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Is buttermilk another term for sour milk or some part of sour milk?

https://en.wikipedia.org/wiki/Buttermilk says:

Originally, buttermilk referred to the liquid left over from churning butter from cultured or fermented cream. Traditionally, before the advent of homogenization, the milk was left to sit for a period of time to allow the cream and milk to separate. During this time, naturally occurring lactic acid-producing bacteria in the milk fermented it. This facilitates the butter churning process, since fat from cream with a lower pH coalesces more readily than that of fresh cream. The acidic environment also helps prevent potentially harmful microorganisms from growing, increasing shelf-life.[3]

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Buttermilk is the byproduct of butter making.
Butter is made by agitating cream (-> the fatty part of milk) resulting in clumps of fat and a milky white liquid that contains nearly no fat and some protein. If the cream was soured before (either by aging or by inoculating the cream with lactobacillae), the buttermilk will also be sour. If the butter was made from unfermented cream, the buttermilk will be mild.

Uncultured buttermilk is rarely sold, even if butter is made commercially with regular cream, the resulting buttermilk is soured afterwards.

As buttermilk is made from a fraction of whole milk (the cream), you could say it’s a part of soured milk. I outlined the process of butter making in this answer, that should also help understanding buttermilk.

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  • 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
    May 9, 2021 at 16:14
  • @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 ♦
    May 9, 2021 at 17:13
  • I posted this question a couple of hours ago: cooking.stackexchange.com/questions/115613/…
    – Matthew Christopher Bartsh
    May 11, 2021 at 9:20
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No, it is not.

Let us consider three dairy products:

Fermented Skimmed Milk: when butter is made from raw milk in a hand-churn, a very milky whey is left behind. This leftover skimmed milk can be allowed to ferment slightly. This is "old-fashioned" buttermilk.

Cultured Lowfat Milk: Modern dairies centerfuge their milk to produce a variety of grades of milk. They can take out the 1% or 2% fat content milk, and add a bacterial culture (Lactococcus lactis or Lactobacillus bulgaricus) to it. This is the kind of buttermilk sold most frequently in the USA, and the one expected in most English-language baking recipes.

Soured Milk: Take 1%, 2%, or whole milk. Add an acid to it, such as lemon juice or white vinegar. Wait an hour, or even overnight. The milk will thicken and change flavor. While often used as a substitute but bakers who cannot find either of the other kinds of buttermilk, soured milk is not buttermilk.

So, while soured milk is similar to buttermilk, and is used as a substitute for it, it is not the same thing. Particularly, if you are making baked goods that call for buttermilk, they will turn out slightly different if you use soured milk.

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Just want to add that the bacteria in question must be mesophilic ie room temperature strains such as Lactic Bacteria (LL) Lactoccocus lactis subsp. Lactis, (LC) Lactoccocus lactis subsp. Cremoris, (LB) Lactobacillus delbrueckii subsp bulgaricus, (LH) Lactobacillus helveticus)

and not thermophilic yogurt bacteria such as acidophilus to achieve characteristic buttermilk flavor

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  • "the bacteria in question must be mesophilic ie room temperature strain" - would that include the bacteria the bacteria that causes milk to go off (go sour) when kept to long at ten degrees Celsius in the fridge? Is the 'characteristic buttermilk flavor the same as that of sour milk?
    – Matthew Christopher Bartsh
    May 9, 2021 at 15:59
  • To go off/sour can mean inedible or can mean unsuitable. If my mik turned to yogurt and I poured it into my coffee, the latter. Tart not slimey nor foul smelling nor lumpy probably safe. So many regional terms for dairy products, hard to know what your sour milk is to me. My Oklahoma grandmother asked if I wanted sweet milk or regular. My glass of 'regular' was cultured buttermilk! Glad I didn't pour it on cereal.
    – Pat Sommer
    May 9, 2021 at 23:58
  • Would milk ever spontaneously turn to yogurt in the fridge? In my experience it only goes sour, and there is no mistaking sour milk for good yogurt. Even relatively bitter yogurt never has that (to me) unique and nauseating taste that adding sugar to does nothing to fix. I get that many people seem to think that yogurt and sour milk taste fairly similar. On the other hand, I have never seen anyone drink sour milk, and could not imagine being able to do it without vomiting. Also, to me the 'sourness' of a lemon has little in common with that of sour milk and would not make me feel nauseated.
    – Matthew Christopher Bartsh
    May 10, 2021 at 0:08
  • All depends on microbes present in environment. If yogurt is made daily, increases likelihood acidophilus or bulgaris contaminate milk. But, true, more likely unpalatables grow first.
    – Pat Sommer
    May 10, 2021 at 23:47
  • "Would milk ever spontaneously turn to yogurt in the fridge?" Extremely unlikely: yoghurt is a fermentation product by thermophilic bacteria, i.e. strains that grow in warm conditions. Even if you get them to grow in the fridge (and I wouldn't be too sure that the same yoghurt taste would be the result), that would be very slow, so any contamination that is better adapted to fridge temp would take over because it grows just so much faster in these conditions.
    – cbeleites unhappy with SX
    May 18, 2021 at 13:53

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