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Pasteurization: How Heat Keeps Pathogens at Bay

When Louis Pasteur developed and patented the process of pasteurization in the 1860s, it had nothing to do with milk. He was more concerned with keeping beer from spoiling.

But, by the turn of the century, this method of preservation had been adapted to address common water- and milk-borne diseases such as tuberculosis, diphtheria, severe streptococcal infections, typhoid fever and other foodborne illnesses.

Pasteurizing milk became routine in the U.S. starting in the 1920s. Today, a number of other products on grocery store shelves, including eggs and juices, are also pasteurized.

While pasteurization doesn’t kill all the microorganisms in our food, it does greatly reduce the number of pathogens so that they are unlikely to cause disease. And, like with Pasteur’s beer, it reduces spoilage organisms, extending our food’s “shelf life.”

The method of pasteurization simply involves heating food (usually a liquid) to a specific temperature for a certain length of time and then immediately cooling it. Manufacturers use various time-temperature combinations when treating their products.

“Vat pasteurization” means milk is heated to 63 degrees Celsius for 30 minutes, but “your normal pasteurization is probably going to be for 15 seconds at 72 degrees [Celsius],” says Michele Jay-Russell, a veterinarian and food safety specialist at the Western Institute for Food Safety and Security at University of California, Davis. This is called “high-temperature short-time” (HTST) processing, or flash pasteurization.

At the most extreme, there’s “ultra pasteurization,” which could involve heating to 138 degrees C for 2 seconds. This variation sterilizes food and allows for products to be on shelves instead of the refrigerated section of the grocery store (think boxed milk).

The specific temperatures allotted for pasteurization are based on the ability to kill the most heat-resistant of pathogens, Jay-Russell says. Campylobacter will die pretty quickly at 72 degrees C, she says, but processors need higher temperatures to kill Q fever.

“If you can kill that off, you’ve killed off everything else,” Jay-Russell adds.

Of course, pasteurization is in the news these days because of the debate about raw milk.

The market is growing of consumers seeking unprocessed foods or those wanting to support small farms. And advocates of raw milk defend it for a number of reasons, particularly arguing that pasteurization reduces the nutritional and health benefits of milk.

But, without pasteurization, E. coli, Campylobcater and Salmonella can be much more prevalent in the milk. According to the Centers for Disease Control and Prevention (CDC), there were 148 outbreaks due to consumption of raw milk or raw milk products reported between 1998 and 2011. Among the victims, there were 2,384 illnesses, 284 hospitalizations and two deaths.

Milk is a real breeding ground for pathogens, Jay-Russell says.

“If you get Salmonella or E. coli on a lettuce leaf, it’s not a happy environment for that bacteria. If you put just a couple of cells into raw milk, it’s like a culture medium,” she says.

Because raw milk can be particularly dangerous for young children whose immune systems are especially susceptible to infection, the American Academy of Pediatrics (AAP) issued a policy statement in December stating that pregnant women, infants and children should only consume pasteurized milk and milk products.

The health benefits that proponents say are removed by pasteurization “have not been clearly demonstrated in evidence-based studies and, therefore, do not outweigh the risks of raw milk consumption,” the AAP said. “Substantial data suggest that pasteurized milk confers equivalent health benefits compared with raw milk, without the additional risk of bacterial infections.”

“You have to take steps to keep the risk as minimal as possible,” Jay-Russell says.

© Food Safety News
  • Integrated Food Service Consul

    Just a question; with ultra-pasteurization don’t you also need reduced oxygen packaging (ROP) to make the product shelf-stable?

  • Karen G Lyke

    Pasteurization of milk came from horrendously unsanitary practices in maintaining cows in city conditions. Fed brewery swill and other marginal& inappropriate foods, cows not surprisingly became sick and their milk as well. Dietary content does indeed affect quality of milk. Cows on grass pasture, cleanly maintained produce milk which contains lactobacillus & other beneficial microbes crucial to gut health. Pasteurization eliminates all microbes — as well as a plethora of desirable nutrients including phosphatase, the enzyme necessary to absorb calcium. Pasteurization makes milk go rancid while clean raw milk clabbers to something beneficial, akin to yogurt in taste and microbial content. Microbes ‘follow the food’, show up where their desired nutrients occur; proper microbes in the gut are crucial to health. CAFOs, inappropriate feed, and other unsanitary conditions are the real problem, not raw milk per se.

