Emulsions: Oil and Water Held Apart by Something Standing Between Them

Put the egg yolk, mustard, lemon juice or vinegar in the bowl FIRST, and whisk them together before a drop of oil goes near them. The emulsifier has to be dispersed and ready before there is anything to emulsify. Everything at room temperature. A cold yolk straight from the fridge emulsifies badly, which is the single commonest reason a first mayonnaise fails. Steady the bowl with a damp cloth underneath, exactly as with a cutting board. Both hands are about to be busy — one whisking, one pouring — and a bowl that spins is why people add the oil too fast.

The first spoonful of oil goes in drop by drop, whisking constantly. At that stage there is very little water phase to break the oil into, so oil arriving faster than it can be dispersed simply pools — and once it pools, the emulsion never starts. Once it has visibly thickened, the pace can increase to a thin steady stream. There is now a large emulsion to absorb new oil, and the process becomes forgiving. One egg yolk will hold a surprising amount of oil — well over 200 ml — but not an unlimited amount. Past its capacity the droplets are packed so tightly that they touch and merge, and the sauce breaks no matter how carefully you are working.

A split sauce is not ruined — the ingredients are all still there, just in the wrong arrangement. Start a NEW emulsion and feed the broken one into it as though it were oil. In practice: a fresh yolk, or a teaspoon of mustard, or a tablespoon of warm water in a clean bowl, whisked, and then the split sauce added drop by drop at first. The same rule as before, for the same reason. For a hot butter sauce that has split, a splash of cold water and hard whisking off the heat is often enough on its own. Heat was usually the cause, so removing it is half the cure.

Pasta water thickens a sauce because of dissolved STARCH, which stabilises fat and water together — which is why the water is saved and why the sauce is finished in the pan with the pasta rather than poured over it. A pan sauce is the same trick: the fond dissolved in stock, then cold butter whisked in off the heat so the milk solids and the liquid hold together. Too hot and the butter melts and separates instead of emulsifying. Vinaigrette is the unstable case, deliberately. Oil and vinegar shaken together separate within minutes unless there is mustard or honey in there — which is exactly why the classic recipe includes one, and why shaking it again just before it goes on the salad is a real technique rather than laziness.