Sour Milk - Buttermilk substitute Author: Shawn Prep time: 2 mins Cook time: 5 mins Total time: 7 mins Serves: 1 cup Need Sour Milk for a recipe? It's easy to make and is often used as a substitute for Buttermilk. While not 100% exactly buttermilk it will be similar and will have the acidity that a recipe may require especially for baking. Remember what happens when you combine baking soda with an acid? (think science project volcano) Recipes with baking soda or baking powder that call for sour milk or buttermilk need the acidity to rise.