Why it happens
Mayonnaise is an emulsion, tiny droplets of oil held apart in egg yolk. If the oil goes in too fast, or the ingredients are very cold, the yolk can't coat the droplets quickly enough and they join back together, so the sauce splits.
How to fix split mayonnaise
Whisk a fresh egg yolk in a clean bowl, then slowly whisk the split mayonnaise into it a little at a time until it thickens again.
How you can tell
- The mayonnaise looks thin, oily and curdled instead of thick and glossy.
- Pools of oil sit on top or around the edges.
- It runs off the whisk rather than holding its shape.
How to fix it
Option 1Start again with a fresh egg yolk
- Put a fresh egg yolk in a clean bowl and whisk it briefly.
- Add the split mayonnaise a drop at a time, whisking constantly.
- Once it starts to thicken, add the rest in a thin, steady stream.
Option 2Use a spoonful of mustard or water
- Put a spoonful of Dijon mustard, or a little warm water, in a clean bowl.
- Whisk the split mayonnaise into it very slowly, a little at a time, until it comes back together.
Next time
- Have the egg yolk and oil at room temperature before you start.
- Add the oil drop by drop at first, and only speed up once the sauce has thickened.
- Whisk constantly and steadily while the oil goes in.