Cultured or fermented milk products have been used for centuries. Some cultured milk products commonly consumed in North America are buttermilk, yogurt, acidophilus milk, kefir, and sour cream. The one factor that all cultured milk products have in common is that they have had bacterial cultures added to them in order to ferment their lactose to lactic acid.
The increased acid concentration causes the casein to precipitate out, which changes the milk to a more curd-like consistency. The type of bacterial culture inoculated into the milk largely determines the flavor of the resultant product. Bacteria also influence the quality of fermented dairy products by the amount and type of acids produced. Some protein is also broken down to provide nitrogen for bacterial growth, and this makes the curd softer and more digestible.
