From what I recall, if it's thick, it's cultures skim milk.
Lactic acid if I remember correctly, one of the acids that make corn silage keep.

If it's thinner, it's the real buttermilk.

I like to keep some powder buttermilk on hand.

These premises insured by a Sheltie in Training ,--- and Cooey.o
"May the Good Lord take a likin' to you"