What do cheesecake, ice cream, trifles, Bavarians, crème brûlée, pots de crème, and pudding all have in common? At the most basic level, these creamy desserts are all custards. Technically, a custard is any liquid thickened by eggs, and in most cases that liquid is cream or milk. It’s a marriage made in heaven: The creaminess of the dairy is enhanced by the silky, emulsifying properties of egg yolks to create an irresistible texture. I’ve turned down a cookie, even a slice of cake, but I don’t think I could ever turn away crème brûlée.
As you’ll see, different types of custards are governed by slightly different sets of chemical properties. Knowing the scientific principles behind them—plus a few key cooking tips and tricks—can help you get these luscious desserts right every time.
Slideshow:Get the recipes for more than a dozen scrumptious custard desserts.
Three types of custards
Custards are categorized by how they’re thickened.
Pure or basic custards are thickened and set by eggs alone. These are the most delicate custards; they require careful attention during cooking, which is usually done in the even heat of a water bath, as they can quickly go from undercooked to broken and curdled.
Starch-thickened custards contain ingredients such as flour or cornstarch for added thickening power. These starches give custards more body, making them sturdy enough to endure cooking with direct heat.
Gelatin-set custards have an alluring richness from the structural boost that only gelatin can provide.
The science behind basic custards
What sets them apart? These delicate custards are thickened only with eggs. When heated, the egg proteins slowly unwind from a coil-like shape and elongate. The proteins can then easily catch onto one another to form a gel, which thickens the mixture.
Delicious examples: Crème anglaise, a classic dessert sauce made with lightly sweetened milk and cream and just barely thickened with egg yolks, is the thinnest of all basic custards. If you freeze this custard sauce, you get ice cream. If you add more egg and bake it, you have flan, pot de crème, crème brûlée, or crème caramel. If you pour it over bread cubes and bake it, you’ve made bread pudding.
Getting them right: Sweet custards typically thicken between 160°F and 180°F, well below the boiling point. If you go past that point, the egg proteins lose their shape and can no longer hold liquid, so a baked custard like crème caramel will appear curdled and runny, and a stirred custard sauce like crème anglaise may have bits of scrambled egg in it.
To prevent overcooking, basic custards are usually cooked over a double boiler or baked in a water bath. These techniques ensure slow, even cooking and provide a layer of insulation that cooking directly over a burner simply can’t—it’s just too easy to scorch delicate custards over direct heat. Whether you’re using a double boiler or not, when cooking a basic custard on the stove-top, check its temperature frequently to ensure that it doesn’t go above 180°F. Remove baked custards from the oven when they have just a slight wobble in the center when nudged; residual heat will continue to cook them until fully set.
The Science behind starch-thickend custards
What sets them apart? The addition of wheat flour (or corn or potato starch) gives these custards a full-bodied texture, and the extra starch molecules slow protein coagulation, making them more resistant—though not immune—to overcooking and curdling. While basic custards should never be boiled, starch-thickened ones need to reach a low simmer to ensure that they’re fully cooked.
Delicious examples: Starch-thickened custards take many forms, from pudding to pastry cream and cheesecake. Puddings contain just enough thickener to give them body but not enough to make them stiff. Pastry cream is thicker (it contains more starch), which makes it a sturdy foundation for a fruit tart or a cream pie. Clafoutis, a classic French custard, is on the far side of the starch-thickened custard continuum, since it contains a relatively high ratio of starch to liquid. This allows it to endure the oven’s direct heat with no water bath.
Getting them right: starch-thickened custards must reach a simmer to ensure that the amylase enzyme in the egg yolks has been denatured, or rendered inactive, by heat. This is especially true of stirred custards like puddings and pastry cream, which are easy to under-cook on the stove-top. An undercooked custard may initially appear thick but will slowly turn to soup as the amylase enzyme attacks the starch and breaks the custard down. A good guideline is to cook for 1 to 2 minutes after bubbles appear in the custard, stirring constantly.
If you overcook a starch-thickened custard, you can use a fine strainer to strain out any bits of egg.
The science behind gelatin-set custards
What sets them apart? Like eggs, gelatin contains proteins that bind liquid. But powdered gelatin is a much more powerful thickener than eggs, so it gives the custard enough body to hold its shape when sliced. This type of custard needs to be chilled in order to set.
Delicious examples: Gelatin-set custards start with a basic custard (such as crème anglaise), which may be flavored with fruit purée or chocolate. When gelatin is added to this mixture, it becomes firm enough to set in a crust for an icebox pie. Fold whipped cream into the gelatin-set custard and you have a fluffy, melt-in-your-mouth Bavarian. Bavarians can be poured into decorative molds to set, or used to fill a cake-lined pan. (A classic example is charlotte russe, which is a Bavarian cream set in a mold lined with lady fingers and garnished with whipped cream.) When egg whites are used to lighten the custard, you’ve got a chiffon filling, which makes lighter-than-air pies.
Getting them right: To ensure that the gelatin dissolves evenly without forming grainy lumps, allow it to “bloom,” or soften, by sprinkling it onto a liquid and letting it sit for 5 minutes before heating to dissolve.
Certain fruits, such as figs, pineapple, kiwi, papaya, mango, and guava, should be cooked before being used in a gelatin-set custard to denature enzymes that can break down the gelatin. (This isn’t necessary for canned fruit, which is pasteurized at a high enough temperature to denature the enzymes.)
For maximum volume, let a gelatin-set custard cool and thicken slightly before folding in any whipped cream or beaten egg whites.