Liaison

The culinary sense of the Czech word corresponds to German legieren, which the DWDS explains as binding soups and sauces with egg yolk or flour, recorded in this sense from the 18th century; the word comes from Italian legare and Latin ligare, to bind. Czech dictionaries, the SSJČ and the Academic Dictionary of Foreign Words, list only the metallurgical sense of legovat, adding other metals to an alloy.

French liaison, a binding, is recorded in a culinary sense as early as 1393 in the Ménagier de Paris, and the Trésor de la langue française defines it as adding an ingredient that makes a preparation smooth and thick – roux, egg yolks, butter, cream or starch. For velouté soups Escoffier allows three yolks and one-fifth of a pint (≈ 115 ml) of cream per quart (≈ 1.1 l).

Temperature is decisive. McGee states that in warm egg-yolk sauces the yolk proteins coagulate into small curds at about 70–77 °C. A soup finished with yolk is therefore not allowed to boil again.