Buttermilk is the milky liquid remaining after cream has been churned to make butter. Sweet natural buttermilk comes from fresh cream that has been chilled and churned. Sour natural buttermilk comes from cream ripened (soured) by adding lactic-acid-producing bacteria, called a starter. Natural buttermilk does not keep well as a fresh beverage. But sweet natural buttermilk is sometimes dried into a powder and used commercially in cooking, baking, and making ice cream.