Louis Pasteur is rolling in his grave (he’s buried at the Pasteur Institute)

The recent raw milk trend is made possible by advances in science. Pasteurization was developed because we had almost no understanding of bacteria. Heating up the milk seemed to make it last longer, and safer to drink, and that was that.

Now, a more modern understanding of microorganisms has lead to an interest in preserving commensal bacteria already present in food. Milk in the cow is generally safe to drink, but cows aren't clean on the outside, and milk is often contaminated during the industrial process. So this leads to a situation where bad bacteria are occasionally mixed in with the good.

Fortunately, testing for specific species of bacteria is cheap and reliable. Milk can be tested for the kinds of bacteria we don't want. Contaminated milk can be pasteurized, and sold as regular milk. Safe milk is chilled and sold as raw milk, preserving its microbiota.

Raw milk drinker here. We've been drinking raw milk for some years now. What makes it work is that the milk comes from a farm (which we can visit), which has a small number of cows and follows modern best practices when it comes to milking procedures. Each teat is washed and disinfected before milking. Additionally, the first few squirts of milk are milked by hand, which gives a visual indication of whether that teat is infected or not. If it is infected, it is milked into the ground by hand, and further treatment may be pursued as needed. If, for some reason, something was missed, a small number of people will be affected.

Contrast that to a large dairy operation. Several hundred cows mean higher pressure to get through them all, and a higher chance that the farmer misses something. Half of all cows are in herds of over 900 animals, which means that many operations are now completely automated, removing the visual inspection that a farmer in a small-scale operation performs. That milk is then sent to a milk processing facility that may process upwards of 100,000 gallons per day. Any bacteria from one cow can and will be mixed with the milk from multiple other farms, and can potentially infect tens of thousands of gallons.

Anecdotaly, we have several friends who cannot digest homogenized milk, but do well with raw milk. We are not sure if this is due to the makeup of the milk fat or due to the (usually harmless) microbiota in the milk.