When you cook an egg, the proteins in the egg white, called albumen, undergo a process called denaturation. Denaturation occurs when the protein molecules are unfolded and their chemical bonds are broken, causing the protein to lose its original structure and function.

In the case of egg whites, the denaturation of albumen proteins is caused by the heat from cooking. As the egg white is heated, the protein molecules start to vibrate faster and bump into each other more frequently, causing them to unfold and break apart. This process also leads to the formation of new chemical bonds between the amino acids in the protein molecules, resulting in the firm, white texture of cooked egg whites.

Additionally, the denaturation of albumen proteins causes the egg white to become opaque and white in color. This is because the unfolded protein molecules scatter and reflect light more efficiently than the original, tightly packed protein molecules, resulting in the characteristic white appearance of cooked egg whites.