The Science of Foams

• When you whip egg whites to a foam, you're changing their physical structure. Ordinarily, the proteins in egg whites are bound up in tight coils. The wires of your whisk or stand mixer physically shear the protein molecules, and separate them with pockets of air. The egg proteins bond back together in longer strands, trapping the air into small bubbles. As you continue whipping, the bubbles become smaller and stiffer until you create a light, firm, stable foam.

Cream of Tartar

• The physical action of your whisk isn't the only factor that affects the egg whites and their proteins. Temperature helps the proteins unwind or "denature" more quickly, which is why whites at room temperature foam better than those straight from the refrigerator. Acidity also helps denature the proteins, which is why many recipes call for a small amount of cream of tartar. Cream of tartar is a byproduct of wine making, a shelf-stable powder that adds acidity to your baking without a noticeable flavor. If you don't keep cream of tartar on hand, you can substitute other acidic ingredients.

Substitutes

• Most other acidic ingredients in your pantry are liquids, and most have a distinctive flavor of their own. Lemon juice is usually the best substitute, and recipes that do not call for tartar usually specify lemon juice. Lime juice is also acceptable, though its flavor is more likely to come through in the finished cookie. That is not necessarily bad for your macaroons, because lime and coconut are complementary. You can also use white vinegar or a good-quality white wine or rice vinegar. In baking, you would use three teaspoons of juice or vinegar for every spoonful of tartar, but that is not necessary when you are frothing egg whites. Even a one-to-one substitution will give a better foam.

Copper Bowl

• Another alternative, especially if you work with egg whites regularly, is to invest in a special copper bowl just for whipping egg whites. They are available in good-quality kitchenware stores, and some manufacturers offer a copper bowl for their stand mixers. Copper bowls create stable foam because they react chemically with the egg whites to change the electrical polarity of the protein molecules. Like a balloon stuck to the wall with static electricity, the realigned electrical charge helps the foam form quickly and remain stable.