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Is sour milk, soured milk, and milk that has gone sour, all the exact same thing? Related to this question: Is buttermilk another term for sour milk or some part of sour milk? and especially this answer to it: https://cooking.stackexchange.com/a/115571/93811 and two of the comments appended to that answer, specifically:

"Is soured milk the same as sour milk as in, 'I kept this milk in the fridge too long after opening, and now it is sour, although it was okay yesterday, and so I need to chuck it out and buy some more'. – Matthew Christopher Bartsh yesterday

@MatthewChristopherBartsh Not exactly. “Kept in the fridge and something grew” is the problem - you don’t know what grew. Way back before commercial milk production, there was a good(-ish) change that something desirable grew - souring meant “let stand, it’ll be thick by tomorrow” or it meant “add some of the existing product and let the microorganisms from that multiply and do their thing”. It’s too complicated for a comment to explain why exactly the former is less likely to work today than back in our (great...)grandparents’ time. – Stephie♦

Matthew Christopher Bartsh
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Yes, it is the same. It refers to milk which has been left out until it has gone sour with whatever wild bacteria it has managed to catch, be they pathogenic, healthy, or neutral. It curdles a bit and changes in smell and taste.

If you find a person or author who makes a difference, then you have discovered either some regionally restricted distinction which is not widespread (maybe even restricted to that one author), or a too-literal translation from some other language.

Update, since you seem to be asking about the exact meaning of Stephie's comment. What she means is that back in the day, when milk went sour, the actual risks were lower than when today's milk goes sour. (not to speak of the perceived risks and of the implicit risks/reward calculations people make by gut feeling). The difference in her comment is not between "sour milk" and "soured milk", but between "milk that was freshly milked from a single healthy cow and left at room temperature until it went sour" and "commercially produced and processed milk which was distributed to me and left in the fridge until it went sour". There are no terms in the English language which make this distinction, both are called "sour milk" or "soured milk" - so the product is probabilistically different, but the words are not.

