Many baking recipes call for sour milk, sometimes as a substitute for buttermilk. Buttermilk and sour milk are slightly acidic and are required to balance the acidity in the recipe’s ingredients. Milk that has gone sour on its own is spoiled and is unsafe to use. To make 1 cup (250 mL) sour milk for baking, use 1 tbsp (15 mL) vinegar or lemon juice and enough milk to equal 1 cup (250 mL). Stir and let stand for 5 minutes before using. This will give the right amount of acidity for the recipe.