I often get asked questions about what buttermilk is and how to substitute for it because people don’t keep it “on hand.” The best answer to this is to say that buttermilk is a wonderful dairy product that makes for some delicious baked goods, and it should always be kept on hand if you like to bake more than just the occasional batch of chocolate chip cookies. Buttermilk is thick and has a tangy, buttery flavor – sort of like a cross between melted butter and sour cream. Despite its thickness, it is low in fat. Some people like to drink it plain, but it is usually best utilized in cakes, pancakes, breads and other dishes where it lends a lot of butter flavor without the extra fat of more butter or full-fat sour cream.
Buttermilk is so named because it was originally the liquid left over after the butter-churning process was complete. These days, buttermilk is cultured, made by adding lactic acid bacteria to [pasturized] nonfat or lowfat milk to thicken it and give it a tangy flavor. The easiest substitute is to add 1 tbsp of vinegar to 1 cup of regular milk to curdle it and give it a sharper flavor. This also provides a similar level of acidity to buttermilk, so it will work in recipes in the same way (it doesn’t have the buttery flavor of buttermilk, however).
Buttermilk is usually sold in smaller containers than regular milk and it is usually quite inexpensive (usually around a dollar, in my next of the woods). It has a longer shelf life than regular milk, so you have more time to use it up. With cakes, pancakes, breads, salad dressings to choose from, there are plenty of options to choose from.