Custard is a range of preparations based on milk and eggs, thickened with heat. Most commonly, custard refers to a dessert or dessert sauce, but custard bases are also used for quiches and other savoury foods. As a dessert, it is made from a combination of milk or cream, egg yolks, sugar, and vanilla. Sometimes flour, corn starch, or gelatin are also added.
Custard is usually cooked in a double boiler (bain-marie) or heated very gently on the stove in a saucepan, though custard can also be steamed, baked in the oven with or without a hot water bath, or even cooked in a pressure cooker. Cooking until it is set without cooking it so much that it curdles is a delicate operation, because only 5-10°F (3-5°C) separate the two. A water bath slows heat transfer and makes it easier to remove the custard from the oven before it curdles.
Depending on how much egg or thickener is used, custard may vary in consistency from a thin pouring sauce (crème anglaise), to a thick blancmange like that used for vanilla slice or the pastry cream used to fill éclairs.
Custard is an important part of dessert recipes from many countries.
While 'custard' may refer to a wide variety of thickened dishes, technically (and in French cookery) the word custard (crème or more precisely crème moulée) refers only to an egg-thickened custard.
When starch is added, the result is called pastry cream (crème pâtissière), which is made with a combination of milk or cream, egg yolks, fine sugar, flour or starch, and usually a flavoring such as vanilla, chocolate, or lemon. Crème pâtissière is a key ingredient in many French desserts including millefeuille (or Napoleons) and filled tarts. It also used in Italian pastry and sometimes in Boston cream pie.
When gelatine is added, the result is crème anglaise collée.
When starch is used alone as a thickener (without eggs), the result is referred to as a blancmange.
Instant and ready-made 'custards' such as are also marketed, though they are not true custards if they are not thickened with egg; Bird's Custard is a well-known brand of packaged custard mix. In the United Kingdom, school custard is a common name for the 'custard' (usually made from cornflour) served for pudding at schools. Its poor quality and thick consistency are often the source of jokes. Pink school custard is made by combining Angel Delight (strawberry) with custard mix, generally starch-based packet custard.
Cooked (set) custard is a weak gel which is viscous and thixotropic; while it does become easier to stir the more it is manipulated, it does not, unlike many other thixotropic liquids, recover its lost viscosity over time.
A suspension of uncooked imitation custard powder or starch mixed with water in the right proportions has the opposite rheological property: it is negative thixotropic, or dilatant, which is to say that it becomes more viscous when under pressure. It is often used in science demonstrations of non-Newtonian liquids: see Oobleck. The British popular-science program Brainiac: Science Abuse demonstrated dilatancy dramatically by filling a swimming pool with this mixture and having presenter Jon Tickle walk across it; this was misleadingly called "walking on custard." A similar exhibition was performed on the Discovery Channel series MythBusters, in which co-host Adam Savage traversed a tank filled with water and cornstarch.