Buttermilk is a fermented dairy product with two main types: traditional and cultured. Traditional buttermilk is the liquid leftover after churning butter from cultured or fermented cream. It is a thin, slightly sour liquid that was historically a byproduct of butter-making and is still common in some parts of the world like India, Nepal, and the Middle East. This type contains naturally occurring lactic acid bacteria that ferment the milk, giving it a tangy flavor and helping preserve it

. Cultured buttermilk , which is what you typically find in most grocery stores today, is made by adding specific lactic acid-producing bacteria cultures (such as Lactococcus lactis or Lactobacillus bulgaricus) to low- fat or skim milk. This fermentation process thickens the milk, lowers its pH (making it more acidic), and gives it a characteristic tangy taste. Cultured buttermilk is not a byproduct of butter-making but is produced intentionally for consumption and cooking

. Nutritionally, buttermilk is low in fat and calories but rich in calcium, riboflavin, and vitamin B12. It is commonly used in baking (e.g., biscuits, pancakes) because its acidity reacts with baking soda to tenderize and leaven baked goods. It also adds moisture and a creamy texture to recipes and can be used in marinades, soups, and dressings

. In summary, buttermilk is a tangy, fermented dairy product either traditionally leftover from butter churning or modernly cultured by fermenting milk with beneficial bacteria, prized for its flavor and culinary uses.