Why Eggs Turn Solid When You Cook Them

Egg white is about 90 percent water, and nearly all the rest is protein: long chains of amino acids folded into compact balls and dissolved in the water. In the raw egg the balls float past each other freely, which is why the white pours. The folds are held in place by weak bonds that need only a little energy to break.

Heat supplies that energy. As the temperature rises the folded proteins shake themselves loose and unravel into long strands, a change called denaturation. The strands then bump into each other and bond, forming a tangled three-dimensional mesh that traps the water inside it. Liquid becomes gel, and with more heat the mesh tightens into a firm solid. The change is a network forming, not water disappearing, which is why a perfectly cooked egg is still moist.

An egg contains several proteins and each unfolds at its own temperature. In the white, ovotransferrin starts to set at about 62 degrees Celsius, while ovalbumin, the most abundant white protein, needs about 84 degrees. That is why a white goes milky first and firm only later. The yolk's proteins set between about 65 and 70 degrees.

Those numbers explain the cook's whole repertoire. A soft-boiled egg is one pulled from the water when the white has passed 62 degrees but the yolk has not reached 65. A Japanese onsen egg, held at 63 to 65 degrees for an hour, has a yolk that is barely set and a white that is soft and custardy, a combination impossible to get by boiling. A poached egg works because the water is kept just below the boil, so the outside sets before the inside overcooks.

Keep heating and the mesh keeps tightening. It squeezes out the water it was holding, which is why an overcooked scrambled egg is both rubbery and sitting in a puddle. A hard-boiled egg cooked too long develops a green-grey ring around the yolk, which is iron sulphide: sulphur released from the white's proteins meeting iron in the yolk. It is harmless, and it does not happen if the egg is cooked for the right time and cooled quickly.

The trick behind good scrambled eggs is the same physics in reverse. Low heat, constant stirring and removal from the pan while they still look slightly under-done let the mesh form gently and hold on to its water. The eggs finish cooking on the plate.

Heat is not the only thing that unfolds a protein. Acid does it too, which is why lime juice turns raw fish opaque in ceviche and why a splash of vinegar in poaching water helps the white set faster. Mechanical force does it as well: whisking egg whites unfolds the proteins and traps air between them, and the unfolded strands then bond around the bubbles to make a foam stable enough to hold peaks. Meringue is that foam, dried by gentle heat.

Custards use the same setting proteins diluted with milk or cream, and the dilution shifts the setting temperature upward and the texture towards silk. A creme caramel sets at around 80 degrees; a few degrees more and the mesh tightens too far and the custard curdles, which is why custards are baked in a water bath that cannot exceed 100 degrees. The temperatures worth remembering:

• •62 degrees: egg white begins to set
• •65 to 70 degrees: yolk sets
• •80 degrees: custard sets, and curdles not far above
• •84 degrees: egg white fully firm
• •Prolonged boiling: water squeezed out, grey ring forms

Eggs solidify because heat unfolds their proteins and the strands bond into a mesh that traps the water. White and yolk set at different temperatures, which is what makes soft-boiled, poached and custard possible, and pushing past those temperatures tightens the mesh until the egg turns rubbery.