Cooking science

How eggs set

Egg white and egg yolk are different mixtures of different proteins and they set at different temperatures. Every style of cooked egg — soft boiled, poached, fried, coddled, confit yolk — is a way of exploiting that gap.

A gap you can cook in

Egg white begins to thicken and set at a lower temperature than yolk does. Between the two there is a window where the white is set and the yolk is still liquid, and that window is what a soft-boiled or poached egg lives in.

Push past it and the yolk sets too — first to a soft fudge, then to a firm crumbly solid. Push much further and both go rubbery and the yolk develops a grey-green ring where sulphur from the white reacts with iron in the yolk.

Different proteins within the white also set at different points, which is why an egg white goes from clear liquid to cloudy to opaque to firm in stages rather than all at once.

What each style is doing

Soft-boiled or poached

White set, yolk liquid. Working in the gap between the two.

Hard-boiled

Both fully set. Overshoot and you get a rubbery white and a grey-green ring.

Fried, sunny side up

Direct heat sets the base of the white while the top and the yolk lag. The reason the white can be crisp and the yolk raw.

Scrambled and custard

Diluted with milk or cream, which raises the setting temperature and gives a much wider window.

Coddled or 63-degree eggs

The whole egg held in the narrow band where yolk thickens and white barely sets — the reverse of everything else.

Meringue

Set by whisking rather than heat, then dried in a low oven.

What moves the setting point

• Dilution. Milk, cream or stock raises the temperature at which the mixture sets and widens the window. This is why a custard is more forgiving than a plain scramble.
• Sugar. Raises the setting temperature further, which is why a sweet custard needs more heat than a savoury one.
• Salt and acid. Lower it. A splash of vinegar in poaching water helps the white set faster.
• Fat. Interferes with the network and gives a softer set.
• Starch. A little cornflour in a custard makes it very difficult to curdle.
• Speed. Fast heat sets the outside long before the middle. Nearly all egg cookery wants gentle heat.

Practical consequences

1. Cook eggs gently. Almost every egg fault is heat that was too high rather than time that was too long.
2. Stop early. Eggs carry a great deal of residual heat and continue setting off the heat.
3. For a set white and a liquid yolk, work in the gap: a short cook at a moderate temperature, then straight into cold water if you are not eating it now.
4. For a custard, dilute and sweeten — both widen the window — and use a water bath so the mixture never exceeds boiling point.
5. Add a little cornflour to a custard if it keeps curdling. It is not cheating.
6. For scrambled eggs, take the pan off the heat repeatedly rather than turning it down.
7. Accept that a grey-green ring is cosmetic. It means overcooked, not unsafe.

Common questions