Cream of tartar is an acid powder, potassium hydrogen tartrate, which is a byproduct of wine-making. When cream of tartar combines with baking soda -- a base or alkali -- during baking, carbon dioxide is released. It creates bubbles that make baked ingredients lighter and more tender. Cream of tartar is also often called for in recipes made with beaten egg whites, or candy recipes. If you don't have any available in your kitchen or can't find cream of tartar at the market, there are other ingredients you can use to replace it in your recipes.
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Cream of tartar makes beaten egg whites more stable and makes the resulting foam whiter. Egg whites are slightly alkaline and cream of tartar makes them more acidic. You won't lose as much volume when folding egg whites into other ingredients when you use cream of tartar. This is true of meringue as well as baked products such as angel food cake or macaroons. Cream of tartar also stabilizes sugar, keeping it from crystallizing, making sweet meringue more stable. It retards browning, so that baked goods stay creamy and light-colored.
When beating egg whites, if you don't have any cream of tartar, use a little lemon juice or vinegar. If your coconut macaroon recipe calls for 1/4 tsp. of cream of tartar, use 1/4 tsp. or less of lemon juice or vinegar. Beat the egg whites for a minute so they break up and start foaming. Then add the lemon juice or vinegar. Add sugar and other ingredients according to your recipe's directions.
If your macaroon recipe calls for baking soda and cream of tartar -- possible if there is an acid ingredient like orange juice in the recipe -- you can substitute baking powder for both ingredients. Baking powder contains both cream of tartar and baking soda. If your recipe calls for 1 tsp. of baking soda and 1/4 tsp. of cream of tartar, for instance, replace them with 1 1/4 tsp. of baking powder.
Avoid using aluminum pots and bowls as the combination of cream of tartar and aluminum makes the egg white an unattractive grey. If your recipe directs you to beat the egg whites in a pan over hot water with the sugar and cream of tartar, either eliminate the cream of tartar or use a stainless steel pan or bowl. If your macaroon recipe doesn't call for cream of tartar, consider adding it anyway to boost the lightness of the cookies, stabilize the egg whites and keep cookies whiter inside for a nice contrast with the toasted exterior.