Rescues · TO EMULSIFY
It never thickened at all and stayed thin and clear.
Why it happens
No agitation, or no emulsifier. A vinaigrette stirred gently and without mustard is just oil sitting on vinegar.
Recovering it
A split emulsion is usually recoverable, because the emulsifier is still present and only the arrangement is wrong. Start a fresh base in a clean bowl — a spoonful of cold water, a fresh egg yolk, or a little of the intact sauce — and whisk the broken mixture into it drop by drop. You are rebuilding around new emulsifier rather than trying to repair the old one. Cold helps: it slows the fat down enough to catch. If the mixture is grainy rather than split, the proteins have set and nothing recovers it.
The mechanism
Fat and water do not dissolve in each other, so left alone they separate into two layers — the arrangement with the least contact between them. An emulsion suspends one inside the other as droplets, and it needs two things at once: mechanical energy to tear the droplets small, and an emulsifier to coat each droplet so it cannot merge back. Emulsifiers are molecules with a water-loving end and a fat-loving end — lecithin from egg yolk, casein from butter and cream, the mucilage in mustard, dissolved starch from pasta water. Whisking without an emulsifier gives you a mixture that separates within a minute. Adding fat faster than the whisk can break it leaves droplets uncoated, they coalesce, and the sauce splits.
The full TO EMULSIFY lesson — how to tell, where to practise it, what it carries to.