Original buttermilk is the liquid product that is left after making butter from cream. It has a higher acid content than regular milk, which causes a tart or sour taste. Adding an acidic ingredient -- such as lemon juice -- to regular milk or a non-dairy milk results in a type of buttermilk, which can be used in cooking and baking. Soy milk makes a good choice because it is high in protein.
Step 1
Measure the amount of original unsweetened soy milk called for in your recipe in a measuring cup.
Step 2
Add 1 tablespoon of lemon juice to every 1 cup of soy milk. If your recipe asks for 2 cups, use 2 tablespoons of lemon juice.
Step 3
Stir the soy milk and lemon juice together. Let the mixture sit at room temperature for 10 minutes. Use the soured soy milk as required in your recipe.