Even if you don't keep cream of tartar in your pantry, you've probably seen it in the baking supplies section of your local supermarket. It's a fine white powder, usually sold in small jars or cans. Cream of tartar is a by-product of wine production, an impurity that crystallizes out of grape juice during fermentation. These crystals are harvested by the wine makers, then refined and ground to a powdery texture for sale. There are relatively few dry, flavorless acidic ingredients bakers can use, which makes cream of tartar a handy ingredient for many purposes in the kitchen.

One of those purposes of cream of tartar is to help egg whites form a light, stable foam. Whether you whip egg whites by hand with a whisk or in the bowl of a powerful stand mixer, the effect is the same. Every time the wires whip through the egg whites, they physically shear the egg whites' proteins and leave behind bubbles of air. The proteins create new bonds, encircling the air bubbles. As the whisk continues to whip the eggs, the bubbles become finer and finer. Acidity speeds the process by weakening the bonds that hold the egg proteins in their original, tightly-packed arrangement.

Many other acidic ingredients can serve the same purpose of helping loosen or "denature" the proteins in the egg whites. If you don't have cream of tartar on hand, you can substitute the same amount of lemon juice or plain white vinegar. In large quantities they can lend a noticeable flavor to your egg whites, but most recipes call for only a fraction of a spoonful. Alternatively, you can use the crystalline powder sold in supermarket produce sections to prevent cut fruit from browning. Those powders consist of ascorbic acid or citric acid, and will also help the egg whites whip up to a high, snowy foam.

Adding an acidic ingredient can make whipping egg whites quicker and easier, but most dedicated bakers know an additional trick or two that can help give a better result. For one thing, room temperature egg whites whip better than chilled whites. Let your eggs sit out for a half hour before whipping them, or separate the whites into a pre-warmed glass or stoneware mixing bowl. Some manufacturers produce copper mixing bowls specifically for egg whites. Those work on a different principle, altering the electrical polarity of the protein molecules to produce a light, stable foam.