Baked custard, flan and cheesecake

Why

Baked custard, flan and cheesecake are held together by egg proteins that unfold in the heat and link into a fine network; milk, cream, cheese and sugar dilute them, so they set at a higher temperature than egg alone, and according to America's Test Kitchen a flan or crème brûlée is done at about 77–79 °C, while above 85 °C the custard curdles. An overheated network tightens and squeezes out water, leaving holes and puddles inside, and a cheesecake rich in cheese also cracks when it is overbaked, cooled too abruptly, or shrinks as it cools while its edge stays stuck to the tin. A water bath cannot exceed 100 °C, so the edge of the dish does not overheat before the centre sets. The custard keeps setting from residual heat and as it cools, so it comes out when the centre only trembles slightly when shaken; Basque cheesecake is a deliberate exception, baked hot for a dark top and a creamy centre. The USDA gives 71 °C in the centre for egg dishes, but cheesecake is often taken out at around 66 °C, so for young children, pregnant women, older people and anyone with weakened immunity bake it longer or use pasteurised eggs.

How to do it

• Stir the mixture gently, do not whisk it, and strain it through a sieve; air bubbles and unmixed egg white cause foam on top and holes inside.
• Set the dishes in a roasting tin, put it in the oven and only then pour in hot water to about half the height of the dishes.
• Bake at the moderate temperature given in the recipe and check towards the end: the centre should only tremble slightly when shaken, not ripple like liquid.
• With a thermometer, aim for about 77–79 °C for flan and baked custard, 66–74 °C for cheesecake as the recipe says, and at least 71 °C for vulnerable groups.
• Loosen the edge of a cheesecake with a knife straight after baking and let it cool slowly, for example in the switched-off oven with the door ajar.
• Let the custard cool at room temperature for no more than 2 hours, then chill it covered in the fridge for at least 4 hours; once cold it unmoulds and slices cleanly.

If it goes wrong

A custard full of holes or weeping will not turn smooth again, but it is edible: pour off the water and serve it chilled with caramel or fruit. Cover a crack in a cheesecake with soured cream, fruit or a glaze; it changes nothing about the taste. Put a custard whose centre still ripples like liquid back in the oven and check it after 5 minutes. If a flan will not unmould, stand the dish in hot water for a moment so the caramel at the bottom loosens.