    • Michael Bulger

      In humans, phosphatase is present in all tissues of the body. That includes the intestines. You don’t need to ingest it with milk in order for the calcium to be absorbed.

    • Ed Davis, Food Freedom Project

      Good detail on the difference between raw and pasteurized milk. Nutrition is not an exact science. The dog food from China with melamine added to increase protein numbers is an example where food is not just protein, carbohydrate, fat, vitamins, and minerals the FDA has determied are needed. (http://mobile.nytimes.com/2013/10/26/health/fda-moves-to-regulate-food-for-animals.html). The packaging and type of each makes a difference.

      One of the recent outbreaks occurred this summer Dakota County, Minnesota. The investigations found the farm was a small “CAFO” of a 60 cow herd (http://m.startribune.com/?id=204570561). The cows were not pastured. The sampling found the bug in a milk container and in bulk tank samples. It was not found in the farm samples. Consequently, I fear the raw milk was contaminated AFTER extraction. Was it a handling error or an issue with raw milk?

      Contact me if you want a copy of the investigation results.

  • tsiebertz

    Great article, Lydia. I’m glad food science and sanitation have come this far so we can all enjoy safe and delicious milk :). Where would we be without Louis Pasteur!?

  • paul

    Pastuerisation at 72 for 15 secs is only equal to a 1.5 log decrease for L.mono. it does not eliminate all bacteria. In milk it is to control Tuberculosis – 6D kill.

  • Mike_Mychajlonka_PhD

    This article contradicts itself. It states that Pasteurization does not kill all the bacteria present. This is quite true. Later on, however, this same article speaks of “Ultra-Pasteurization” as a process capable of effecting the “sterilization” of food. This is not true. The obvious and common meaning of the term “sterilized” is that the product so treated is devoid of life and that all contaminants have been destroyed. Federal regulations allow “Grade A” raw milk to have as many as 100,000 bacterial colony forming units per milliliter. At this levels of bacterial load, sterilizing milk (with the usual safety margins) would require a decimal reduction of twelve logs (or more). Pasteurization schemes involving temperatures lower than the boiling point of water generally deliver a decimal reduction of between 5 and 8 logs. The critical part here is that most (but not all) pathogens may be destroyed by such treatment. Milk also contains bacterial spores, which are quite tough. You may boil most spores for an hour or more without causing much (or any) change in their viability. Ultra-Pasteurization, however, occurs at temperatures above the boiling point of water and so may kill bacterial spores. It also causes browning reactions in milk and negatively affects flavor in addition to other effects. The unfortunate term of “Commercial Sterilization” is a packaging and shelf life concept that is often confused with actual sterility. Commercial sterilization simply means that a producer has decided that a certain low rate of spoilage in a product is acceptable. Processing conditions may then be applied in such a way as to deliver an intended shelf life of a product with no more than the “acceptable” rate of spoilage. So, while “Ultra-Pasteurization” may deliver a given level of “Commercial Sterilization,” that does not mean that such a product is “sterile” within the generally accepted meaning of that term.

  • J Bentley

    Is the following statement from the Pasteurization article true?

    “If you get Salmonella or E. coli on a lettuce leaf, it’s not a happy environment for that bacteria. If you put just a couple of cells into raw milk, it’s like a culture medium,” she says.

    It clashes with a statement made by Mark McAffee during a meeting with the Nevada Dairy Commission in Las Vegas in 2006. Mr. McAffee and his family run the largest raw milk dairy in the United States located on 500 acres of certified organic pasture near Fresno, California.

    “If you put E. coli in our product, it dies. If you put Salmonella in our product, it dies.”

    I was sitting behind him when he made that statement.

    In gratitude for your kind attention,

    J Bentley