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.

Measure the amount of original unsweetened soy milk called for in your recipe in a measuring cup.

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.

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.