This is, technically, the liquid remaining when cream has been churned into butter. However, commercial buttermilk is in fact pasteurized skim milk that has been thickened and altered with a bacterial culture to replicate traditional buttermilk. The bacterial culture transforms the milk sugar into lactic acid to simulate the slightly sour flavor of natural buttermilk. Cultured buttermilk tends to be thicker with a more pronounced tartness; natural buttermilk has a slightly thinner consistency and a milder sour flavor. Buttermilk’s acidity hinders bacterial growth, which in turn adds to its ability to withstand prolonged refrigeration. Buttermilk is also available in a dry, powdered form.