A custard should be smooth and silky. When it curdles, it turns grainy, with little lumps of cooked egg sitting in a thin, watery liquid. It is nearly always the egg and the heat, though milk that has started to turn can curdle when it is heated too.
My crème brûlée is a custard set with nothing but egg yolks, no flour and no cornflour, so it has no safety net. Every step of that recipe is there to stop it curdling. Here is why.
The short answer
A custard curdles when the eggs get too hot. Heat makes the egg proteins join together. A little joining sets the custard. Too much and they pull together into hard lumps and squeeze the liquid out. That is the grainy, watery result. My zabaglione is the same idea at an even lower heat: the yolks start to thicken at about 50C, which is why it cooks over a pan of water.
What is going on
On its own an egg sets at a fairly low temperature. In a custard the egg is spread through a lot of milk, cream and sugar, so its proteins have to travel further to find each other. That raises the setting point to around 78 to 80°C, and it makes the set much more delicate. There is only a small gap between just set and curdled.
The other catch is that it does not stop the second you take it off the heat. The setting carries on for a while. If it was heating fast, it can run straight past set and into curdled before you notice.
How to stop it curdling
- Add the hot to the cold. Pour the hot cream into the yolks slowly, stirring all the time. Tipping the yolks into hot cream, or the cream in all at once, cooks the first bits of egg on contact.
- Do not boil the cream. I take mine off at about 80°C, as the first bubbles form at the edge.
- Use a water bath in the oven, and do not cover the whole tin. The water keeps the outside of the custard below boiling, so the edges cannot race ahead of the middle, but a sealed tin traps the steam and the water comes to the boil.
- Go gently, and do not turn the heat up. The gentler the heat, the more time you have to catch it at the right moment.
- Take it out while it still wobbles. It carries on setting as it cools, and it is firmer again once it is chilled.
Why some custards are harder to curdle
A spoonful of flour or cornflour protects a custard. The starch swells at about the same temperature the egg sets, soaks up some of the heat and gets in the way of the egg proteins pulling too tightly together. That is why a custard thickened with cornflour can even be boiled. Chocolate and cocoa help in the same way, because they contain starch.
Can you save a curdled custard?
It depends on which kind. A stirred custard that has just started to go lumpy can often be rescued by straining the lumps out through a sieve. A baked custard that has gone grainy cannot be put right. It is still safe to eat, it just will not be smooth.
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