Thickening with starch

Why

Starch granules don't dissolve in cold water, they just disperse in it; in hot liquid they absorb water, swell and thicken the sauce. Tip starch straight into something hot and the surface of every clump gelatinises instantly, sealing dry powder inside and creating a lump – which is why starch is always mixed into cold liquid first. Potato starch thickens below boiling point and gives a clear, glossy, slightly stringy sauce, but its large swollen granules are easily broken by long boiling and stirring, and the sauce thins out again; cornflour needs to come briefly to the boil, is cloudier but more stable and sets into a gel as it cools, which makes it good for puddings. For both, acid (lemon, vinegar, tart fruit) and long boiling cut the starch chains into shorter pieces that thicken less.

How to do it

• Mix the starch with two to three times as much cold water or milk and stir again just before using, as it settles.
• Pour it into the hot liquid in a thin stream, stirring constantly.
• Just bring potato starch to the boil and take it off the heat straight away.
• Simmer cornflour for about a minute, stirring gently, until the sauce thickens and turns glossy.
• Add lemon juice, vinegar or tart fruit only after thickening.
• Once thickened, stir the sauce only gently; vigorous whisking thins it.

If it goes wrong

Rescue a sauce that is thinning with another batch of starch mixed into cold water – pour it in, just bring it to the boil and then stop stirring. Whisk out lumps or pass the sauce through a sieve. Starch broken down by long boiling with acid won't thicken again, but a fresh batch will.