Wine-making Roots

• Cream of tartar is more formally known as tartaric acid and, oddly enough, it's a by-product of wine making. Tartaric acid is one of the components of grape juice. As wine ferments, the acid separates out in the form of small crystals. These are refined and purified, then ground to a fine powder for culinary purposes. The silky-smooth powder is important because it's one of the few acidic ingredients available to home cooks in a dry, tasteless form. This makes it a valuable component in many recipes.

As a Leavening Ingredient

• In doughs and batters, cream of tartar is usually added to act as a partner to baking soda. Soda is an alkaline substance, and in the presence of moisture and acidity, it reacts chemically to produce fine bubbles of carbon dioxide. These make the baked goods rise, becoming light and fluffy. The issue is adding just the right quantity of acidic ingredients to balance the soda. Typically, liquids such as buttermilk, lemon juice or even molasses are used, but these have strong flavors. That's why some recipes complement these acids with cream of tartar, to activate the soda without adding flavor. If you have no tartar, use lemon juice instead or substitute buttermilk for sweet milk.

In Meringues

• Cream of tartar is a relatively uncommon ingredient in most baking recipes, but you'll see it regularly in those that call for beaten egg whites. A small amount of acidity has the effect of unwinding or "denaturing" the protein molecules in the egg whites, helping them whip more quickly into a high, fluffy meringue. The resulting foam also lasts longer than one without added tartar. If you're making a meringue or a meringue-based sponge cake and don't have cream of tartar, substitute lemon or lime juice to achieve the same effect.

In Candy-Making and Sugar Syrups

• Aside from meringues, recipes that call for cooked sugar most often call for tartar. When you're boiling sugar for candy-making, caramel or sugar syrups, it has an innate tendency to crystallize once a certain percentage of the water has cooked out. That's frustrating -- and easily avoided. Some sugars -- called "invert" sugars -- naturally remain liquid. You can turn ordinary sugar into invert sugar by subjecting it to heat and acidity, from a source such as cream of tartar. If you don't have tartar at your disposal, a few drops of lemon juice or white vinegar will work. You can also add an invert sugar, such as corn syrup or glucose, which serves the same purpose.