rumtscho
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    But, that doesn't make it *exactly* the same. Like sour cream vs cream that's gone sour - one was intentional & further pasteurised to prevent it going any further; the other was left in the fridge until something random grew in it. – Tetsujin May 11 '21 at 09:00
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    @Tetsujin I have never seen somebody make this distinction by using the word "sour" for milk. Milk which has had some culture added has a different name in English, depending on the culture/process: yogurt, buttermilk, kefir, etc. Other languages do use the term, that's why I mentioned the too-literal translation. – rumtscho May 11 '21 at 09:04
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    tbh, I've never seen "sour milk" for sale anywhere, but sour cream I can get in any supermarket. – Tetsujin May 11 '21 at 09:06
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    @Tetsujin exactly. The term "sour milk" is not used for any desirable product which we can get, neither is the term "soured milk". Both are used only for what has happened to go sour on its own. It so happens that the English language developed differently for cream than for milk, and one intentionally-cultured variation is called "sour cream". Such divergence in terms is common. – rumtscho May 11 '21 at 09:32
  • "Yes, it is the same. It refers to milk which has been left out until it has gone sour with whatever wild bacteria it has managed to catch, be they pathogenic, healthy, or neutral. It curdles a bit and **changes** in smell and taste." , you wrote. How many people think the 'change' in smell and taste is an improvement or is even remotely acceptable? "back in the day, when milk went sour, the actual risks were lower than when today's milk goes sour...", you wrote. Do you mean it was common for people to drink sour milk back then? I think I would throw up if I dared to try that. – Matthew Christopher Bartsh May 15 '21 at 03:19
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    Yes, it was absolutely acceptable and common. In fact, my own grandparents' generation did it, and it was widespread. And if it was too-far-gone, it was "hidden" by using it as a baking ingredient instead. It is absolutely normal that tastes and attitudes towards food are part of culture and not absolute - you shouldn't use your own experience to predict other people's preferences and reactions to certain foods. – rumtscho May 15 '21 at 11:51
  • @MatthewChristopherBartsh see [this](https://en.wikipedia.org/wiki/Soured_milk). If you have only come across commercially processed (e.g. pasteurized or UHT) rotting milk, I can understand your aversion, that’s yuck! But just as sour cream is “off in a good way” (joining ranks with other “off” foods like cheese, sauerkraut, wine, vinegar ...), sour milk can be an appreciated food item and for example be found in German cuisine (sweetened, with breadcrumbs or cinnamon), all over Scandinavia, or rather, everywhere where farmers kept cows. It’s typically not drunk, but spooned, like yogurt. – Stephie May 15 '21 at 19:55
  • And note that spontaneously fermented milk products are made from raw milk (relying on the biome in the milk and the environment). For pasteurized milk, you need inoculation with the target bacteria, or the chance of the milk simply rotting is very high. The pasteurization can also be done to “clear the ground”, e.g. in yogurt making before adding the starter or reserved yogurt. – Stephie May 15 '21 at 20:13
  • @rumtscho I am not talking about using sour milk as a baking ingredient, something must happen to it when that is done as there is no hint of a taste of sour milk in scones made with sour milk. How on earth could your grandparents generation drink sour milk? Here is someone *trying* to drink sour milk. https://www.youtube.com/watch?v=68CYpccOvZY "Spoiled Milk Challenge Fail". – Matthew Christopher Bartsh May 18 '21 at 08:07
  • @Stephie The Wikipedia article you linked to has, "Before the invention of refrigeration, raw milk commonly became sour before it could be consumed, and various recipes incorporate such leftover milk *as an ingredient*." No mention of drinking it. – Matthew Christopher Bartsh May 18 '21 at 08:19
  • @Stephie: one of the environmental factors was: people "selected" in favor of good/OK strains. When the milk went sour in a bad way, you'd clean everything thoroughly in order to get rid of that one. Keeping a local good/OK strain around will make the milk go sour faster than having no such strain (and lower bacteria counts), but if it's sufficiently predominant, the result will still be edible - this is a sensible tradeoff if "not going sour" is not a viable option e.g. because of lack of refridgeration, pasteurization or microfiltration. – cbeleites unhappy with SX May 18 '21 at 13:26
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    @cbeleitesunhappywithSX exactly. Like catching wild yeasts for sourdough in a baker’s kitchen or under a flowering shrub (hello, elder!) or stirring the beer with always the same stick (which transferred the yeast from one batch to the next) - or extra cleaning and scrubbing if you had „bad spirits“. A milk farmer once told me that the mandatory equipment cleaning agents and cleaning regimen plus the immediate chilling of the milk is one of the reasons why the spontaneous fermentation goes awry more frequently today than in the past. I guess she had a point there. – Stephie May 18 '21 at 13:31
  • @ Matthew: in my native languange Sauermilch = literally sour milk (regionally also Dickmilch, thick milk - possibly what rumtscho was thinking about as too literal translation), you can buy it in the supermarket. It is nowadays a specific product like yoghurt, but fermented with a different species (wikipedia tells me lactococcus lactis vs streptococcus thermophilus + lactobacillus bulgaricus). Of course, the local DIY home strains were likely yet different ones - but they certainly were more carefully selected than "whatever falls in there first in the fridge" for modern spoiling of milk. – cbeleites unhappy with SX May 18 '21 at 13:32
  • @Stephie: Yes. The other point being that spontaneous fermentation doesn't happen as often/as soon - which is the purpose of the cleaning. I seem to have heard of putting leftover milk into the same (possibly wooden) bowl to *make* such "sour milk". That would provide a proper home for the strain used for inoculation, like the wooden spoon. – cbeleites unhappy with SX May 18 '21 at 13:34
  • @Stephie I read the Wikipedia article called "Soured milk" that you linked to. I also read the talk page. I learned a lot, and reading here also helped. It seems there is a sense of 'sour milk' that I had never heard of, which means fermented milk (yogurt, for example). I don't know how I missed it. Maybe this is a recent addition to the list of senses of 'sour milk'. It seems a lot of people are saying 'spoiled milk' to mean what I mean when I say 'sour milk'. To me 'sour milk' (i.e. milk that had soured) had always been milk that had a foul, sour taste and smell because it had spoiled. – Matthew Christopher Bartsh May 18 '21 at 14:18
  • @Stephie I never for a moment thought yogurt was a type of sour milk but rather a type of fermented milk, like cheese. Maybe I will start saying 'spoiled milk' instead of saying 'sour milk'. Language is always changing. "Language is not a lantern slide but a moving picture", someone said once. By searching for "drinking spoiled milk" I found this much better video. This guy tries to drink *gallon* of (very) sour milk. I found it hard to watch. "Can A Human Drink A Gallon of Spoiled Milk w/o Vomiting into A Washing Machine? | L.A. BEAST" https://www.youtube.com/watch?v=lJ_K960dG4c . – Matthew Christopher Bartsh May 18 '21 at 14:57
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    Rule of thumb: if it’s sour because it’s a carton of store-bought milk forgotten in the fridge, forget it. If it’s sour because it’s a batch of raw milk left at room temperature, it’s dubious. If you added the bacteria on purpose, enjoy! – Stephie May 18 '21 at 15:08
  • I found this interesting: https://microbenotes.com/spoilage-of-milk-and-milk-products/ – Matthew Christopher Bartsh May 18 '21 at 15:23
  • @MatthewChristopherBartsh you seem to still be trying to find a term which is universally understood to imply only one sense of souring of milk. This doesn't exist. All three terms (sour, spoiled and fermented) milk can be used interchangeably, and while you will find people who make distinctions between them, not all people make the same kind of distinction. For example, my mother is much more likely to say "the X has fermented" than "the X has spoiled" when she finds some food with signs of unwanted bacterial activity, and shesometimes forgets it also means purposeful fermentation. – rumtscho May 18 '21 at 15:42
  • @rumtscho The words 'sour', 'soured', and 'fermented' all seem to have way too many meanings for my liking. It seems to me that it has been a long time since I came across a set of words with so many confusing meanings. Maybe it's because there is an element of subjectivity to what milk tastes like. 'Spoiled milk' is fairly unambiguous, and seems to be the word favored by the scientists so I like it, except that it doesn't specify or connote the highly distinctive smell and taste of sour milk. For example, if milk spontaneously turned to some sort of cheese I would maybe not call it sour. – Matthew Christopher Bartsh May 18 '21 at 16:16
  • @MatthewChristopherBartsh: All three terms have distinct meanings, and are used with milk very much along the general lines of their meaning: sour is precise in the sense that it describes actually acidity building up and the pH dropping. Spoiled means gone wrong. I'd disagree that it is unambiguous in taste, since different causes that all spoil milk cause different taste (think e.g. moldy vs. bitter) Fermentation refers to microbial or enzymatic processes. These terms "meet" here, since the microbial fermentation produces acid, thus the milk goes sour - and that likely means it's spoiled. – cbeleites unhappy with SX May 18 '21 at 20:24