A sauce needs body to cling to food. Cooks thicken with starch, with egg yolks, or by reducing.
Key idea
Roux: equal weights of fat (usually butter) and flour, cooked together. White roux cooks a few minutes without color (for béchamel); blond until golden (velouté); brown until nutty brown (espagnole). Darker roux has more flavor but less thickening power. Slurry: cornstarch or arrowroot stirred into cold liquid, then whisked into a simmering liquid. Liaison: egg yolks and cream, tempered with hot liquid and added at the end without boiling.
| Roux per cup of liquid | Result |
|---|---|
| 1 tablespoon each butter and flour | Thin sauce or cream soup |
| 2 tablespoons each | Medium sauce (classic béchamel) |
| 3 tablespoons each | Thick sauce, as a binder for croquettes |
- Combine hot roux with cool liquid, or cool roux with hot liquid, whisking, to avoid lumps.
- Simmer a roux-thickened sauce for at least a few minutes, often 20 or more, to cook out the raw flour taste.
- A cornstarch slurry thickens as it comes to a boil; boiling it a long time or stirring hard can thin it again.
- Temper a liaison: whisk a little hot liquid into the yolks first, then whisk the mixture back in, and keep it below a simmer.
Why it works
Starch granules swell with water as they heat and burst near a boil, thickening the liquid. Flour must be coated in fat (roux) or mixed in cold water first so the granules separate; dumped into hot liquid, the outside of each clump swells and seals the dry flour inside.
Worked example: Scale a roux
A medium béchamel uses 2 tablespoons each of butter and flour per cup. How much of each for 6 cups?
- $2 \times 6 = 12$ tablespoons each, which is ¾ cup.
Answer: 12 tablespoons (¾ cup) of butter and 12 of flour.
Common mistake
Boiling a liaison. The yolks scramble into flecks.
Practice at home
- Make three small roux (white, blond, brown) and thicken one cup of milk or stock with each, using the same weight. Compare thickness and flavor.
In Meal Prep
- Béchamel Sauce: a white roux at work
The science behind it
- Heating curves: how heat flows into a sauce