Cooking science

Proteins unfolding with heat

A raw protein molecule is a long chain folded into a specific shape. Heat shakes it apart, the chain unfolds, and the exposed parts link to their neighbours. That single sequence explains a cooked egg, a set custard, a seared steak and a firm piece of fish.

Unfolding, then linking

Protein molecules are held in their folded shape by many weak bonds. Heat provides enough energy to break those bonds and the chain unfolds — that is denaturation.

An unfolded chain exposes parts that were previously tucked inside, and those parts readily bond to the equivalent parts of neighbouring chains. The molecules link into a network — that is coagulation, and it is what turns a liquid into a solid.

The reason it cannot be undone is that the new bonds between molecules are as stable as the old ones within them. Cooling does not reverse it, because there is nothing driving the network apart.

Other things denature protein

Heat is the most obvious and not the only one.

Acid

Ceviche, gravlax cured with citrus, milk curdled with lemon. The proteins denature without heat and set in the same way.

Salt

At high concentration. Part of what curing does.

Mechanical action

Whisking egg white unfolds proteins at the air-water interface. That is why a meringue is stiff without ever being heated.

Alcohol

Denatures protein readily, which is why spirits are used in some cures.

Enzymes

Cut the chains rather than unfolding them — a different mechanism, and the one behind pineapple and papaya tenderising meat.

Freezing

Ice crystals disrupt structure physically, which is why thawed food often weeps.

Where it decides the outcome

• An egg setting, from liquid to soft to firm to rubbery as the network tightens.
• Custard thickening, and curdling if the network tightens too far and squeezes the liquid out.
• Meat firming and shrinking, as muscle fibre proteins contract.
• Fish going from translucent to opaque, which is the visible signature of protein setting.
• Milk skin on hot milk, where surface proteins denature and link.
• A meringue holding its shape, set by whisking rather than heat.
• Cheese made with acid rather than rennet, such as paneer or ricotta.

Using the sequence

1. Recognise that setting is a range, not a point. Egg goes from liquid to soft-set to firm to rubbery, and each stage is a different dish.
2. Stop earlier than looks right for anything protein-set. It continues to set with residual heat.
3. Use gentle heat for anything delicate. A custard over direct heat sets locally and curdles; over a water bath it sets evenly.
4. Add fat where you want a wider window. Fat interferes with the network and makes it more forgiving.
5. Add starch for the same reason — a spoonful of cornflour in a custard makes it far harder to curdle.
6. Use acid deliberately where heat would be wrong: ceviche, a citrus cure, or setting a fresh cheese.
7. Accept that overcooked protein cannot be reversed, and plan for that rather than hoping.

Common questions