True buttermilk is made from whole milk and is the direct byproduct of butter making. Virtually all of the buttermilk marketed today is artificially soured skim milk and thus not authentic buttermilk. Dairies produce it by adding harmless bacteria to the milk. This thickens the milk and creates a tangy flavor.
Real buttermilk - which is nowadays difficult to find - is preferable for cooking and drinking because it has a richer taste, fuller body, and higher nutritive value than the imitation.
** Asian Cooking