Emulsifying

Why it needs help

Oil and water separate because the molecules are attracted to their own kind and not to each other. Shaking a jar disperses the oil into droplets, and left alone those droplets find each other again and merge back into a layer.

An emulsifier is a molecule with one end that likes water and one that likes fat. It sits on the surface of each droplet and stops them merging. That is the whole mechanism.

The other half is mechanical: the droplets have to be made small in the first place, and smaller droplets stay suspended for longer. That is why whisking hard, or using a blender, produces a more stable emulsion than stirring.

What to use as the emulsifier

Egg yolk

Lecithin, and the strongest common emulsifier. Mayonnaise, hollandaise, aioli.

Mustard

Both an emulsifier and a flavour. The reason almost every vinaigrette recipe includes it.

Honey

Mild emulsifier, and it sweetens. Useful in a dressing.

Starch

Pasta water, a little cornflour, or a starchy purée. Physically gets in the way of droplets merging.

Butter and cream

Milk proteins do the work. This is what finishing a pan sauce with cold butter relies on.

Garlic paste

Pounded to a paste with salt, garlic emulsifies well — the basis of a true aioli.

Nothing at all

A shaken vinaigrette holds for a minute or two. Fine if you dress the salad immediately.

Building one that holds

1. Have everything at the same temperature. Cold oil into warm yolk, or the reverse, breaks emulsions.
2. Start with the water phase — vinegar, lemon juice, egg yolk — and the emulsifier, and combine them first.
3. Add the fat slowly at the start. The first few drops are where the emulsion is built, and adding faster than the emulsifier can coat the droplets is the commonest cause of failure.
4. Once it visibly thickens and pales, the emulsion exists and you can add the rest in a steady stream.
5. Whisk continuously. Stopping lets droplets find each other.
6. If it becomes too thick to work, loosen it with a few drops of water or lemon juice and carry on.
7. Season at the end and taste. Emulsions need more salt and acid than expected because the fat mutes both.

Rescuing a broken one

The same method works for mayonnaise, hollandaise, aioli and beurre blanc.

• Stop immediately and take it off any heat.
• Start again in a clean bowl with a fresh emulsifier — a yolk, a teaspoon of mustard, or a tablespoon of warm water.
• Whisk the broken mixture into it drop by drop at first, exactly as you added the fat originally.
• Once it re-emulsifies, add the rest in a stream.
• A stick blender in a tall narrow jug rescues almost anything mechanically.
• For a hot butter emulsion, whisking in a piece of cold butter often brings it back on its own.
• If it broke from overheating and looks grainy, sieve the rescued sauce to remove the set curds.

Common questions