Buttermilk doesn't contain butter; it's the slightly sour liquid left over from the butter-making process. Most buttermilk contains bits of milk fat (or curds) and is slightly thicker than milk but not as heavy as cream. The process of making processed, or cultured, buttermilk is similar to that of yogurt. Lactobaccillus is added to whole milk and allowed to ferment for 12 to 14 hours at low temperature. Salt and butter flecks may be added to the final product for flavor and texture. But it is possible to make a buttermilk substitute at home.