Custard is a variety of culinary preparations based on a cooked mixture of milk or cream and egg yolk. Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce (crème anglaise) to a thick pastry cream (French: crème pâtissière) used to fill éclairs. Most common custards are used as desserts or dessert sauces and typically include sugar and vanilla. Custard bases may also be used for quiches and other savory foods. Sometimes flour, corn starch, or gelatin is added as in pastry cream or crème pâtissière.
Custard is usually cooked in a double boiler (bain-marie), or heated very gently in a saucepan on a stove, though custard can also be steamed, baked in the oven with or without a water bath, or even cooked in a pressure cooker. Custard preparation is a delicate operation, because a temperature increase of 3–6 °C (5–10 °F) leads to overcooking and curdling. Generally, a fully cooked custard should not exceed 80 °C (176 °F); it begins setting at 70 °C (158 °F). A water bath slows heat transfer and makes it easier to remove the custard from the oven before it curdles.
Mixtures of milk and eggs thickened by heat have long been part of European cuisine, since at least Ancient Rome. Custards baked in pastry (custard tarts) were very popular in the Middle Ages, and are the origin of the English word 'custard': the French term 'croustade' originally referred to the crust of a tart, and is derived from the Italian word crostata, and ultimately the Latin crustÄre.
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