Educated Cook

Sauce

How to thicken a sauce: starch or reduction

Thicken a sauce by reducing it, with a cornstarch slurry, or with a flour roux: what each does, the ratios, and how to fix a thin or lumpy one.

Published

01

What

Reduction: simmer the sauce until it has less water and coats a spoon. Starch: cornstarch whisked in as a slurry, or flour cooked into a roux first. Which one you pick changes the look, the flavor and how the sauce reheats.

Illinois Extension notes that cornstarch is pure starch, while flour is starch plus gluten, so flour has less thickening power. Roughly 1 tablespoon of cornstarch or 2 tablespoons of flour thickens 1 cup of liquid.

02

Why (ELI5)

ELI5

Cornstarch and flour are starches. Mixed into liquid and heated, they swell and gelatinize, which thickens the sauce. Reducing does it another way: simmering drives off water until what’s left is more concentrated.

03

How

Reduce it

Simmer until the sauce lightly coats a spoon. Draw a finger through it: a clean line means it’s there (University of Wyoming Extension’s spoon test). Taste before you salt, because reduction concentrates salt along with everything else. This is the pan sauce method, and it adds no starch.

Cornstarch slurry: glossy and quick

Heat the liquid first. In a small bowl, mix 1 tablespoon of cornstarch with 1 to 2 tablespoons of cool water to make a slurry, then pour it slowly into the hot liquid, whisking until it thickens (University of Wyoming Extension). Cornstarch gives a clear, shiny sauce, and it suits pie fillings and fruit sauces (Illinois Extension). It’s also gluten-free.

Flour roux: opaque and reheats well

A roux is flour cooked in fat, and it can run from white to deep brown. Warm the fat, sprinkle in the flour and whisk until it looks like wet sand, about a minute or two (University of Wyoming Extension). Add hot liquid gradually, about half a cup first, whisking constantly to avoid lumps, then bring to a simmer.

Flour makes a cloudy, opaque sauce, which suits fat-based sauces such as a cheese sauce (Illinois Extension). Wyoming Extension’s classic gravy ratio is 3 tablespoons flour, 2 tablespoons fat and 1 cup hot liquid. That’s more flour than Illinois Extension’s 2 tablespoons per cup, so start low and add more if you need it.

Which to use, and how much

  1. 01Decide: reduce, slurry or roux.
  2. 02Slurry: mix cornstarch with cool water first, then stir it into hot liquid.
  3. 03Roux: cook flour in fat until it looks like wet sand, then add hot liquid a little at a time.
  4. 04Simmer while whisking until the sauce coats a spoon.
  5. 05Taste, then season.

Reheating the sauce later? Wyoming Extension says flour-thickened gravy reheats much better. Cornstarch-thickened gravy turns thinner and can look uneven after reheating. Want it clear and glossy? Use cornstarch. Want it rich and opaque? Use a roux.

04

Common mistakes

Too thin (flour sauce): Wyoming Extension suggests mixing equal parts flour and soft butter into a paste, then whisking in a tablespoon or two at a time into the boiling sauce, and cooking it at least 5 minutes to lose the raw-flour taste. Too thin (cornstarch sauce): stir a tablespoon of cornstarch with a little cool water into a smooth slurry first, then whisk it into the simmering sauce (University of Wyoming Extension). Too thick: you used too much thickener. Whisk in more liquid, such as stock. Water works but waters down the flavor. Lumpy: pour it through a fine-mesh strainer, or blend it smooth. Don’t sprinkle dry cornstarch or flour straight into a hot pan. Mix it with cool liquid or fat first.