Cultured Butter

Milk and cream are subjected to heat treatment to kill the bacteria that would otherwise thrive in their nutrient-rich environments. According to the International Dairy Foods Association, cream labeled “pasteurized” is generally heated and held at 161°F for 15 seconds. Increasingly common is a method called ultrapasteurization, in which the cream is held at 275°F or higher for just 1 or 2 seconds. This results in a longer shelf life, but in our experience the butter it makes is not quite as flavorful as butter made with regular pasteurized cream, possibly because the culturing bacteria find ultrapasteurized cream a less hospitable place to work their charms. 
The ideal temperature range for churning butter is 55 to 60 degrees F/10 to 15.5 degrees C. At colder temperatures, the fat is too firm, and the thick cream just sticks to the sides of the food processor bowl instead of churning with the blades. At warmer temperatures, too much of the fat is liquid instead of crystallized, which results in a greasier final product. In step 2, you can chill the cream in the refrigerator or over an ice bath. You can also refrigerate the cultured cream for several days if that's convenient; just be sure to cover it tightly to keep odors out. When you're ready to turn it into butter, take the mixture out of the refrigerator and, if necessary, let it warm to 55 to 60 degrees F/10 to 15.5 degrees C.
Cream aged for one day will make a sweetish butter with just a hint of tang. As the aging process continues—especially at warmer room temperatures—the cream gets more and more pungent. After a week it may smell almost too strong for some tastes, but most of what you smell resides in the liquid portion that gets separated out, leaving the butter surprisingly mellow. Homemade cultured butter also makes incredible browned butter. Try it on your next pasta dish.