Cooking an egg

A raw egg white is clear and runny. Cook it and it turns white and firm, and it stays firm when it cools. Its proteins have unfolded and tangled together.

Egg white is about 90% water and 10% protein. A protein, as you have seen, is a long chain of amino acids folded up in 3D, and hydrogen bonds help hold each chain folded. In raw egg white, the protein chains are folded into compact balls that float apart in the water, and light passes straight through.

Heat makes the chains jiggle harder. From about 60 °C the shaking breaks the hydrogen bonds, and the chains unfold. Side groups that were tucked away inside each ball, many of them oily, now stick out, and they cling to the side groups of neighbouring chains. The chains tangle into a mesh that holds the water in its gaps, so the liquid turns into a soft solid. The mesh scatters light, so the clear white turns white. Chemists call this unfolding denaturing.

The peptide bonds stay as they were, so every chain keeps its amino acids in the same order. Cooling leaves the chains tangled, and a boiled egg stays boiled.

White first, then yolk

An egg holds many different proteins, and each one unfolds at its own temperature.

In boiling water the heat reaches the white first, so a few minutes give a set white round a runny yolk.

Acid and salt

Amino acids, as you have seen, carry charges in water: -⁠NH₃⁺ at one end and -⁠COO⁻ at the other. The side groups of a protein carry charges too, and in raw egg white most of the proteins carry more negative charge than positive, so they push each other apart. A splash of vinegar adds H⁺ ions, which turn some -⁠COO⁻ groups back into -⁠COOH. With less charge to push them apart, the proteins come together sooner and set at a lower temperature. The ions of salt crowd round the charges and help in the same way. Cooks add vinegar and salt to the water for poached eggs, so the white sets quickly into a neat shape.