Buttermilk and sour milk can be used interchangeably when you are making
quick breads. Chemically, vinegar would probably work - but your taste
buds not favor it as much as some good buttermilk. The sour interacts with
the baking soda to release carbon dioxide which is what causes the
rising. Since it happens relatively quickly, you want to keep the time
short between mixing the baking soda with the sour milk and getting it to
the oven to bake.
If you have a recipe for yeast bread that uses buttermilk for flavor, sour
milk would work for that too, but there the carbon dioxide comes from
fermenting due the yeast acting on the sugar (found both in flour and sugar).
Richard L Walker <rlwalker2@cox.net>
Pensacola FL U.S.A. 32504-7726