sour milk or buttermilk

Buttermilk is a fermented dairy product produced by the bacterial fermentation of milk, traditionally the liquid left behind after churning butter from cream, though modern commercial buttermilk is typically cultured from regular milk using specific lactic acid bacteria (Lactococcus lactis). The fermentation process converts lactose into lactic acid, giving buttermilk its characteristic sour, slightly tangy flavor and thinner consistency than whole milk. Traditional buttermilk is thinner and less acidic than cultured buttermilk, which is more widely available commercially and has a more pronounced sour taste and creamy texture. The acidity level typically ranges from 0.8 to 1.0% lactic acid by volume.

Buttermilk is essential in baking, where its acidity reacts with baking soda to produce carbon dioxide, creating tender crumbs and lift in cakes, biscuits, pancakes, and quick breads. Beyond baking, it is used as a marinade base for fried chicken and other meats, as a tenderizing agent due to its acidic nature, and as a base for salad dressings and sauces. In traditional cuisines, particularly in Southern American, Indian, and Irish cooking, buttermilk appears in savory dishes, soups, and beverages. It can also be used as a direct substitute for milk in certain applications where the slight tang is desirable or even advantageous.