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

Question

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]

Answer 1

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.

Answer 2

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.

Answer 3

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