Louis Pasteur, a French chemist and microbiologist, developed the heat treatment process that eliminates harmful bacteria from milk. In 1864, he created a method of heating beverages to a specific temperature for a set period of time, killing dangerous microorganisms while keeping the liquid safe to drink. His original target was beer and wine, not milk, but the technique proved just as effective for dairy and eventually became one of the most important public health advances in modern history.
How Pasteur’s Discovery Happened
Pasteur was investigating why beer and wine spoiled during fermentation when he realized that microscopic organisms were responsible. By heating these beverages to controlled temperatures, he could destroy the bacteria causing spoilage without ruining the flavor. He called the process “pasteurization,” and it quickly proved useful far beyond the brewing industry.
At the time, milk was a major source of illness. Diseases like tuberculosis, typhoid fever, and brucellosis spread easily through unpasteurized dairy. Once researchers recognized that Pasteur’s heating method worked on milk too, the technique was adapted for dairy production. Commercial pasteurizing machines were introduced by 1895, roughly three decades after Pasteur’s initial discovery.
Alice Evans and the Push for Mandatory Pasteurization
Pasteur invented the process, but it took decades of advocacy before pasteurization became standard practice for milk. One of the most important figures in that fight was Alice Catherine Evans, an American microbiologist working at the National Institutes of Health. Her research proved the direct link between raw milk and brucellosis, a serious bacterial infection that caused recurring fevers, joint pain, and long-term complications in thousands of people.
Evans never earned a Ph.D., which delayed acceptance of her findings. The dairy industry and some in the scientific community resisted her conclusions for years. Eventually, her evidence became too strong to ignore, and her work was a driving force behind laws requiring milk pasteurization across the United States.
How Heat Kills Bacteria in Milk
Pasteurization works by heating milk to a temperature that destroys the proteins bacteria need to survive. When milk reaches temperatures between 60°C and 72°C (140°F to 161°F), the heat unfolds the structure of proteins inside bacterial cells, effectively disabling them. Without functional proteins, the bacteria can no longer reproduce or cause infection.
Two main methods are used today. High Temperature Short Time (HTST) pasteurization heats milk to at least 72°C (161°F) for 15 seconds. This is the standard method for most fresh milk sold in grocery stores. Ultra-pasteurization heats milk to at least 138°C (280°F) for 2 seconds, which kills virtually all microorganisms and extends shelf life significantly. That’s why ultra-pasteurized milk can last weeks longer than regular pasteurized milk, even before you open it.
What Pasteurization Prevents
Raw milk can harbor a long list of dangerous pathogens, including bacteria that cause salmonella, listeriosis, E. coli infections, and brucellosis. Before pasteurization became widespread, milk was one of the most common sources of foodborne illness in the United States and Europe. Children were especially vulnerable.
Today, dairy products account for only about 1% of reported foodborne illness in the U.S., according to the American Society for Microbiology. Of that small fraction, roughly 70% of cases trace back to raw milk, not pasteurized products. That statistic captures just how effective the process is: pasteurized milk is responsible for a tiny sliver of an already small number.
Does Pasteurization Change Milk’s Nutrition?
One common concern is that heating milk destroys its nutritional value. A large meta-analysis of existing research found that pasteurization does reduce levels of certain vitamins, particularly vitamins B1, B2, C, and folate. Vitamins B12 and E also decreased, while vitamin A levels actually increased slightly after heat treatment.
However, the overall nutritional impact is minimal. Most of the affected vitamins are naturally present in milk at relatively low levels, so the losses don’t meaningfully change milk’s role in your diet. The one exception worth noting is vitamin B2 (riboflavin), since milk is a significant dietary source of it. Even so, pasteurized milk remains a rich source of calcium, protein, and other essential nutrients. The tradeoff between a small reduction in heat-sensitive vitamins and the elimination of potentially deadly bacteria is, by any public health measure, overwhelmingly favorable.