-cup buttermilk or sour milk

Buttermilk is a tangy, fermented dairy product traditionally produced as the liquid byproduct of butter churning, though modern commercial buttermilk is typically cultured skim milk fermented with lactic acid bacteria. Sour milk, closely related, is milk naturally or artificially soured through lactose fermentation, developing a characteristically tart flavor and thickened texture. Both products contain lactic acid, which lowers pH and creates a distinctly acidic profile with a subtle sour taste and smooth, pourable consistency that ranges from thin to moderately thick depending on fermentation time and culturing method.

Traditional buttermilk from butter-making contains significantly less fat than sour milk, as most butterfat has been removed in the butter-churning process. Modern cultured buttermilk is made by adding lactic acid-producing cultures to milk, creating a more consistent product. The tangy flavor profile complements both savory and sweet applications, with subtle complexity that develops during fermentation.

Buttermilk and sour milk are essential in baking, particularly for quick breads, pancakes, waffles, and biscuits, where their acidity reacts with baking soda to create lift and tender crumb structure. They are widely used in Southern American cuisine, Indian cooking (as traditional lassi and dal bases), and Middle Eastern cuisines. Beyond baking, these ingredients appear in marinades for fried chicken and other meats, dressings (buttermilk ranch), and as direct dairy beverages. The acidity tenderizes proteins while adding subtle flavor depth, making buttermilk a functional ingredient in both sweet and savory cooking that extends shelf stability through acidification.