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.
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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.