Thickening Sauces

A sauce can be thickened in several ways, and each gives a different result. Choosing the right one is most of the skill.

A roux

Equal weights of butter and flour cooked together for a minute or two, until it smells biscuity, then liquid whisked in. It makes white sauce, cheese sauce, velouté and most gravies, and it reheats well.

Lumps come from adding liquid too fast. Add the first of it a splash at a time, whisking it smooth each time; once half is in, the rest can go faster. Warm liquid is easier than cold.

A flour-thickened sauce needs a few minutes' simmering to lose its raw, floury taste. It also thickens as it cools, so stop when it is a little thinner than you want it.

Cooking the roux longer, until nut brown, gives a deeper flavour but less thickening. That is the base of a gumbo.

Cornflour and other starches

Cornflour thickens quickly and leaves a glossy, clear sauce, which suits stir-fries, fruit and sweet sauces. Always mix it first with a little cold water, then stir it into the hot liquid and bring it to the boil.

It thins again if boiled for long or stirred hard, and does not freeze well. Arrowroot is similar and stays clear in acidic sauces.

Reduction

Simmering a sauce uncovered drives off water and concentrates both flavour and body. It is how pan sauces and good gravies get their richness.

Season only once it has reduced: the salt concentrates along with everything else.

Finishing with butter or cream

Whisking cold butter, a piece at a time, into a reduced pan sauce off the heat gives it shine and a velvety body. Do not let it boil afterwards or the butter separates.

Double cream can be boiled and reduced safely; single cream and yoghurt curdle, so stir them in off the heat.

Rescues

Lumpy: whisk hard, or push it through a sieve, or blitz it with a stick blender.

Too thin: simmer it down, or whisk in a paste of equal butter and flour (beurre manié) a little at a time.

Too thick: whisk in more of the liquid, a splash at a time.