Acidity and a liquid are the two most important parts of a buttermilk substitute, whether it is made from dairy or not. Ideally, one that tastes and looks like Buttermilk. Usually, you can mix a small amount of acid, like lemon juice, with a liquid, like milk or soy milk. This mixture curdles quickly and works well in recipes that call for Buttermilk, but it might not be pleasant to drink on its own.