Yogurt is made from all sorts of milk; sheep and goat were probably the first. Reduced-fat milks make especially firm yogurt because manufacturers mask their lack of fat by adding extra milk proteins, which add density to the acid-coagulated protein network. (Manufacturers may also add gelatin, starch, and other stabilizers to help prevent separation of whey and curd from physical shocks during transportation and handling.)
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.