A teaspoon of a liqueur or a tablespoon of thin cream poured over food could be considered a sauce, but a true sauce is thick enough to have body and to adhere to the food. There are various ways to achieve this thickening.
A sauce is reduced to give it a more concentrated flavor, as well as to thicken it. Boil the sauce, uncovered, over low heat, stirring frequently, until the desired quantity is obtained. You may boil it down only a little or by half or more. Sauces that are to be reduced must be seasoned cautiously because the seasoning will also be concentrated. Often this procedure will result in an adequate thickening.
Some of the great classic sauces are thickened with egg yolk only; no flour is used. Examples are hollandaise and bearnaise and the best-known cold sauce, mayonnaise. In a sauce such as Mornay, where egg yolks are used, the amount of flour is reduced.
Any mixture enriched or thickened with egg yolks must never be allowed to boil, because this could cause the sauce to curdle. Low heat is essential. If the sauce is too hot, even if it is not boiling, the egg may cook into little hard lumps before it can expand and mix completely with the other ingredients. Remember, it takes only seconds to cook an egg once it is out of its shell.
This mixture, called liaison in classic terminology, is used to thicken the rich, delicious veloute sauce and its variations. The yolks and cream are well mixed, and the little hot veloute is stirred in to warm the yolks and allow gradual expansion. Then the liaison is stirred into the sauce and everything is brought to a boil. In this case, the egg yolks can be brought to a boil because a liaison is used to enrich sauces made with starch of some kind, and this combination needs to be cooked at a higher heat than sauces that are emulsions.
Grated cheese is used to thicken cheese and Mornay sauces. Bread crumbs thicken sauce polonaise, so often served with cauliflower. They are also the thickener as well as the chief ingredient for bread sauce, which the English serve with roast chicken.
The French chef's method of finishing a sauce is to remove it from the source of heat when ready to serve, then add butter and stir until melted. The heat of the completed sauce melts the butter; it is not necessary to return it to heat. This is called "buttering a sauce"; it gives both gloss and richness. If you want a light and fluffy sauce, omit the butter and stir in 3 tablespoon of whipped cream instead.
The best-known puree for a sauce is onion, which makes sauce soubise, but other purees of vegetables, nuts and fruits can be used to thicken sauces. Your imagination will give you many ideas along these lines.
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