Milk is known as a highly nutritious food, rich in vitamins and minerals. But the ubiquitous dairy beverage has spilled into the headlines in the past year, most recently because of recalls for unpasteurized or raw milk in California.
We asked two experts from NC State University’s Department of Food, Bioprocessing and Nutrition Sciences to help us understand the science behind pasteurization and milk safety, from the cow to the consumer. Lynette Johnston is an assistant professor and food safety Extension specialist. She grew up on a dairy farm. Gabriel Keith Harris is a professor and co-director of undergraduate programs in food science.
What happens to milk before it reaches consumers in a grocery store?
Lynette Johnston: The dairy farm milks cows, and the milk is cooled as soon as possible, collected in a bulk tank and then transported to a processor. This raw milk is kept under refrigeration until the point of pasteurization. Pasteurization is a heating process that kills all potential pathogens that might be present in raw milk. There is a significant level of safety that we’re gaining by the pasteurization step. The milk then is bottled or processed into ice cream, cheese, or other products, refrigerated and distributed.
Just because milk is pasteurized doesn’t mean that it’s been heat treated to a point like canned food that can be kept at room temperature for a long time. We still have to keep it under refrigeration to prevent further growth of any remaining spoilage bacteria that survive the pasteurization step.
What is it about milk that makes it something that we need to be careful with?
Gabriel Keith Harris: Milk basically has all of the things that any self-respecting bacteria would want to grow in. It’s got sugar, it’s got protein, it’s got fat, it’s got minerals, growth factors. It’s got everything that will make it possible for bacteria to grow very quickly. And there is a lot of water. So you have all of the necessary nutrients for bacteria to grow quickly. If bacteria make their way into this super-rich nutrient medium, and especially if the temperature is in the right range — their comfort zone where they can grow very quickly — then things can go from good to not good in a very short period of time because these microbes multiply so quickly.
Milk pasteurization is a beautiful thing because it knocks out those microbes which are most likely to cause harm. And the good thing about that is that they are generally more heat sensitive than the ones that just spoil the milk. If you drink pasteurized milk that’s way past its expiration date, it’s going to really be horrible smelling and horrible tasting, and you’ll just spit it out immediately. It’s bad from a spoilage perspective, not bad from a send-you-to-the-hospital perspective. So that’s a huge difference.
What’s the history behind pasteurizing milk?
Harris: Pasteurization is a process that Louis Pasteur, who was a French chemist and microbiologist, originally came up with for beer and wine to keep them from souring. Pasteurization was then transferred over to other liquid materials. NC State University was one of the first places that milk was pasteurized in the State of North Carolina, in 1918. It’s not back in the 1860s when Pasteur was doing his initial work — it’s actually quite a bit later. But it was a good thing because milk was dangerous to consume before pasteurization.
Johnston: About 25% of foodborne illnesses were associated with dairy products around that time. Now dairy products account for less than 1% of reported outbreaks, and we can thank dairy farmers and processors for complying with the Grade A Pasteurized Milk Ordinance, passed in 1924, for that. In 2024, we celebrated its 100th anniversary.
Interestingly, public health activists and medical doctors pushed for increased sanitary conditions on dairy farms. They were the ones firsthand who saw children dying because of raw milk they were consuming, and they brought an understanding of the public health burden. After the U.S. Public Health Service passed the Pasteurized Milk Ordinance in 1924, we saw a significant reduction in tuberculosis cases. Today, when outbreaks do occur from consuming raw dairy products, they are typically associated with pathogenic bacteria like E. coli, Listeria monocytogenes, Salmonella and Campylobacter species.
What happens during pasteurization?
Harris: If you were to visit the dairy plant at NC State University, you’d see that we have a bunch of tanks, and some of them are labeled with an “R,” which stands for “raw,” because we’re keeping raw milk separate from pasteurized milk. We store raw milk until it can be processed, and we keep it at refrigeration temperature to discourage the rapid microbe growth that could happen at higher temperatures. And then we do something which is not related to safety, but related to making a variety of products, which is to separate the milk. You take that raw milk and separate it into skim and cream, so you can make dairy products of any fat level, from skim milk, which has almost no fat, to ice cream, which is high fat.
