Buttermilk is traditionally the liquid remaining after churning butter from cream. However, modern buttermilk is usually made by introducing specific lactic acid bacteria to milk, which ferments the milk and gives it its tangy flavor. The acid in the buttermilk when combined with baking soda, produces carbon dioxide gas, which helps the waffles rise and become light and fluffy. It also helps inhibit excessive gluten formation and add a subtle tangy flavor to waffles.