Roux thickening

What it is

A roux is flour cooked in fat. It is the thickener for béchamel, velouté, many gravies, gumbo and some Anglo-Indian dishes. It ranges from white (cooked briefly) to blond, brown and dark brown; darker roux has more flavour but less thickening power.

Why cooks use it: Coating flour in fat separates the starch grains so they thicken liquid without lumps, and cooking removes the raw flour taste.

The science

When the starch granules in flour are heated in liquid they swell and gelatinise, thickening the sauce; full thickness develops close to the boil. Browning the roux breaks down some starch, so dark roux thickens less. Mixing cold liquid into hot roux (or hot into cool) gives the starch time to disperse before it swells, preventing lumps.

The Maillard reactionStarch gelatinisationFats and oilsHydrocolloids: thickeners and gelling agents

Step by step

1. 1

   Melt equal weights of butter (or oil) and flour over medium-low heat.

2. 2

   Stir constantly: 2–3 minutes for white roux, 5–8 for blond, 15–30+ for brown and dark roux.

3. 3

   Take the pan off the heat and add liquid gradually (cold or warm liquid into hot roux), whisking until smooth.

4. 4

   Return to the heat and bring to a boil, whisking, then simmer at least 5–10 minutes.

5. 5

   Adjust thickness with more liquid; season and strain if needed.

Heat and timing

Time: Roux 2–30 minutes; sauce simmer 5–20 minutes.

How you know it’s right

• White roux smells like baked pastry, not raw flour
• Sauce coats the back of a spoon
• Sauce thickens noticeably as it boils