Buttermilk of times past was the liquid left after butter was churned. Today it is made commercially by adding special bacteria to nonfat or lowfat milk, giving it a slightly thickened texture and tangy flavor. Some manufacturers add flecks of butter to give it an authentic look.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.