Buttermilk (sometimes referred to as "sour milk") is an ingredient that is commonly used in baking. If you don't have buttermilk on hand, but it is a required ingredient in a recipe, there are several options for successful substitutes.
Lemon juice or vinegar
Either the juice from a lemon or vinegar will work as a buttermilk substitute. For every cup of buttermilk needed, 1 tbsp of white vinegar or fresh or concentrated lemon juice is mixed with milk and allowed to sit for 10 minutes before using.
Yoghurt or sour cream
Replace the buttermilk in recipes with 236 ml (1 cup) of plain yoghurt. To thin out the yoghurt, use 2 parts yoghurt and 1 part milk. Low-fat yoghurt and sour cream can also serve as cup-for-cup substitutes for buttermilk.
Reconstitute powdered buttermilk whenever fresh buttermilk is unavailable. For every 236 ml (1 cup) of buttermilk, substitute 236 ml (1 cup) of cold water and mix it thoroughly with 32 g (1/4 cup) of the buttermilk powder until it is completely dissolved.
Cream of tartar
For a recipe calling for 236 ml (1 cup) of buttermilk, stir in 1 1/4 to 1 1/2 tsp cream of tartar. Allow the cream of tartar and milk mixture to rest for 10 minutes before using it in your recipe.