cupful sour milk

Sour milk is milk that has undergone spontaneous or controlled acidification, resulting in a tangy, fermented dairy product with a distinctive sour flavor and thickened consistency. This occurs when lactic acid bacteria naturally present in milk or deliberately introduced through fermentation convert lactose into lactic acid, lowering the pH and causing the milk proteins (casein) to coagulate. The result is a product distinct from buttermilk (a byproduct of butter-making) and yogurt (which involves specific starter cultures and often heating). Sour milk was historically produced through natural fermentation before refrigeration was commonplace, and it remains a staple in many cuisines, particularly Northern European and Eastern European traditions. Modern sour milk may be produced by adding an acid (vinegar or lemon juice) to fresh milk or through controlled fermentation.

Sour milk serves as a leavening agent and flavor enhancer in baking, particularly in quick breads, pancakes, cakes, and biscuits, where it reacts with baking soda to produce carbon dioxide and rise. Its acidity tenderizes baked goods and adds subtle tang to finished products. In savory applications, sour milk is used in traditional dishes across Scandinavian and Eastern European cuisines, often in sauces, dressings, and as a topping. It can be used as a substitute for buttermilk or Greek yogurt in recipes, though with adjustments to liquid ratios if needed. Sour milk also aids in marinades, particularly for meat, where its acidity helps tenderize proteins.