Liaison — Egg Yolk and Cream Thickening

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Liaison is the classical French technique for enriching and lightly thickening a sauce using a mixture of egg yolk and heavy cream whisked together and incorporated into the sauce just before service. It is the defining finish for sauce allemande, sauce suprême (in some formulations), blanquette de veau, and dozens of cream soups. The technique relies on egg yolk's proteins, which coagulate gently when heated, thickening the sauce while the cream adds richness and a silky mouthfeel. The ratio is 3 egg yolks to 100ml of cream for every litre of sauce — though this varies with the desired richness. The critical technical challenge is temperature management: the liaison must heat enough for the yolk proteins to thicken (above 65°C) but never reach a boil (above 80°C, the yolks scramble into visible curds that ruin the sauce's texture). The technique is performed by tempering: a ladleful of hot sauce is whisked into the yolk-cream mixture to raise its temperature gradually, then the tempered mixture is whisked back into the pot of sauce, which is stirred continuously over gentle heat until the sauce thickens perceptibly — usually 2-3 minutes. The heat is then cut immediately. Once a liaison is added, the sauce must never boil again. The finish should be imperceptible as egg — the sauce simply becomes more velvety, more golden, and slightly thicker.

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