Pasteurization heats milk to a specific temperature for a set duration to kill harmful bacteria and viruses, making it safe to drink. The most common method, called High-Temperature Short-Time (HTST) pasteurization, heats milk to at least 72°C (161°F) for 15 seconds. This brief burst of heat destroys 99.999% of dangerous bacteria while keeping the milk’s overall taste and nutrition largely intact.
Which Pathogens Does It Kill?
Raw milk can harbor bacteria like E. coli O157:H7, Salmonella, and Listeria monocytogenes. Standard pasteurization achieves what’s called a 5-log reduction, meaning it eliminates 99.999% of these organisms. The process was originally designed around a heat-resistant bacterium that causes Q fever, so the temperatures required to kill that particular pathogen also wipe out virtually every other dangerous microbe found in milk.
Pasteurization also handles viral threats. When the H5N1 bird flu virus was detected in dairy cattle in 2024, the FDA and USDA conducted multiple studies to confirm that HTST pasteurization completely inactivated viable H5N1 in milk. The FDA tested 297 commercial pasteurized retail products and ran a separate validation study using real-world pasteurization equipment, finding no viable virus in any pasteurized sample.
How Processors Verify It Worked
Dairy plants don’t test every batch for bacteria directly. Instead, they check for an enzyme naturally present in raw milk called alkaline phosphatase. This enzyme is slightly harder to destroy with heat than the pathogens themselves, so if it’s gone, the dangerous organisms are certainly gone too. Pasteurized milk typically shows alkaline phosphatase levels well below 20 milliunits per liter, far under the 350 milliunits per liter cutoff that would signal a problem. If levels come back above that threshold, the batch failed pasteurization.
What Happens to Milk Proteins
Milk contains two main families of protein: caseins and whey proteins. Caseins are heat-stable and survive pasteurization without significant changes. Whey proteins are more sensitive. The 72°C temperature causes some whey proteins to unfold and clump together, a process called denaturation. The percentage of soluble whey protein drops measurably after HTST treatment.
Not all whey proteins respond the same way. Lactoferrin (an immune-supporting protein) and certain immunoglobulins like IgA and IgM show significant reductions after pasteurization. Immunoglobulin G, along with the two most abundant whey proteins, remain relatively stable because they’re more heat-resistant. So pasteurization does reduce some bioactive compounds in milk, but it doesn’t strip out whey protein wholesale. The major proteins you’d get from drinking milk or using whey powder are still present.
Effects on Vitamins and Minerals
Calcium, phosphorus, and other minerals pass through pasteurization unchanged because heat doesn’t alter mineral content. Most vitamins also survive well. The main casualties are small losses of vitamin C and some B vitamins, which are heat-sensitive. These losses are modest, and milk isn’t a primary dietary source of vitamin C anyway, so the practical nutritional impact is minimal.
How It Changes Flavor
HTST pasteurization produces only subtle flavor changes. Most people can’t distinguish HTST-pasteurized milk from raw milk in taste tests. The brief heating can generate trace amounts of sulfur-containing compounds, which contribute a very faint “cooked” note, but this is far milder than what you’d notice in ultra-high temperature (UHT) milk.
The flavor difference becomes more relevant in cheesemaking. Raw milk carries a diverse population of native bacteria that produce a wider range of aroma compounds as cheese ages. Pasteurization kills nearly all of these native organisms, so cheesemakers rely on added starter cultures instead. Over weeks of ripening, cheeses made from raw milk develop a more complex flavor profile because “non-starter lactic acid bacteria” from the original milk continue breaking down fats and proteins in ways that starter cultures alone don’t replicate. This is why some artisan cheeses are made from raw milk, though they must be aged long enough for safety.
HTST vs. UHT Processing
The HTST method (72°C for 15 seconds) is what produces the milk in the refrigerated section of your grocery store. It extends shelf life to roughly two to three weeks when kept cold. Spore-forming bacteria can survive pasteurization in dormant form, and these are what eventually cause refrigerated pasteurized milk to spoil.
Ultra-high temperature processing heats milk to around 135-150°C (275-302°F) for just two to five seconds. This kills virtually everything, including spores, which is why UHT milk can sit on a shelf unrefrigerated for months in aseptic packaging. The tradeoff is a more noticeable cooked flavor and a brownish tint from the higher heat interacting with milk sugars and proteins. UHT processing also causes more extensive whey protein denaturation than HTST.
What Pasteurization Doesn’t Do
Pasteurization is not sterilization (unless you’re talking about UHT, which comes close). Standard HTST milk still contains some harmless, heat-resistant bacteria and bacterial spores, which is why it needs refrigeration and still has an expiration date. Pasteurization also doesn’t remove antibiotics, pesticide residues, or other chemical contaminants. Those are managed through separate testing and regulation before milk ever reaches the processing plant.
It also doesn’t add anything to milk. Pasteurization is purely a heat treatment. Vitamins like A and D that appear on milk labels are added separately through fortification, a completely different step in processing.