Buttermilk

Although “butter” is part of the name, butter-milk is low in fat, with just 1 percent butterfat. Buttermilk gets its name because it is the liquid left from the butter-making process. It has a thick texture (and it’s a good idea to shake the carton well before pouring) and a tangy acidity to it, making it a perfect ingredient to activate leavening in cakes. It is typically not used in custards since it curdles once heated.