cream of tartar
A fine white powder derived from a crystalline acid deposited on the inside of wine barrels. Cream of tartar is added to candy and frosting mixtures for a creamier consistency, and to egg whites before beating to improve stability and volume. It's also used as the acid ingredient in some baking powders.
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.