Egg white is a mixture of protein (10%) and water. Egg whites have myriad kitchen uses: they help dry out and crisp baked goods; in deep-fry batters, whites repel grease, making a lighter product. But egg whites are best known for how they whip up to lofty heights in soufflés, meringues, angel food cakes, and much more.
Most recipes call for large egg whites. If the recipe doesn't specify, assume large.An extra-large egg yields 2-2/3 Tbs. whites A large egg yields 2-1/4 Tbs. whites A medium egg yields 2 Tbs. whites
Packaged egg whites can take the place of fresh eggs in many recipes.
When it comes to choosing eggs for whipping, professional opinions differ. As eggs age, the whites become thinner and whip easily to great volume. Fresher eggs are more viscous so they take longer to beat, but some cooks think that the resulting foam is more stable. For making clouds of meringue in particular, you can use older whites for extra volume and add a little cream of tartar to stabilize the foam.
Egg separate best when cold, but egg whites whip up better at room temperature. When separating eggs, it's important that none of the yolks mix with the whites. Yolks contain fat, and even a tiny amount of fat will ruin an egg white's ability to trap and hold air. Before you begin, set three bowls or containers on the counter in front of you. Crack an egg and separate the white into one bowl, put the yolk in the second bowl, and then pour the cleanly separated white into the third. Repeat with each egg. That way, if a yolk breaks or an egg is spoiled, you won't contaminate the entire batch of already-separated whites. If you spot a small speck of yolk in the whites, use a clean egg shell to scoop it out; it will attract the yolk like a magnet.
Separated egg whites will hold for a few day in the refrigerator covered tightly. Egg whites can also be frozen for longer storage.