Cornflour, plain flour or arrowroot — which thickener should I use?

All three thicken by the same mechanism: starch granules absorb water, swell many times their size and crowd the liquid. What separates them is when they do it, how much abuse they survive afterwards, and what the finished sauce looks like. Choosing by those three things rather than by habit is what stops a sauce going watery in the oven or gluey in the fridge.

Step by step

1. Plain flour: robust, opaque, needs cooking.Only part of flour is starch, so you need roughly twice the weight of cornflour for the same thickness. It gives a matt, creamy, opaque sauce. It must be cooked for at least two minutes or it tastes raw. It tolerates long simmering, survives reheating well, and freezes acceptably though it may weep and need whisking.
2. Cornflour: strong, glossy, fragile.Nearly pure starch, so use about half what you would use of flour. Sets clear and glossy, which is why it is used in Chinese-style and sweet sauces. Thickens as it reaches a simmer and needs about a minute at that heat. Boiled hard or held simmering it breaks down and thins, and acid speeds that up. Thins again on reheating and weeps badly after freezing.
3. Arrowroot: strongest, clearest, most delicate.The strongest of the three by weight and the glossiest. Thickens at a lower temperature, so it can be used in sauces that must not boil. It breaks down very quickly with heat, so it goes in at the end off the boil. It is the only one that reliably thickens acidic liquids, and it dislikes dairy — it turns slimy in milk-based sauces.
4. Match the starch to what the sauce has to survive.A lasagne béchamel that will bake for an hour: flour. A stir-fry sauce thickened in thirty seconds: cornflour. A fruit sauce or glaze that must stay clear and is full of lemon: arrowroot. A gravy that will be reheated on Boxing Day: flour.
5. Always make a slurry with cold liquid for cornflour and arrowroot.Dry starch dropped into hot liquid cooks a skin on the outside of each clump and seals dry powder inside. That is exactly what a lump is. Mix to a smooth cream with cold water first, then pour into the simmering sauce while stirring.
6. Give flour a roux, or use a beurre manié.Flour needs to be dispersed in fat rather than in water. Cook it in butter as a roux at the start, or knead it into soft butter as a beurre manié and whisk small pieces into the finished sauce.
7. Judge the thickness at serving temperature.All three continue to thicken as the sauce cools, and cornflour in particular sets noticeably. A sauce that looks right at a rolling simmer will be much thicker on the plate.

Tips

• Cornstarch in an American recipe is cornflour in a British one. Cornflour in an American recipe is finely ground maize — not the same thing at all.
• Gluten-free thickening: cornflour, arrowroot, potato and tapioca starch all work. Plain flour does not, and gluten-free flour blends thicken unpredictably because they are mostly starch already.
• Roughly, 1 tablespoon of cornflour thickens 250ml of liquid to a pouring sauce; 2 tablespoons of plain flour do the same job.
• Never boil an arrowroot-thickened sauce. It thickens and then thins again within a minute of reaching a boil, and it will not come back.
• Sauces to be frozen do best on flour or tapioca. Cornflour-thickened sauces come out of the freezer watery every time.

Alternatives

• Potato starch: Thickens at a low temperature, very glossy and clear, and gives a light texture. Breaks down fast with heat, so it is added at the end. Common in Northern European and Japanese cooking.
• Tapioca starch: Very glossy, slightly stringy, and the best of the common starches for freezing — it does not weep the way cornflour does. Widely used in commercial pie fillings for exactly that reason.
• Reduction: No thickener at all. Concentrates flavour rather than diluting it and cannot go gluey, lumpy or thin on reheating. Slowest, and it concentrates salt with everything else.
• Purée: Blended vegetables, a spoonful of mashed potato, a handful of red lentils cooked to collapse, or bread. Adds body and flavour together, will not thin on reheating, and suits soups and stews better than sauces.

Questions people ask