Custard Science: Egg Proteins and Setting
Custard is a cooked mixture of eggs (or egg yolks), milk or cream, and sugar, set by the controlled denaturation and coagulation of egg proteins. The three primary custard categories: stirred custards (crème anglaise, pastry cream, curd — cooked with stirring, set to a pourable or spreadable consistency); baked custards (crème brûlée, flan, cheesecake — set in the oven by gentle, even heat); and steamed or molded custards (Japanese chawanmushi, crème caramel — set by steam in a water bath). The temperature range of custard setting is narrow and unforgiving: egg yolk proteins begin to coagulate (denature) at approximately 149°F (65°C) and are fully set at 185°F (85°C). Whole egg custards set at slightly lower temperatures due to the additional albumen (egg white) proteins. The key variable is starch: crème pâtissière contains cornstarch or flour, which gelatinizes during cooking and raises the temperature threshold to approximately 185°F — the starch stabilizes the egg proteins and prevents curdling at higher temperatures, making pastry cream much more forgiving than crème anglaise (which contains no starch and curdles if overcooked above 185°F). The water bath (bain-marie) used for baked custards moderates oven heat — the water bath cannot exceed 212°F, surrounding the custard with gentle, even heat and preventing the edges from overcooking before the center sets. A properly set baked custard jiggles uniformly like Jell-O when the pan is gently shaken — liquid in the center means under-baked, no jiggle means over-baked (rubbery, weeping).