Roux

Why

A roux is flour cooked in fat, usually in a ratio of roughly 1 : 1 by weight. The fat coats each flour particle, so it disperses in the liquid on its own instead of clumping; the starch then swells and thickens fully only close to boiling point. Lumps form when flour meets hot liquid too suddenly: the outside of a clump gelatinises instantly and seals dry flour inside – which is why you combine cool roux with hot liquid or hot roux with cool liquid, and add the liquid gradually. The longer and darker a roux cooks, the more the heat breaks its starch into shorter pieces that thicken less: a dark roux smells nutty, but you need more of it for the same thickness. Once thickened, let the sauce simmer gently for a while so the starch swells completely and the raw-flour taste disappears.

How to do it

• Weigh out fat and plain flour in a ratio of roughly 1:1.
• Stir the flour into the fat over low heat; a light roux takes a few minutes, a dark one needs time and attention.
• Pour cool or lukewarm (not ice-cold) liquid into hot roux, or hot liquid into roux that has cooled.
• Add the liquid gradually, the first splash carefully and at arm's length: hot roux, especially a dark one, spatters and the steam burns. Whisk it in at once, not with a wooden spoon.
• Then simmer the sauce or soup gently, usually for 10–20 minutes, stirring often and tasting until the floury taste is gone.
• Use more flour for a dark roux; it thickens less than a light one.

If it goes wrong

Whisk out lumps, blitz them with a stick blender or pass the sauce through a sieve. A floury taste disappears with longer gentle simmering. A burnt roux with black specks and a bitter smell can't be saved – throw it out and start again, next time over lower heat and stirring constantly.