Ingredient

Cornflour

Cornflour is pure starch, and it is the fastest, clearest thickener in an ordinary kitchen. It is also the one that goes wrong most spectacularly, because it clumps the instant it meets hot liquid.

How it thickens, and why it must be slurried

Cornflour granules are packed starch. In hot water they absorb liquid, swell enormously and burst, releasing long starch molecules that tangle and hold the water — which is what thickening is. It happens fast, at around 60 to 70°C, and it is dramatic: a small amount thickens a lot.

That speed is the problem. Tip dry cornflour into a hot sauce and the outside of each little heap gelatinises instantly, sealing the dry powder inside. That is a lump, and no amount of whisking will open it. The only reliable fix is to disperse the granules in cold liquid first — a slurry — so each one is separated before any of them meets heat.

Two parts cold water to one part cornflour is the usual ratio. Whisk it smooth, pour it into the simmering liquid while stirring, and let it come back to a simmer for a minute so it reaches full thickness and loses any chalkiness.

Cornflour versus flour as a thickener

Thickening power

Cornflour is roughly twice as strong as plain flour by weight, so use about half as much.

Appearance

Cornflour sets glossy and translucent; flour sets opaque and matt. Choose by what you want the sauce to look like.

Flavour

Cornflour is neutral. Flour tastes raw unless cooked out, which is why a roux gets a few minutes in the pan.

Stability

Flour-thickened sauces reheat and freeze better. Cornflour-thickened sauces thin out if boiled hard or held hot, and weep on freezing.

With acid

Acid breaks down cornflour thickening. Add lemon juice at the end, off the heat.

Gluten

Cornflour contains none, which is often the deciding factor.

Its other jobs

Thickening is only the obvious use.

• Crisp coatings. Tossed with cornflour before frying, tofu, chicken and vegetables develop a much crisper shell than flour alone gives. A mix of about three parts flour to one part cornflour is a good general batter.
• Tenderising in stir-fries. Meat tossed with cornflour and a little liquid before cooking — velveting — comes out noticeably softer and the coating thickens the sauce at the end.
• Shortbread and biscuits. Replacing a small proportion of the flour with cornflour reduces gluten and gives a shorter, more crumbly texture.
• Stabilising eggs. A little cornflour in a custard or a lemon curd makes it far harder to curdle, because the starch physically gets between the proteins.
• Stopping icing sugar caking, which is why most icing sugar already contains some.

The failures

• Lumps — added dry. Slurry it. See lumpy gravy.
• Sauce goes thin again — boiled too hard or held hot too long. The starch network breaks down. Thicken at the end, keep it below a rolling boil, and serve it.
• Chalky or floury taste — not simmered long enough after adding. Thirty seconds to a minute at a simmer fixes it.
• Weeping on reheating — cornflour does not survive freezing and reheating well. Use flour or a roux for anything you intend to freeze.
• Cloudy where you wanted clear — that is flour, not cornflour. Use cornflour for a glaze or a Chinese-style sauce.

Common questions