From
http://frugalliving.about.com/od/makeyourowningredients/qt/Cream-Of-Tartar-Substitute.htm Cream of tartar is the powdered form of tartaric acid, a substance that forms at the bottom of barrels when making wine. It is added to recipes for three reasons:
to stabilize egg whites (when whipping them for meringue)
to act as a leavening agent in cookies and other baked goods (when used in conjunction with baking soda)
to make frosting and icing creamy (it prevents the sugar from crystallizing)
Cream of Tartar Substitutes
If you don't have any cream of tartar in your pantry, you can use one of these substitutes in its place:
For beating egg whites - use an equal amount of white vinegar or lemon juice, or omit the cream of tartar entirely
As a leavening agent - replace the baking soda and the cream of tartar in the recipe with baking powder. One teaspoon of baking powder replaces 1/3 tsp of baking soda and 2/3 tsp cream of tartar
For frosting - leave the cream of tartar out. No substitution is necessary
