Liaison

A liaison is a mix of egg yolks and heavy cream used to thicken and enrich a sauce or soup in the last minutes of cooking. It gives a silky texture and a pale golden color. It is classic in cream soups, chicken fricassée, blanquette de veau and sauces built on velouté. The word is French for "binding".

How to do it

1. Whisk 1 egg yolk with 60 ml (1/4 cup) of heavy cream for every 250 ml (1 cup) of liquid you want to thicken.
2. Take the soup or sauce off a boil and lower it to a gentle simmer.
3. Ladle about a third of the hot liquid slowly into the yolk mix while whisking. This is called tempering.
4. Pour the warmed mix back into the pot, stirring.
5. Heat gently, stirring, until the sauce thickens slightly. Do not let it boil.
6. Serve soon.

How to tell it is done

The sauce coats the back of a spoon, looks glossy and smooth, and has no specks of egg.

Why it works

Egg yolk proteins unfold and link together when heated, which thickens the liquid. Cream dilutes the yolks so they set more gently. Tempering raises the temperature of the yolks slowly so they thicken instead of scrambling.

Substitutes

For a thickener that tolerates boiling, use beurre manié or a starch slurry. These do not add the same richness.

Common mistakes

• Adding cold yolks straight to hot liquid, so they curdle into strands.
• Boiling the sauce after adding the liaison. Most egg sauces start to curdle above about 82°C (180°F).
• Reheating leftovers too hard. Warm them slowly and stir.

In recipes

"Finish with a liaison" or "bind with egg yolks and cream" means this method. Greek avgolemono works the same way, using egg and lemon juice instead of cream.