KnowHow Team explains: Egg yolk is colloidal in nature. A substance is said to be a colloid when the size of the particles dispersed in the solvent (liquid) is bigger than that of the solute (solid) molecules in a true solution (any type of sugar or salt dissolved in water) but smaller than in a coarse suspension (in which the dissolved particles are visible, for instance, sand in water).

Egg yolk is made up of a protein that is not only a colloid but is also hydrophillic (water loving) in nature. An envelope of water prevents the particles from coming in contact with each other — a condition necessary for coagulation or aggregation. These colloidal particles are also charged particles, i.e. each particle carries the same charge. It’s a well-known fact that like charges repel, therefore no two particles could coalesce because the moment they come close to one another they would be repelled apart.

The yolk by virtue of being a hydrophillic colloid, consists of charged protein particles surrounded by a shell of water. When an egg is heated in hot water (boiled or poached) or fried in hot grease, the charge neutrality of the particles is disturbed and the water envelope evaporates. This eliminates the two factors, which prevents the coming together of the particles. As the stability of the colloidal particles is disturbed, the yolk solidifies.

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Md. Rehan Yousuf, Calcutta