Cooking science

Gelation

A gel is mostly liquid held motionless by a sparse three-dimensional network. Jelly is around ninety-nine per cent water; what makes it a solid is a scaffold occupying the other one per cent. Almost everything that sets in a kitchen works this way.

What a gel actually is

Take a liquid and introduce long molecules that can link to one another at intervals. If enough links form, the molecules build a continuous network through the whole volume, and the liquid can no longer flow because it is trapped in the mesh. That is a gel.

Because the network is sparse, a gel is overwhelmingly liquid by weight and behaves like a solid only up to a point. Push it hard enough and the network breaks — which is why a jelly wobbles, why a set custard can be stirred back to a sauce, and why gels are described by how much force they take to break rather than by a simple hard-or-soft.

The main setting agents, and how they differ

Gelatine

Protein from collagen. Melts at body temperature, which is why it dissolves in the mouth. Sets slowly, is clear, and is destroyed by fresh pineapple, papaya and kiwi, which contain protein-digesting enzymes.

Pectin

From fruit cell walls. Needs sugar and acid as well as heat. Does not melt in the mouth. This is what sets jam.

Agar

From seaweed. Sets much firmer than gelatine, at a much higher temperature, and does not melt until well above body temperature — so it feels less luxurious and holds up in the heat.

Starch

Swollen granules crowding each other, rather than a true chemical network. Sets on cooling and firms further over days.

Egg protein

Unfolds with heat and links irreversibly. Cannot be re-melted, which is why a set custard cannot be un-set.

Alginate

Sets on contact with calcium rather than with temperature. Sets cold, and instantly.

Reversible and irreversible

The most useful practical distinction is whether the network can be undone. Gelatine, agar and starch gels are thermally reversible: heat them and they melt, cool them and they set again, more or less indefinitely.

Egg gels are not. Heat unfolds the protein and the new bonds are permanent, so an overcooked custard cannot be rescued by cooling and a hard-boiled egg never becomes runny again. Pectin gels sit in between — once set they cannot simply be re-melted, though they can be re-boiled with more liquid.

This is why a recipe that can be fixed by gentle reheating and one that cannot look so similar from the outside. It depends entirely on which agent is holding it together.

Why gels weep

Liquid leaking out of a set gel is called syneresis, and it has a small number of causes.

• The network contracted. Over time, or with heat, the links tighten and squeeze liquid out — the same thing that makes an overcooked custard grainy.
• Too much setting agent. A dense network is a stiff, brittle one that expels water rather than holding it.
• Freezing. Ice crystals physically tear the network, so most gels are ruined by freezing and thawing.
• Acid or enzymes added after setting. Both can cut the network.
• Time. Starch gels in particular go on tightening for days, which is the same process that stales bread.

Common questions