Culinary function

Thicken

Thickening means making a liquid resist flowing without turning it into a solid. It is distinct from gelling: a thickened sauce still pours, and a gel holds its shape when you tip the bowl.

What makes a liquid thick

Viscosity comes from resistance to molecules sliding past each other. Anything that gets in the way of that sliding thickens a liquid, and ordinary cooking uses four quite different ways of doing it.

Long molecules tangling — starch, pectin, gums. The commonest route, and the one behind gravy, custard and most sauces.

Suspended particles crowding — a purée, ground nuts, breadcrumbs, blended vegetables. Physically obstructive rather than chemically clever.

Emulsified droplets — butter, cream, egg yolk. Millions of fat droplets in the way of one another.

Simply less water — reduction, which raises the concentration of everything already there.

Thickening versus gelling

Thickened

Pours, coats a spoon, flows back into a level surface. A gravy, a custard, a velouté.

Gelled

Holds its shape, has a set structure, and fractures rather than flowing. A jelly, a panna cotta, a set custard.

The same agents can do both

Gelatine, pectin and starch all thicken at low concentrations and gel at higher ones. It is a matter of degree.

Why the distinction matters

A sauce that has gelled is a mistake, and a jelly that has only thickened has failed. The target concentration is different.

Choosing a route

• Reduction where you have time and want no added ingredient. Concentrates flavour, and salt with it.
• Roux where you want an opaque, stable sauce that reheats and freezes. Béchamel, gravy, cheese sauce.
• Cornflour slurry where you want gloss and translucency and speed. Chinese-style sauces, glazes, fruit fillings.
• Purée where you want body with no starch flavour. Soups especially.
• Butter or cream where richness is wanted as well as body. Pan sauces.
• Egg yolk where you want silkiness — and gentle heat, because it curdles.
• Pasta water for pasta, where the released starch emulsifies as well as thickens.

Why a sauce thickens more as it cools

Almost every thickened sauce is thicker cold than hot, and the amount catches people out. Starch networks continue to associate as the temperature falls, fat firms up, and gelatine — present in any stock-based sauce — begins to set.

The practical consequence is that you should stop while the sauce is still slightly thinner than you want it. A gravy adjusted to exactly the right consistency in a hot pan will be noticeably heavy on the plate, and one adjusted in the jug will be stiff by the time it reaches the table.

It also means the test has to happen at serving temperature. Dropping a spoonful onto a cold saucer for a few seconds tells you far more than looking at the pan does — the same trick jam makers use for a setting point.

The thickeners, and what each is for

• Reduction — no thickener at all. Concentrates flavour as well as body. The best answer when there is time.
• Roux — flour cooked in fat. Robust, opaque, classic for gravies, béchamel and stews. Needs a proper simmer to lose its raw taste.
• Cornflour slurry — starch slaked in cold liquid. Fast, glossy, sets clear. Breaks down with long cooking and with freezing.
• Beurre manié — raw flour and butter kneaded together, whisked in at the end. Thickens and enriches at once.
• Egg yolk — thickens gently and richly, and curdles if boiled.
• Purée — blended vegetables, bread, nuts or pulses. Adds body and flavour together, and is how most traditional cuisines thicken.
• Fat emulsion — cold butter or cream whisked in off the heat. Gives gloss and body rather than viscosity.

Common questions