Buttermilk is a tangy, slightly thick cultured-milk product — thinner than yogurt, tarter than fresh milk — whose lactic sourness and gentle acidity make it a workhorse of baking and Southern American cooking. Modern "cultured buttermilk" is low-fat milk fermented with lactic-acid bacteria; its clean sour bite tenderises batters, activates baking soda and brines meat, while its clinging body coats and marinates.