What my grandmother called buttermilk and what we can now buy at the grocery store are two very different things. In Gran's time it was the liquid left over from making butter - today it's a cultured product more closely related to yoghourt or sour cream. However what they both have in common is higher acidity, and recipes that call for buttermilk rely on this acid content to make the recipe work
1 Cup (16 tbs) (method 1)
Cream of tartar
1⁄4 Tablespoon (method 1)
1 Tablespoon (method 2)
1 Cup (16 tbs) (method 3)
1) Method 1:
In 1 cup milk add 1 3/4 tablespoons cream of tartar and stir to mix.
2) Method 2:
In 1 cup milk add 1 tablespoon vinegar and stir to mix.
3) Method 3:
For each 1 cup buttermilk called for in a recipe, use 1 cup plain yoghourt.
If your recipe require adding buttermilk and you don't have that in your stock, then what to do? Here is a great solution for this problem. In this video, top chef describes 3 alternatives for making buttermilk substitute, which is surely going to help you.