One of the coolest things about eggs is their ability to make glossy, snowy-white foams. Whipped egg whites are the core ingredient in everything from fruit-filled Pavlovas to the entire family of sponge cakes and angel food cakes. Recipes for these baked goods often call for cream of tartar or meringue powder, two confusingly similar white powders.
Cream of Tartar vs. Meringue Powder
Truthfully, cream of tartar and meringue powder have very little in common except that they are white powders, and they're often used in the same kinds of recipes.
Cream of tartar is a finely powdered form of tartaric acid, which precipitates naturally out of wine as it ages. You may have even had the experience of finding a few mouth-puckering crystals of it at the bottom of your glass. It's refined and packaged for use by bakers and pastry chefs, who appreciate it as one of the few acids that's a dry powder rather than a liquid like vinegar or lemon juice. It's often called for in recipes using whipped egg whites, because a bit of acidity helps the meringue form.
Meringue powder, on the other hand, is made up of dried egg whites and a few other ingredients added to keep it from clumping or spoiling. It's a shelf-stable powder that bakers can use to make meringues and meringue-based recipes as needed, without the perishability and food safety issues you get with fresh eggs.
Making Royal Icing
For an example of a recipe that can use either of these powders, consider royal icing. This is the glossy cookie icing you often see on holiday cookies, tinted to make holiday designs in a variety of smooth, shiny colors. It pipes beautifully, so cake decorators also use it for detail work like the delicately lacy designs you sometimes see on wedding cakes.
Traditional recipes for royal icing call mostly for egg whites and sugar, but they often include a pinch or two of cream of tartar to help the egg whites make an airy foam. You'll start by whipping the whites and the cream of tartar together; then add powdered sugar – a lot of powdered sugar – slowly until it reaches the right glossy, slow-flowing consistency.
If you don't like the idea of working with raw eggs, you can also make royal icing with meringue powder. In this case, start by beating the meringue powder with water, until it's reconstituted and foamy, then begin adding the sugar.
Cream of Tartar Substitute for Meringue
If you're making meringue, or any other meringue-based recipe, you also have the option of starting from meringue powder or fresh egg whites. In the case of fresh egg whites, your recipe will frequently call for a small quantity of cream of tartar. If that's not something you keep in your pantry, you can substitute either lemon juice or distilled white vinegar.
They're not as concentrated as cream of tartar, so you'll need to double the amount called for in the recipe. If your recipe includes 1/2 teaspoon of cream of tartar, for example, you'd use a full teaspoon of lemon juice or vinegar.
Meringue Powder Substitute
If your recipe calls for meringue powder and you don't happen to have any on hand, you have a couple of options. Freeze-dried egg whites are certainly available, but if you don't have meringue powder, you probably won't have those either. Liquid egg whites are usually the easiest option.
You'll need one egg white for every two tablespoons of meringue powder called for in your recipe, to create the same amount of foam. If you're working with leftover egg whites from other baking, or with pasteurized egg whites from a carton, that's about three tablespoons. Your egg whites won't have the other ingredients meringue powder uses to stabilize the foam, so again, you'll want to add a bit of acidity in the form of cream of tartar or lemon juice.