Custard
Custard is a mixture of milk or cream, eggs, and usually sugar, cooked gently until the egg proteins set into a smooth, thick sauce or soft solid. It is the base of crème anglaise, crème brûlée, flan, quiche, bread pudding, ice cream, and pastry cream.
How eggs thicken milk
Egg proteins start out as tightly folded chains. As they heat, they unfold (denature) and then link with one another (coagulate), forming a loose mesh that traps the surrounding liquid. A little linking gives a silky sauce; more gives a sliceable, set custard.
The trick is control. Heat too far and the proteins tighten until they squeeze the liquid back out. You get curds floating in thin, watery liquid, or a baked custard full of bubbles and holes.
What changes the setting point
- Dilution: more milk per egg spreads the proteins out, so they set at a higher temperature and more softly.
- Sugar: surrounds proteins and delays bonding, raising the setting temperature and making custard more forgiving.
- Yolks vs whites: yolks set softer and richer; whites set firmer. Yolk-only custards are silkier.
- Starch: flour or cornstarch gets in the way of protein bonding, so custards like pastry cream can be boiled without curdling.
Two families
Stirred custards are cooked on the stove, stirred constantly, and stay pourable or spoonable. Examples: crème anglaise, ice-cream base, pastry cream, lemon curd.
Baked custards are poured into dishes and cooked undisturbed, usually in a water bath. Examples: crème brûlée, crème caramel, pot de crème, cheesecake, quiche.
Temperatures and ratios
| Custard | Typical ratio | Done at |
|---|---|---|
| Crème anglaise | 5–6 yolks per 500 ml milk/cream | 82–84 °C / 180–183 °F |
| Pastry cream | 4–5 yolks + 30–40 g cornstarch per 500 ml milk | Boil 1 min |
| Crème brûlée | 5–6 yolks per 500 ml cream | 77–80 °C / 170–175 °F centre |
| Flan / crème caramel | 3 whole eggs + 2 yolks per 500 ml milk | 77–80 °C / 170–175 °F centre |
| Quiche | 3 eggs per 375 ml milk/cream | 77–80 °C / 170–175 °F centre |
For egg safety, any custard should reach at least 71 °C / 160 °F.
Baking tips
- Bake at 150–165 °C / 300–325 °F in a water bath that comes halfway up the dishes.
- Pull it when the edges are set but the centre still jiggles like gelatin. Carryover cooking finishes the job.
- Strain the mixture before baking for the smoothest texture and skim off foam.
Stirred custard tips
- Temper the eggs: whisk a little hot milk into the yolks before adding them to the pot.
- Use medium-low heat and a heatproof spatula, scraping the bottom and corners.
- Have a strainer and a bowl over ice ready to stop cooking the moment it is done.
Questions
At what temperature does custard set?
Yolk-only stirred custards thicken at about 77–84 °C / 170–183 °F. Baked whole-egg custards set at around 76–80 °C / 170–175 °F in the centre. Above about 85 °C / 185 °F, egg proteins tighten too much and the custard curdles.
Why did my custard curdle?
It got too hot, too fast. The egg proteins bonded so tightly that they squeezed out liquid and clumped. For a stirred custard, strain it and blend it smooth; for baked custard, lower the oven and use a water bath next time.
Why use a water bath for baked custard?
The water around the dish cannot get hotter than about 100 °C / 212 °F, so it shields the edges from the oven's dry heat. The custard cooks evenly from edge to centre instead of overcooking at the sides.
Why does pastry cream need to boil when other custards don't?
Pastry cream contains starch, which protects the eggs from curdling and needs boiling to thicken fully. Boiling for about 1 minute also destroys an enzyme in yolks that would otherwise break down the starch and thin the cream.