Custard is a culinary preparation made by blending eggs with milk or cream. Custard is thickened by the coagulation of the egg proteins, which is achieved by gently heating the custard in some way.
Types of Custard
Custards can be cooked on the stovetop, or in a double-boiler, in which case they are called stirred custards. Examples of these are zabaglione, Bavarian cream, and crème Anglaise.
Baked custards include custard pies (and tarts), crème brûlée, flan and cheesecakes. These are usually baked directly in a water bath, called a bain-marie, or sometimes with a large pan of water in the oven.
Boiled custards, such as crème patissière, are prepared with some sort of added starch, such as flour or cornstarch.
How is Custard Made?
The combination of eggs and cream shows up everywhere in the culinary arts. The ratio of eggs to cream can vary, but they all work the same way.
Custard can be cooked in a bain-marie in the oven, or on the stovetop. Cooking custard in a bain-marie helps keep the cooking air moist and heats gently so that the custard doesn't curdle or crack.
Sometimes a starch, such as flour or cornstarch, is added to the custard to stabilize it. When you do this, you don't need as many eggs. Pastry cream (sometimes called crème patisserie), which is used as a filling for classic desserts like cream puffs and éclairs, is made this way.
A custard can have various consistencies, from nearly liquid, as in crème Anglaise; to just barely set, as in crème brûlée; to quite stiff, as in pastry cream.
Custard is mainly used as a dessert, or as a base for a dessert, or as a dessert sauce. But note: custard can also be savory. Quiche is an example of a savory custard baked in a pie crust. And believe it or not, a frittata is a savory custard that's cooked directly in a deep skillet.
Sweet custards are more common, though, and they can take the form of a filling for a pastry, like crème patissière, or pastry cream, which is a stirred custard with added flour; or a sauce that's poured over a dessert, such as crème Anglaise, which is simply a thin stirred custard; as well as the main element of the dessert itself, like cheesecake or creme brulée.
Custard can also be frozen. Do you know what you get when you freeze custard? That's right, ice cream. Not all ice creams contain eggs, but the best ones do. Not only for richness, but also for smoothness. Adding eggs to the custard helps prevent those little ice crystals from forming when it freezes.
The key with cooking custard is to cook it gently. That means slowly, at a low temperature, using indirect heat. A double-boiler like the kind you'd use to melt chocolate or make hollandaise sauce is useful. Cooking custard too quickly, or at too high a temperature will cause the egg proteins to curdle.
This means you'll get something with a texture that resembles scrambled eggs, which is not what you want. Scrambled eggs are great, but custard should be smooth.
Likewise the low temperature. A crème brûlée is cooked in a 300 F oven, which is warm enough for the egg proteins to firm up without curdling. And the water bath ensures that the tops of the custard don't dry out and crack. When it's done, the center of a crème brûlée should still jiggle slightly (but not the edges).
But liquid custards like crème Anglaise are cooked much lower, like 180 F. This allows them to thicken while remaining pourable.
Because of its high protein content, custard needs to be stored in the refrigerator. The best way to do it is to allow it to cool thoroughly, then cover it with plastic wrap and refrigerate for up to three days.
Custard doesn't freeze well; or rather, custard can undergo undesirable texture changes when frozen and then defrosted, particularly baked custards. But obviously ice cream is a custard, and it freezes just fine.