To sour reconstituted milk, just add a little vinegar to it and stir it up. For instance, if a recipe calls for 1 cup of sour milk or buttermilk, then measure a tablespoon of vinegar into a measuring cup. Add reconstituted milk to reach the 1 cup mark. Stir the milk gently. In a moment or two, it will sour. This can replace soured milk or buttermilk in baking recipes. Click here to purchase