What is a roux, and why do sauces split?

A roux is flour cooked in fat, used to thicken sauces. Understanding how it works explains most sauce failures.

How it thickens. Flour contains starch granules. Heated in liquid, they absorb water and swell — gelatinisation — trapping liquid and thickening it.

The fat's role is separation. Coating starch granules in fat before liquid arrives keeps them apart, so each hydrates individually. Add flour directly to hot liquid and the outside of each clump gelatinises instantly, sealing dry flour inside — which is exactly what lumps are.

The colours, and what they trade:

White roux — cooked briefly, just enough to remove raw flour taste. Maximum thickening power. Béchamel.

Blond roux — cooked longer, lightly coloured, slightly nutty. Velouté.

Brown and dark roux — cooked considerably longer, deeply flavoured, and much weaker as a thickener, because prolonged heat breaks down the starch. Cajun cooking uses very dark roux and compensates with far more of it.

The rule: darker means more flavour and less thickening.

Why sauces go lumpy: adding liquid too fast, or liquid and roux at similar temperatures. The traditional guidance is a temperature difference — hot roux with cold liquid, or cold roux with hot liquid — added gradually and whisked continuously.

Why sauces split, which is a different failure:

Emulsion sauces — hollandaise, beurre blanc, mayonnaise — hold fat suspended in water with the help of an emulsifier such as egg yolk. They break when too hot (the egg proteins coagulate), when fat is added too fast for the emulsion to absorb, or when the ratio exceeds what the emulsifier can hold.

Dairy sauces curdle when boiled, or when acid is added — the proteins coagulate and separate.

Rescuing a split emulsion: start again with a fresh yolk or a spoonful of water in a clean bowl and whisk the broken sauce into it very slowly, rebuilding the emulsion.