True buttermilk is the liquid whey left after cream

has been churned to create butter. Traditionally this
whey was allowed to ferment into a slightly
thickened, sour liquid that would keep longer than
fresh milk. These days, though, buttermilk is made
from regular milk by dosing it with Streptococcus
lactis, a bacteria that consumes lactose, the main
sugar in milk, and produces lactic acid, which adds
tartness to the buttermilk, as well as causing casein,
the primary protein in milk, to curdle, thickening, or
clabbering, the milk.
In some recipes, it’s possible to substitute
artificially clabbered milk—milk to which an acid
like vineger or lemon juice has been added to thicken
it—for buttermilk, but you’ll always be left with a
telltale flavor from the added acid. Much better is to
substitute another soured dairy product. When I
have no buttermilk on hand, I’ll use yogurt, sour
cream, or even crème fraîche diluted with milk.

DAIRY

PRODUCT

TO  SUBSTITUTE

FOR 1   CUP OF

BUTTERMILK

Yogurt ⅔ cup yogurt