How do I stop a sauce or custard forming a skin?
The evaporation and protein coagulation that produce a skin, and the three reliable ways to stop it.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 3 min
Short answer
Press cling film or greaseproof paper directly onto the surface, or float a thin layer of melted butter or milk on top. A skin forms because the surface dries and the proteins there coagulate, so anything touching the surface prevents it.
Skin forms on white sauce, custard, gravy and hot milk for the same reason: water evaporates from the surface, the proteins and starch left behind concentrate, and they set into a film.
Step by step
- Cover the surface, not the pan.A lid on the pan traps steam that condenses and drips back, but the surface still dries at the edges. Contact with the surface itself is what works.
- Press film or paper directly on.Cling film or a disc of greaseproof laid straight onto the sauce, right to the edges, with no air gap. The most effective method for custard and pastry cream.
- Or float a layer of fat.A knob of butter dropped on top of a hot sauce melts into a film that seals it. Stir it in before serving. Standard practice for a white sauce being held.
- Or float a little milk.A thin layer of milk poured over hot custard or bechamel. Whisk it in when you come back to it.
- Stir occasionally if it is on the heat.Movement stops the surface drying long enough to set. A gentle stir every couple of minutes is enough.
- Keep it covered while cooling.Cooling is when most skins form, because there is no movement and the surface is still evaporating. Cover it before you walk away, not after.
- Whisk a skin back in if one forms.On a savoury sauce, whisk hard or use a stick blender and it usually disappears. On custard, lift it off with a fork instead — whisked in, it gives lumps.
- Sieve if whisking does not work.A fine sieve removes any remaining fragments. It takes thirty seconds and nobody will know.
Tips
- Contact is the point. Film hovering above the surface does nothing.
- Butter on top of a white sauce is the traditional method and it works perfectly.
- Lift a custard skin off rather than whisking it in — it will not dissolve.
Common mistakes
- Covering the pan with a lid instead of the surface — The surface still dries and sets, and the condensation running back down the lid waters the sauce down at the edges. The film has to touch the sauce.
- Whisking a custard skin back in — The proteins have already coagulated, so they break into small rubbery lumps rather than dissolving. Lift it off and sieve what is left.
If it doesn't work
Skin on a white sauce
Cause: Surface dried — Fix: Whisk hard or blend briefly, then cover the surface with film or butter.
Skin on custard
Cause: Cooled uncovered — Fix: Lift it off with a fork and pass the custard through a sieve.
Skin returns each time you check it
Cause: Covering the pan, not the surface — Fix: Press film onto the sauce itself, right to the edges.
Watery patches under the lid
Cause: Condensation dripping back — Fix: Use surface contact rather than a lid, or wedge the lid slightly ajar.
Questions people ask
Is the skin on custard safe to eat?
Yes — it is just coagulated protein. Some people like it. It is a texture problem rather than anything else, so remove it if you want a smooth custard.
Why does hot milk form a skin so quickly?
Milk proteins concentrate at the surface as water evaporates, and they set within a minute or two on hot milk. Stirring, covering the surface or keeping it below a simmer all slow it.