Roux: The Classic Sauce Thickener
Roux is a classic thickener made by cooking equal amounts of butter and flour together. A white roux (cooked 1-2 minutes) suits a milk-based béchamel sauce, a blond roux (5-6 minutes) suits sauces that need a bit more depth, and a brown roux (10-15 minutes) thickens less but adds a toasted, nutty aroma to darker sauces.
Adding the liquid gradually to the roux, rather than pouring the roux into hot liquid, prevents lumps; stir each addition in fully before adding the next, letting it blend completely with the liquid first.
Practical Tip
If you want a starch-based thickener like cornflour, mix it smoothly with cold water in a separate bowl first, then add it to the boiling liquid; dry starch added straight to hot liquid clumps instantly.
Emulsion: Holding Oil and Water Together
Mixtures like mayonnaise and a classic vinaigrette are emulsions — they hold together oil and water-based liquids (lemon juice, vinegar) that would not normally mix. The lecithin in egg yolk is a natural emulsifier that binds the two together.
When building an emulsion, the oil should go in a thin stream, little by little, while whisking constantly; if the oil is poured in all at once, the mixture "breaks" — the oil and water separate again.
If an emulsion breaks, don't panic: take a small amount of fresh egg yolk or water and slowly whisk the broken mixture back into it as a new base.
Reduction: Thickening by Boiling Down
Reduction means boiling a liquid (wine, stock, cream) uncovered so the water boils off and it thickens; the flavour concentrates as the water evaporates. A litre of wine reduced to around 200-250 millilitres becomes noticeably thicker and more intensely flavoured.
Adding salt early during a reduction can be risky, since the salt concentration also rises as the liquid reduces, and the sauce can end up too salty. It is safer to season with salt once the reduction is finished.
The Risk of Curdling in Cream and Yoghurt Sauces
Cream can simmer at high heat without trouble, but lower-fat dairy like yoghurt and milk tend to curdle with a sudden change in temperature. Warming a yoghurt-based sauce gradually with a spoonful of hot liquid before adding it to a hot dish — tempering — stops it from getting a sudden heat shock and splitting.
Stirring a small amount of flour or starch into a yoghurt-based sauce stabilises its proteins and makes it more resistant to high heat, cutting the risk of curdling; this method is common in yoghurt-based soups.
Balance the Flavour When Seasoning a Sauce
When a sauce tastes flat, the first instinct is to reach for salt, but what is usually missing is acid or sweetness. A little lemon juice or vinegar can add depth, while a pinch of sugar can soften the sharpness of an overly acidic or tomato-based sauce.