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Eggs, Liaison of, for Thickening Sauces
When eggs are used for thickening great care is required to keep the sauce from boiling, and the least cessation from stirring during the heating process will spoil the whole contents of the saucepan. Make a liaison as follows. Beat well the yolks of three eggs, and add to them three small wine-glassfuls of cream or two and a half glassfuls of milk. When well mixed, strain through a sieve.
A liaison is nothing more than egg yolks loosened with cream or milk, stirred into a hot sauce off the heat to give it body and gloss. The instruction that matters is the first one: it must never boil.
- The hinge: keep it below a simmerYolks begin to thicken from around 150F (65C) and will curdle not far above that, so the working window is narrow. Once they curdle the sauce turns grainy and whisking will not bring it back. Finish the sauce over low heat or off the burner entirely, and pull it the moment it coats a spoon.
- A wine-glassfulIn Victorian and Edwardian usage a wine-glass is about 2 fl oz, so three small glassfuls of cream is roughly 5-6 fl oz (150-175 ml) and two and a half of milk about 5 fl oz (150 ml). The cream version is more forgiving, since fat slows coagulation.
- Temper before addingWhisk a ladleful of the hot sauce into the yolk mixture first, then return the lot to the pan. Tipping cold yolks straight into hot liquid is another way to lose the sauce, alongside the boiling and the pause in stirring the recipe warns of.
- Straining is not optionalThe sieve catches the chalazae and any bits of albumen clinging to the yolks, which would otherwise show up as white threads in the finished sauce. Strain the liaison before use, not after.
- Stir with a spatula, not a whiskA flat wooden spatula or silicone spoon keeps contact with the base and corners of the pan, where the sauce scorches first. A whisk aerates and misses the corners.
- Yield and useThis quantity will thicken and enrich roughly a pint (500 ml) of stock-based sauce, blanquette or veloute. Once a liaison has gone in, the sauce cannot be reheated hard or held long over direct heat.
- Keeping it warmIf the sauce must wait, set the pan in a bain-marie of hot -- not boiling -- water and cover the surface with a butter paper or film to stop a skin forming.
Liaison is simply the French for a binding or joining, and in classical kitchen French it covers any thickener -- roux, starch, blood, ground almonds -- though used on its own it often came to mean the egg-and-cream finish specifically. A liaison of this kind is what turns a plain veloute into a sauce allemande and what finishes a blanquette de veau or a poulet a la creme. The warning against boiling is repeated insistently in period cookery books, and the coal ranges of the day gave the cook far less control over low heat than a modern hob does; keeping a pan genuinely below a simmer meant moving it off the plate or setting it in water.