Egg yolks, like flour and cornstarch, are a thickening agent for sauces, custards, and the like. For fact collectors, it might be noted that 2 egg yolks are approximately equal to 1 tablespoon of flour as a thickener.
Unlike flour and cornstarch, egg yolks must be treated with special care, for an obvious reason. High heat affects both yolks and whites in the most visible manner when scrambled together in a skillet. In exactly the same way high heat affects egg yolks in a sauce. If the yolk blended in the sauce is brought to an excessive temperature, the yolk “scrambles,” just as it does in the skillet, and causes the sauce to curdle. Therefore, when yolks are used to thicken sauces they must be cooked over very gentle heat.
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