Cooking science

Starch granules swelling

A starch granule is a tight package of molecules that is nearly inert in cold water. Heated in the presence of water it absorbs, swells enormously and bursts — and that swelling is what thickens every sauce, every gravy and every pan of rice.

Nothing, then everything

In cold water, starch granules sit there. They absorb a little and change almost nothing about the liquid — which is why a cornflour slurry is thin and pourable.

Heated past a certain point, the granules begin absorbing water rapidly and swell to many times their original size. As they swell they crowd each other and the liquid thickens sharply. Heated further, they rupture and release long starch molecules into the liquid, which thickens it further still.

This is why sauces thicken suddenly rather than gradually. Nothing much happens, and then over a few degrees everything happens at once.

Why the practical rules exist

Slake starch in cold water first

Dry starch hitting hot liquid swells on contact and the outside of each clump seals, trapping dry powder inside. That is a lump, and it is permanent.

Different starches swell at different temperatures

Cornflour, arrowroot, potato and tapioca all behave differently. Arrowroot sets clearer and breaks down faster.

It needs enough water

Starch cannot swell in a dry mixture. A roux works because the flour is dispersed in fat first and the liquid is added gradually.

Acid and sugar interfere

Both weaken the thickening. A very acidic or very sweet sauce needs more starch.

Over-boiling breaks it down

Ruptured granules eventually fragment and the sauce thins again. This is why a cornflour sauce held too long goes watery.

It sets further on cooling

Judge consistency hot; it will be thicker cold.

Where it shows up

• Sauces and gravies, thickened deliberately.
• Rice absorbing water and going from hard to soft. The same swelling, in a grain rather than a slurry.
• Pasta water going cloudy, as starch leaves the pasta and swells in the water. That is what makes it useful for finishing a sauce.
• Potato going from crunchy to soft, and going gluey if the granules are ruptured mechanically.
• Bread crumb setting, as the starch gelatinises and holds the structure.
• Custard thickened with cornflour, where starch does the work instead of egg.
• Roux-based sauces, where the flour is coated in fat so the granules disperse before they swell.

Thickening without lumps

1. Slake the starch in cold liquid until completely smooth. This is non-negotiable.
2. Bring the sauce to a simmer before adding it, then take it off the boil.
3. Pour the slurry in slowly while stirring, so it disperses before it swells.
4. Return it to a gentle heat and let it come back to a simmer. It thickens as it does.
5. Give it a minute before deciding whether to add more. Starch takes a moment to reach full thickening.
6. Do not boil hard afterwards, and do not hold it at temperature for long. Both break the granules down and thin it again.
7. If lumps form anyway, whisk hard and then push it through a sieve.

Common questions