Most “buttermilk” sold in the United States is not buttermilk at all. True buttermilk is the low-fat portion of milk or cream remaining after it has been churned to make butter. Traditionally, that milk or cream would have begun to ferment before churning, and afterwards the buttermilk would continue to thicken and develop flavor. With the advent of centrifugal cream separators in the 19th century, buttermaking produced “sweet” unfermented buttermilk, which could be sold as such or cultured with lactic bacteria to develop the traditional flavor and consistency. In the United States, a shortage of true buttermilk shortly after World War II led to the success of an imitation, “cultured buttermilk,” made from ordinary skim milk and fermented until acid and thick.