Cooking science

Starch gelatinisation

Gelatinisation is the moment a starch granule gives up its structure. It is what thickens a sauce, cooks a potato, sets a custard and turns raw flour into bread crumb — and once it has happened it cannot be reversed.

What happens to the granule

Raw starch is packed into microscopic granules with a partly ordered internal structure. Cold water barely penetrates them, which is why raw flour and raw potato are gritty and why a cornflour slurry stays thin.

Heat plus water changes that. Above a temperature that varies by starch — broadly somewhere between 55 and 80°C — water forces its way in, the ordered regions come apart, and the granule swells enormously, sometimes to many times its original volume. Long starch molecules leak out into the surrounding water, where they tangle with each other and with the swollen granules, and the whole mixture thickens.

The ordered structure does not come back on cooling, which is why gelatinisation is described as irreversible. What does happen on cooling is a different, slower reordering — retrogradation — which is a separate process and the reason bread stales.

What it explains

A thickening sauce

Granules swell and burst, releasing polymer that tangles and raises viscosity. This is what a roux, a slurry and a custard all do.

A cooked potato

The starch inside the cells gelatinises and the cells separate, which is what makes a floury potato go fluffy.

Cooked rice

Each grain absorbs water and its starch gelatinises. Surface starch that escapes is what makes rice sticky, which is why rinsing helps.

Bread crumb

Starch gelatinises in the oven and sets around the gluten network, fixing the loaf in shape.

Why a sauce needs to be cooked out

The thickening is not complete until the granules have properly swollen — an under-cooked flour sauce is both thin and tastes raw.

Why it needs water

Dry starch heated alone does not gelatinise. It dextrinises instead, which is what browning flour in a dry pan does.

Using it deliberately

• Disperse the starch before heating. A slurry in cold water, or flour coated in fat as a roux. Dry starch dropped into hot liquid gelatinises on the outside of each lump and seals the inside dry.
• Bring it properly up to temperature. Different starches gelatinise at different points, and a sauce held below that never thickens fully.
• Do not boil a cornflour sauce hard or hold it hot. The tangled network breaks down again and the sauce thins.
• Acid weakens it. Add lemon juice at the end, off the heat.
• Rinse rice you want fluffy; do not rinse risotto rice. The same released starch is unwanted in one and the whole point of the other.
• Remember it is why pasta water works. The starch that leached out is an emulsifier as well as a thickener.

Different starches, different behaviour

They gelatinise at different temperatures and produce noticeably different results, which is why they are not interchangeable.

Cornflour

Gelatinises around 62 to 72°C. Sets glossy and translucent, thickens strongly, and breaks down if boiled hard or held hot.

Wheat flour

Gelatinises around 52 to 85°C depending on the fraction. Opaque and matt, weaker per gram, and much more stable to reheating and freezing.

Potato starch

Gelatinises at a lower temperature, thickens very fast and very clear, and breaks down quickly. Good for a last-minute glaze.

Arrowroot

Very clear, tolerates acid better than cornflour, and goes slimy if overcooked.

Rice flour

Thickens weakly and stays stable. Common in gluten-free baking and in some Asian sauces.

Waxy versus ordinary

Waxy maize and waxy rice starches have almost none of the straight-chain fraction, so they resist retrogradation and are far better in anything that will be chilled or frozen.

Common questions