milk or soured milk or butter milk or curdled milk

Buttermilk is the acidic liquid remaining after butter is churned from cream, or alternatively, a cultured dairy product produced by fermenting milk with lactic acid bacteria. Historically, traditional buttermilk was the natural byproduct of butter production from whole milk or cream, characterized by its thin consistency and slightly tangy flavor from residual butterfat and milk solids. Modern cultured buttermilk, the more common commercial product, is produced by inoculating milk with Lactococcus lactis or similar cultures, resulting in a consistency similar to plain milk but with a distinctive sour, slightly thick body. Traditional buttermilk contains approximately 0.5-1% fat, while cultured buttermilk may contain up to 2% fat depending on the milk source.

In traditional dairying across Northern Europe, the Middle East, and South Asia, buttermilk was a crucial by-product that reduced waste and provided a shelf-stable source of nutrition. The flavor profile ranges from mildly tangy to distinctly sour, with subtle notes of lactic acid and fermented milk.

Buttermilk is essential in baking, particularly in American and British cuisine, where its acidity reacts with baking soda to create lift and tenderness in pancakes, biscuits, cakes, and quick breads. The acid denatures gluten strands and tenderizes baked goods while adding subtle tang. In savory cooking, buttermilk is used in marinades for fried chicken, as a base for dressings, and in traditional cuisines of South Asia (lassi, Indian breads) and the Caucasus. It is also used in cold soups, as a cooking medium for grains, and as a curing or brining agent. Buttermilk's low fat content and high lactic acid concentration make it valuable in both traditional and modern cooking for flavor development and texture modification.