Cream of tartar - potassium hydrogen tartrate - is an ingredient familiar to experienced bakers, but rarely included in modern recipes. A by-product of the wine-making industry, this white, powdery acid is derived from a crystalline substance that collects on barrels used for fermentation and aging.
Cream of tartar stabilizes and adds volume to egg whites for meringues; adding 1 teaspoon (or 3.1 grams) to a cup (480 grams) of egg whites makes it almost impossible to end up with a dry, over-beaten mixture. It inhibits crystallization in sugar syrups (mint juleps, anyone?) and is used as a leavening ingredient in combination with baking soda. If you have no baking powder on hand, a mixture of one part cream of tartar to two parts baking soda is a good substitute, but work fast; this is a single-action leavening product, unlike double-acting commercial baking powder.
But wait! There's more! Cream of tartar isn't just for baking - it's a great cleaning agent. A few tablespoons mixed with hot water or hydrogen peroxide removes stains from aluminum pans and rusty drains. Make a scrubbing paste with lemon juice for shiny, clean copper kettles, bowls and sauce pans.
And if you have children, keep some on hand to mix up some homemade play dough.
Cream of tartar - look for it in your grocer's spice section. It lasts indefinitely when stored in a cool, dry cupboard in an air-tight container.
Cream of tartar * potassium hydrogen tartrate, a carboxylic acid * molecular structure KC4H5O6
References:
Beranbaum, Rose Levy, The Cake Bible. New York NY: HarperCollins, 1998.
Herbst, Sharon Tyler and Herbst, Ron, The New Food Lover's Companion. Hauppauge NY: Barron's Educational Series, 2007.
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