Why do my sauces and custards curdle?

Curdling is protein coagulating too fast — eggs scrambling, or dairy proteins clumping when hit by acid or high heat. Both are prevented by controlling the temperature and the rate at which things are combined.

What you'll need

• Whisk (optional)
• Thermometer (optional)
• Heatproof bowl (optional)

Step by step

1. Cook anything egg-based gently.Eggs begin setting around 65°C and scramble above about 75°C. That is a narrow window, and direct high heat crosses it instantly.
2. Temper eggs before adding them to hot liquid.Pour a little of the hot liquid into the beaten eggs while whisking, then return the mixture to the pan. Adding eggs straight into hot liquid scrambles them.
3. Use a bowl over simmering water for custards.It limits the temperature the mixture can reach. Slower, and it removes most of the risk.
4. Add acid to dairy off the heat and at the end.Lemon, wine or vinegar into hot cream or milk curdles it. Take the pan off the heat, let it cool slightly, then add.
5. Use full-fat dairy where a sauce is heated.Higher fat content is considerably more stable. Single cream, milk and low-fat yoghurt all curdle far more readily when heated or acidified.
6. Stabilise with a little starch.A teaspoon of cornflour whisked into cream or yoghurt before heating substantially reduces the chance of splitting.
7. Do not boil anything containing eggs or yoghurt.A bare simmer at most. Boiling is what pushes both past the point of no return.
8. Rescue a curdled custard by blending or sieving.An immersion blender often brings a lightly curdled custard back. Sieving removes the lumps if not. Neither works on a badly scrambled one.

Tips

• Yoghurt added to a hot dish is best stabilised with cornflour, or stirred in off the heat right at the end.
• A custard is done when it coats the back of a spoon and a drawn finger leaves a clear line. That happens well below boiling.
• If a sauce is going to be reheated, that is another opportunity to curdle. Reheat egg and dairy sauces very gently.

Common mistakes

• Adding eggs straight into hot liquid — They scramble on contact. Tempering — adding hot liquid to the eggs gradually — is what prevents it.
• Adding lemon or wine to hot cream — Acid plus heat curdles dairy proteins. Add acid off the heat once it has cooled slightly.
• Using low-fat dairy in a heated sauce — Fat stabilises the emulsion. Low-fat cream, milk and yoghurt curdle far more readily when heated or acidified.

Questions people ask