Cooking encyclopedia When it goes wrong
Also called curdled; broken emulsion
The droplets got large enough to find each other. It can usually be rebuilt.
Do not whisk harder — that is not the problem. Start a fresh emulsifier in a clean bowl: a spoonful of the water phase, or a fresh yolk, or a teaspoon of mustard, and whisk the broken sauce into THAT, slowly, as though it were the oil.
An emulsion is fat in fine droplets held apart by an emulsifier. Too much fat too fast, too little water phase, or a temperature shock leaves droplets big enough to merge. Rebuilding works because you are re-dispersing existing fat into a new, adequately emulsified base.
Mayonnaise, aioli, hollandaise, butter sauces, vinaigrettes.
Whisking a broken sauce harder; adding more oil to fix it; heating a split butter sauce, which finishes it off.
Meridian cooking encyclopedia — Watchman Productions