on color, but it does result in the loss of some aroma compounds and reduction of vitamin C and carotene (a form of vitamin A).

Vegetable pasteurization causes some tissue softening and nutrient changes. Some nutrient levels are diminished, while others are increased.

Recent Advances

In the modern era, pasteurization refers to any process used to disinfect food and inactivate spoilage enzymes without significantly diminishing nutrient levels. These include non-thermal as well as thermal processes. Examples of newer commercial pasteurization processes include high pressure processing (HPP or pascalization), microwave volumetric heating (MVH), and pulsed electric field (PEF) pasteurization.

Sources

  • Carlisle, Rodney (2004). Scientific American Inventions and Discoveries. John Wiley & Songs, Inc., New Jersey. ISBN 0-471-24410-4.
  • Fellows, P.J (2017). Food Processing Technology Principles and Practice. Woodhead Publishing Series in Food Science, Technology and Nutrition. pp. 563–578. ISBN 978-0-08-101907-8.
  • Rahman, M. Shafiur (1999-01-21). Handbook of Food Preservation. CRC Press. ISBN 9780824702090.
  • Smith, P. W., (August 1981). "Milk Pasteurization" Fact Sheet Number 57. U.S. Department of Agriculture Research Service, Washington, D.C.
  • Wilson, G. S. (1943). "The Pasteurization of Milk." British Medical Journal. 1 (4286): 261, doi:10.1136/bmj.1.4286.261