Even slightly sour milk has a horrible, nauseating taste, while plain yogurt tastes either great, or at least nowhere near as bad. A dozen sources (all of them) say that the sour odor and taste of sour milk and also of yogurt is caused by the presence of lactic acid. Does sour milk have something else in it besides lactic acid that gives it the horrific taste and odor that yogurt (which has plenty of lactic acid it it) almost never has. Or is there more than one kind of lactic acid?
Is the taste of sour milk due to lactic acid and one or more amino acids?
"CHARACTERISTICS OF MILK OFF-FLAVORS... Typical Milk: No criticism. Very little distinct odor, pleasantly sweet and clean with no aftertaste... Acid: Basic taste sensation. Sour, tart, may cause tingling sensation on tongue. “Cultured milk” or “sour” odor may be present. Cause: Growth of lactic acid producing organisms such as Lactococcus lactis, due to poor refrigeration, especially when temperatures exceed 70°F (21°C)."
Like everyone, the Cornell author explains the sour taste and odor of sour milk by referencing only lactic acid.
https://www.nationalgeographic.com/culture/article/when-good-milk-goes-bad is no different when talking about slightly sour milk, as opposed to the much later (very) spoiled moldy milk, which involves yeasts and molds and ...danger.
https://www.reddit.com/r/chemistry/comments/j15maj/what_does_lactic_acid_smell_like/ says pure lactic acid does not smell bad. Aha! On the other hand, it does say whether or not it smells like sour milk. What a pity.
To those that nitpick that it is subjective to say that sour milk tastes awful, I say this: 1. Taste and odor is subjective, obviously. 2. I Googled 'Sour milk tastes okay' and there was not one relevant hit so people are not in any way divided on this. 3. This is an SE question, not an academic paper.
I found out recently (with a lot of help from the folks at Seasoned Advice -thanks guys) that what I call 'sour milk' a lot of people call 'spoiled milk', and what I call 'fermented milk products' a lot of people call 'soured/sour milk products'. I think this has been a cause of quite a bit of talking at cross purposes involving me, both here and at Seasoned Advice. But hopefully my meaning is clear now.