[alphazard]

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.

[maxerickson]

If they are commensal, what's it matter?

https://en.wikipedia.org/wiki/Commensalism

biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed.

[NoPicklez]

Because the good bacteria present in the milk is commensal, but the bad bacteria introduced outside aren't.

[maxerickson]

Commensal means something like "not harmful" rather than "good". It's interesting that the language of raw milk has adopted such a precise term.

[NoPicklez]

My point being that if raw milk has bad bacteria introduced from outside of the cow and it is not pasteurized, that bacteria has been shown to be harmful for us