What Is Buttermilk? Ask Paul

In the most traditional method, the milk is allowed to stand for a day or two after being collected. During this time, the cream rises to the surface of the container, where it can be easily separated out. Naturally occurring lactic acid bacteria also lightly ferment the cream, imparting a pleasant tang and the flavor complexity we associate with butter.

The separated cream, which is made up of 30 to 40 percent fat, is then placed in a churn and agitated. The microscopic particles of fat suspended throughout the cream slam into each other again and again and gradually start to stick to each other. The fat aggregates together in larger and larger clumps until it’s all been separated out from the liquid and can be drained, pressed gently, and formed into blocks: That’s butter.

The now fat-free liquid that remains is buttermilk. The acidity from fermentation gives it a tart taste and also causes the protein in it to gel, giving it a creamy viscosity.

For better or worse, the traditional method is now seldom seen. It’s quicker to separate cream mechanically, without waiting and without fermentation. Such fresh sweet cream produces mild, sweet butter, but it doesn’t produce buttermilk worth the name.

For that reason, today’s familiar buttermilk is not a by-product of the butter-making process but a product in its own right. It’s made from ordinary skim milk, to which manufacturers add cultures of lactic acid bacteria. The bacteria give the milk its thickness and distinctive flavor as well as the tangy acidity that makes it a valuable ingredient in baking because it reacts with baking soda

Does it have to be skim milk that gets cultured? 

Not at all. Skim milk is traditional because the original buttermilks were fat-free; but whole milk can be cultured just as well. The result is a product that’s becoming more popular in markets and recipes: whole cultured buttermilk.

Even cream can be cultured with the same bacteria, yielding tart and luxurious crème fraîche. You can make crème fraîche at home by adding a spoonful of storebought buttermilk (any kind) to heavy cream and letting it sit on the counter for a day or two.

Come to think of it, once you’ve done that, you’re most of the way to making your own cultured butter! And DIY buttermilk . . . which brings us full circle.