buttermilk or sour milk

Buttermilk is a fermented dairy product produced as a byproduct of butter churning or, in modern commercial production, through the controlled bacterial fermentation of milk. Traditionally, buttermilk resulted from the liquid remaining after cream was churned into butter; contemporary cultured buttermilk is created by inoculating milk with lactic acid bacteria cultures (typically Lactobacillus lactis), which ferment lactose and produce lactic acid. The fermentation process lowers pH, creates a tangy flavor profile, and produces a slightly thickened consistency. The resulting product contains 0.5–1% fat (in traditional versions) or variable fat content in cultured variants, with a distinctly acidic, slightly sour taste and thin, pourable consistency.

Sour milk, a related but distinct product, develops when milk naturally sours through spontaneous bacterial fermentation or can be intentionally created by adding acid (lemon juice, vinegar) to milk. While historically a common household staple, sour milk differs from cultured buttermilk in production method and bacterial cultures involved.

Buttermilk is fundamental to baking and cooking across numerous culinary traditions, valued for its acidity, which activates leavening agents like baking soda and contributes to tender crumb structure in quick breads, pancakes, biscuits, and cakes. In savory applications, it serves as a marinade base, particularly in Southern American and Indian cuisines, where it tenderizes poultry and adds tang to fried chicken, tandoori preparations, and dressings. Buttermilk is also used in soups, salad dressings, and cold beverages. Its acidic properties make it useful for curdling when higher acid content is needed, and it pairs well with herbs and spices in marinades and sauces.