How do you make buttermilk from unpasteurized milk?

To make buttermilk from unpasteurized milk, add an acid such as lemon juice or white vinegar to the milk. Use about 1 tablespoon of acid for each cup of milk. Stir the mixture and let it sit at room temperature for about 10-15 minutes until it thickens slightly and curdles. This homemade buttermilk can then be used in recipes as a substitute for store-bought buttermilk.