When baking, Kraft recommends substituting an equal amount of lemon juice in place of the cream of tarter. The juice provides the acidity needed to activate the ingredients and create the air pockets that prevent a dense finished product. If a recipe calls for 2/3 teaspoon of cream of tarter and 1/3 teaspoon of baking powder, cooks can simply substitute 1 whole teaspoon of baking powder.
Many recipes using beaten egg whites also call for cream of tarter to help the eggs hold their shape. In these recipes, substitute lemon juice or leave the ingredient out entirely.
Frosting recipes include cream of tarter to keep sugar from crystallizing, so it is advised not to use a substitute in this case.