How to thicken a sauce

The six methods

Reduction

Boil off water. Concentrates flavour along with everything else, including salt. Slowest, best-tasting, and the only one that adds no ingredient.

Roux

Flour cooked in fat, then liquid whisked in. Opaque, stable, reheats and freezes well. Needs a few minutes of simmering to lose the raw flour taste.

Slurry

Cornflour or flour dispersed in cold water, stirred into simmering liquid. Fast, glossy and translucent with cornflour. Breaks down if boiled hard or held hot.

Beurre manié

Soft butter and flour kneaded to a paste and whisked into a finished sauce. Last-minute, and it enriches at the same time.

Purée

Blend some of the solids back in, or add potato, bread, lentils or a vegetable purée. Adds body and flavour with no starch taste.

Emulsion

Whisk in cold butter, cream, egg yolk or pasta water. Thickens by suspension rather than by starch. Glossy and rich, and it can split.

Choosing one

• A pan sauce after searing — reduction, then cold butter whisked in. Nothing else gives that gloss.
• Gravy — a roux made in the roasting tin, or a slurry if you are short of time.
• A Chinese-style or glossy stir-fry sauce — cornflour slurry. It is the only one that gives that translucent shine.
• Béchamel, cheese sauce, lasagne — roux. Opaque and stable is exactly right.
• Soup — purée some of it, or add a floury potato. Starch thickeners in soup usually taste of starch.
• A stew that is too thin — take the lid off and reduce, or beurre manié at the end.
• Pasta sauce — pasta water and hard tossing. The starch in it emulsifies the fat.
• Anything you will freeze — roux. Cornflour weeps on thawing.

The two that go wrong

Both fail the same way — dry starch meeting hot liquid — and both are prevented the same way.

1. For a slurry: whisk the starch into cold liquid until completely smooth. Never add dry starch to a hot pan.
2. Pour it into the simmering sauce in a thin stream while stirring.
3. Simmer for a minute or two afterwards so it reaches full thickness and loses any chalkiness.
4. For a roux: melt the fat, add an equal weight of flour, and cook it, stirring, for at least a minute — longer for a darker roux.
5. Add the liquid gradually, whisking hard, so each addition is fully incorporated before the next.
6. Simmer for several minutes to cook out the flour.
7. If lumps form either way, a sieve or a stick blender fixes it in seconds.

Rules that apply to all of them

• A sauce thickens as it cools. Stop slightly thinner than you want it.
• Add thickener gradually. Thin to thick is easy; thick to thin means adding liquid and re-seasoning.
• Season after thickening, not before, especially with reduction.
• Cornflour is about twice as strong as flour by weight.
• Acid weakens cornflour. Add lemon juice at the end, off the heat.
• Do not boil a sauce thickened with egg or cheese. It will split.

Common questions