The white

The white accounts for two thirds of the shelled egg's weight. It is made of 90% water. The rest is made up of proteins, minerals, fatty materials, vitamins, and glucose.

The proteins in the whites play important roles in protecting the embryo. Some proteins block the action of digestive enzymes. Others bind tightly to vitamins and/or iron. One protein inhibits the reproduction of viruses, and another digests the cell walls of bacteria. The egg white is a chemical shielf against infection and predation.

The following proteins are important for the cook:

• Ovomucin accounts for <2% of albumen but has the most influence on the eggs culinary value. Ovomucin pulls together the proteins into an organized structure. It is what makes the thick egg white thick. It gradually desintegrates, so older eggs have a more runny texture.
• Ovalbumin is the most dominant egg protein. Its role is not entirely understood but it is thought to help inhibit protein-digestive enzymes. It is the only egg protein to have reactive sulfur groups, which strongly contributes to the flavor, texture, and color of cooked eggs. Ovalbumin becomes more heat resistant as the egg ages, so fresher eggs need less cooking than older eggs.
• Ovotransferrin holds tightly to iron. It is the first protein to coagulate when an egg is heated, thus it determines the setting temperature. Whole eggs set at higher temperature than egg whites because ovotransferrin binds to the iron in the yolk and becomes more resistant to coagulation. Ovotransferrin changes color when bound to metals, which is why whites whipped in a copper bowl turn golden and whites supplemented with ground-iron turn pink.

Egg whites are highly immunogenic. Portions of ovalbumin tend to be responsible. Sensitivity to egg whites develops early in life. Pediatricians recommend that infants and children under 1 do not consume egg whites.