How do I thicken a sauce without ruining it?

The five thickening methods, which suits which sauce, and why reduction beats them all when there is time.

Difficulty: beginner
Time: 20 min
Read: 3 min

Short answer

Reduce it if you have time — it concentrates flavour as well as thickening. Otherwise use a cornflour slurry for a glossy finish, a roux or beurre manié for a richer one, or blend in some of the solids.

Every thickener changes the sauce as well as its consistency, so the right choice depends on what you want the sauce to taste and look like.

Step by step

Reduce it where you can.Simmer uncovered and let water evaporate. It thickens and concentrates flavour simultaneously, which no added thickener does. It just takes time.
Use a cornflour slurry for speed.Two teaspoons of cornflour mixed smooth with cold water, whisked in, then simmered for a minute. Glossy, clear, and very quick.
Use beurre manié for richness.Equal parts soft butter and flour kneaded to a paste, whisked into the simmering sauce in small pieces. It thickens and enriches at once.
Use a roux for a stable base.Flour cooked in fat before liquid is added. The right approach when the sauce is built from scratch rather than corrected at the end.
Blend some of the solids.For soups and stews, removing a ladleful and blending it back in thickens without adding anything. It keeps the flavour pure.
Consider a starchy addition.Potato, lentils, bread or a spoonful of the pasta water. Traditional and effective, particularly in rustic dishes.
Add cream or egg yolk for a rich finish.Off the heat for egg yolk, tempered first with a little hot sauce, or it scrambles. Cream needs a brief simmer to thicken.
Add thickener gradually.Sauces continue to thicken as they cool, so stop while it still looks slightly thin. Overthickened sauce is much harder to correct than thin sauce.
Simmer after adding starch.Both flour and cornflour need heat to reach full thickening power and to lose any raw taste. A minute or two at a simmer.
Taste after thickening.Thickeners dilute flavour, particularly a slurry. Check the seasoning again once the consistency is right.

Tips

Reduction is the only method that improves flavour as well as thickening.
Stop while it still looks slightly thin. Sauces thicken as they cool.
Taste again after thickening — starch dilutes the seasoning.

Common mistakes

Adding dry flour or cornflour straight to a hot sauce — It clumps instantly, and the lumps are then sealed and cannot be whisked out. Always make a slurry with cold liquid first.
Thickening to the final consistency while hot — Sauces thicken substantially as they cool, so what looks right in the pan is often too thick on the plate.

If it doesn't work

Sauce still thin after adding cornflour

Cause: Not simmered — Fix: Bring it to a simmer for a minute — cornflour needs heat to thicken.

Sauce gone gluey

Cause: Too much starch — Fix: Thin with stock or water and adjust the seasoning.

Cloudy sauce

Cause: Flour rather than cornflour, or overboiled — Fix: Cornflour gives a clearer result. Avoid hard boiling after thickening.

Sauce split after adding cream

Cause: Boiled too hard — Fix: Add cream off the heat or at a bare simmer. A little cold cream whisked in can bring it back.

Tastes floury

Cause: Starch not cooked out — Fix: Simmer for a few more minutes.

Questions people ask

What is the difference between flour and cornflour for thickening?

Cornflour thickens more per gram, gives a glossy clear result and needs less cooking. Flour gives an opaque traditional sauce and needs several minutes to lose its raw taste.

What is beurre manié?

Equal parts soft butter and plain flour kneaded to a paste, then whisked into a simmering sauce in small pieces. It thickens and enriches at the same time.