Egg whites have a nearly magical ability to trap air in the form of a wispy, ethereal foam. They give angel food cake its characteristically airy texture, lighten mousses and chiffon pies, and even serve as a dessert topping in the form of meringue. This makes egg-white foams a fundamental baking skill, but it's not quite as straightforward as you'd think. There are a few "gotchas" that can keep your whites from stiffening properly, and several pro tips to help you master the technique.
Egg whites whip best when they're not too cold, so starting with eggs straight from the refrigerator -- especially if you're whipping by hand -- can become a test of patience. You'll find the whites whip better and faster if you bring them first to room temperature. Letting the eggs sit on your counter isn't an ideal scenario from the food safety perspective, so it's usually best to rest your bowl of separated egg whites in hot water for a few minutes. This warms the whites quickly, so there's limited risk of bacterial growth.
Before you start whipping, check your egg whites for any traces of yolk. The yolks are high in fat, and fat prevents the whites from foaming. If even a small amount of yolk gets into your whites as you separate the eggs, set them aside and use them for something else. Professionals usually separate one egg at a time into a small bowl, and inspect the white carefully before tipping it into the main mixing bowl. Adopting this technique can save you a lot of time and eggs.
It's important to have a perfectly clean bowl for whipping egg whites. If the bowl has any soap residue from washing, or if it has a film of fat from an earlier step in your recipe, your foam won't stiffen. Clean your bowls carefully and rinse them thoroughly before you start whipping the egg whites. The same holds true for your beaters or whisk.
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If you have budget for it, and enough storage space in your kitchen, pick up a copper bowl for whipping egg whites. The copper reacts chemically with protein molecules in the egg whites to form a stronger bond, helping them whip to a lighter, longer-lasting foam. You can even find copper bowls for most major brands of stand mixers, if you're willing to search.
Many recipes call for whisking salt or acidic ingredients with the egg whites. These help the protein molecules in the egg whites to denature, or unwind. The denatured strands of protein bond better with each other, so the whites foam more quickly and retain air better. Some recipes suggest a splash of vinegar or lemon juice, but both of those add a flavor to your whites that isn't always welcome. Cream of tartar, an acidic powder, is just as effective at helping the egg whites stiffen, but adds no flavor. If you're improvising rather than working from a written recipe, cream of tartar is usually your best bet. Usually 1/2 teaspoon is enough for 2 to 4 egg whites.
Sugar also helps, and many recipes call for part of their sugar to go into the egg whites once they've reached the soft-peak stage. Sugar makes the bubbles stickier and more durable, so the foam is less likely to deflate when you fold the egg whites into your batter. Feel free to put a tablespoon or two of a recipe's sugar into the egg whites, even if it's not part of the written directions.
Tips
If you've ever added corn syrup to your kids' bubble-blowing soap solution, adding sugar to the egg whites is exactly the same principle.