Cooking science

Protein coagulation

Almost everything heat does to food that is not browning is protein coagulation. It is why an egg sets, why meat firms and then dries, why milk curdles and why fish flakes — one process, in different materials.

Unfolding, then joining

A protein in its natural state is a long chain folded into a specific compact shape, held by relatively weak internal bonds. Heat gives the chain enough energy to break those bonds and unfold — this is denaturation — exposing parts that were tucked inside.

Those exposed parts readily bond to the same parts on neighbouring unfolded proteins. As more and more of them link, they form a continuous network that traps the water around it. Liquid becomes solid. That is coagulation.

Keep heating and the network keeps tightening. The links multiply, the mesh contracts, and the water it was holding is squeezed out. That final stage is what over-cooking is, in an egg, a steak, a custard and a piece of fish alike — and it is not reversible, because the bonds do not come undone on cooling.

The same process, different materials

Egg white

Sets at a relatively low temperature, going from clear liquid to opaque solid. Over-heated, it turns rubbery and weeps.

Egg yolk

Sets higher than the white. That gap is what makes a soft-boiled egg and a custard possible.

Meat

Muscle proteins contract as they denature, squeezing out moisture. This is why a steak gets firmer and drier the longer it cooks.

Collagen

The exception that behaves in the opposite direction — it contracts at first, then, with long moist cooking, converts to gelatine and softens. This is why braising works.

Milk

Its proteins are stabilised in suspension. Heat and acid together destabilise them, which is curdling — and also, deliberately, cheese making.

Fish

Coagulates at a lower temperature than meat, which is why fish cooks so fast and overcooks so easily.

Gluten

Sets in the oven, which is what fixes a risen loaf in its shape rather than letting it collapse.

What it explains

• Why gentle heat gives better results for eggs, custards and lean meat — the setting range is narrow and low heat gives you time inside it.
• Why resting meat works — the tightened fibres relax slightly and reabsorb some of the liquid they squeezed out.
• Why brining helps — salt dissolves some of the muscle protein, so the network contracts less violently and holds more water.
• Why acid "cooks" ceviche — acid denatures proteins too, without heat. The chemistry is the same up to a point; the flavour chemistry is not.
• Why a custard curdles — the yolk proteins went from thickening to fully networking within a few degrees.
• Why sous vide is precise — holding food at exactly the temperature where the proteins have set and no further is the whole idea.

Why it cannot be undone

Denaturation is sometimes reversible in a laboratory. Coagulation is not, because the proteins have not merely unfolded, they have bonded to each other into a new structure.

That is why there is no fix for an overcooked steak, a rubbery egg or a scrambled custard beyond disguising it, and why every technique in this area is preventive: lower heat, shorter time, a thermometer, taking things off before they look done, and using the residual heat rather than fighting it.

It is also why "it just needs a bit longer" is such a costly instinct. In this one area, more heat is never a correction.

Common questions