Pasteurization itself is passing thin streams of milk past very hot water on the other side of a metal plate, which heats things super fast. If you put a metal spoon in a soup and forget about it and come back, you’re going to burn your hand. We’re taking advantage of the ability of metal to conduct heat efficiently. With hot water on one side and cold milk on the other side, rapidly we’re able to get this temperature up to where we want it at 161 degrees Fahrenheit for 15 seconds. What you’ll see is something that looks like a car radiator. Behind it are a series of tubes going back and forth to hold that milk at the correct temperature for 15 seconds to get rid of the pathogens that we’re concerned with in raw milk.
Can consumers buy raw milk that isn’t pasteurized?
Johnston: It’s under the state’s jurisdiction to choose how they will allow sales of raw milk. Here in North Carolina, you can actually get raw milk, but you have to earn part of that herd through what’s called the herd share program. The way North Carolina looks at this is that if you’re a dairy farmer producing that raw milk, you have the right to consume that raw milk as it is your product.
People might think that choosing raw milk is a healthy option, because it’s directly from the farm. What are the potential health benefits versus the health risk?
Harris: We call this a health halo. Certain things are viewed just as naturally good, and things that basically are as close as possible to the raw state are often viewed that way. So if it comes straight off of a farm, straight out of a field, straight out of a cow, then it must be healthy. The idea is that if you are dealing with something that is natural and unprocessed, not heated, that it’s going to be healthier just because it’s closer to the way that it appears in nature. That’s not always correct from a safety perspective.
A recent study showed an almost 900% increase in risk from drinking raw milk versus pasteurized. Now I think the exhilaration of riding a motorcycle at 100 miles an hour with no helmet is real, but the danger is crazy. You’re weighing things that are very different in terms of risk versus benefit.
To say there’s nothing different about raw milk and pasteurized milk isn’t completely true from a nutritional perspective. It’s almost 100% true from a protein, sugar and vitamin perspective, but levels of a few vitamins are knocked down a few notches with the pasteurization process. So there are some minor changes, but nutritionally, pasteurized milk is really, really close to raw milk. We might need to investigate some of these other potential benefits, but only if that raw milk can be made safe to be consumed by the general public.
We have some other reasons besides bacteria to be cautious about unpasteurized or raw milk. What has been happening with avian influenza, which was found in dairy cows for the first time in March 2024?
Johnston: The U.S. is dealing with a virus known as highly pathogenic avian influenza, specifically the strain H5N1. This is a virus that’s known to infect poultry, where it actually can become deadly in those poultry flocks. In 2024, we saw the transmission of this virus into dairy herds. Fortunately, the virus is not as severe in cattle.
While multiple dairy herds across the country have been infected with H5N1, recently California has been the hardest hit. To date, the California Department of Food and Agriculture has announced two recalls of raw milk based on positive tests for avian influenza. As research continues, we’re learning that mammary glands of cows play a role in the spread of this virus. Therefore, raw (unpasteurized) milk can be a vehicle for the virus to spread on a dairy farm. It is important to note that there is no evidence of H5N1 being spread to humans as a foodborne disease through the consumption of milk. For humans, this virus is currently considered a respiratory virus and not a foodborne pathogen.
The good news is that pasteurization is not only effective on harmful bacteria, but recent studies show that pasteurization is very effective on H5N1 as well. So that’s another reason to consume pasteurized milk. Generally speaking, the food safety risks of consuming raw milk are greater than the nutritional benefit of consuming raw milk.
Harris: And it’s critical to be clear that the idea that you could potentially immunize yourself from this virus by drinking raw milk is a myth. Drinking raw milk which contains the virus is not a way to get protection against it.
Overall, the dangers related to raw milk are related more to those pathogenic bacteria, those microbes that we can easily eliminate with pasteurization and have been eliminating for over 100 years. So the health dangers are really there and we can easily deal with them.
For the latest information on avian influenza, visit:
- U.S. Centers for Disease Control and Prevention
(CDC link to current updates on H5N1)
- North Carolina Department of Health and Human Services
(North Carolina response to avian influenza)
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