When an egg is heated (without the shell in a pan, the yellowish and translucent liquid becomes white and rubbery, the egg yolk becomes solid.
When we heat the white and yellow of an egg, we can observe that the liquid becomes solid. It is said that the egg has coagulated, or else that it has cooked. The proteins of the egg (yellow and white) are at the origin of this coagulation.
White is made up of 12.5% of a protein of the albumin group: ovalbumin, which represents more than 50% of the total protein of white. It is composed of four acid chains amines, which give it a globular form.
When the temperature approaches 60 ° C, the atomic agitation becomes such that the bonds weaker, like hydrogen bonds, break. The protein runs and becomes a long chain of amino acids: this is the denaturation.
In this denatured state, some parts of the protein become accessible and can to meet other protein molecules, with which they will associate. This association of proteins by disulfide bridges is responsible for the phenomenon of coagulation. In addition, the binding of the proteins together reveals a network that traps the water molecules, causing the stiffening of the egg after cooking.