Emulsion

An emulsion is a mixture of two liquids that do not normally mix, usually fat and water, where one is broken into tiny droplets spread through the other. Shaken oil and vinegar form a short-lived emulsion. Mayonnaise, hollandaise, butter, cream and a glossy pan sauce are all emulsions that stay stable much longer.

How to do it

1. Start with the watery part, such as vinegar, lemon juice or egg yolk, plus an emulsifier such as mustard or yolk.
2. Add the fat slowly at first, a few drops at a time, while whisking or blending hard.
3. Once the mixture thickens and turns opaque, add the rest of the fat in a thin stream.
4. Keep the temperature steady: cool for mayonnaise, warm but not hot for butter sauces.

Why it works

Oil and water repel each other, so oil droplets try to join up and float to the top. Whisking breaks the oil into small drops. 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 keeps the drops apart. Common kitchen emulsifiers are lecithin in egg yolk, compounds in mustard and honey, milk proteins in butter and cream, and gelatin in stock.

The more fat drops there are and the smaller they are, the thicker the emulsion. That is why a mayonnaise turns from runny to stiff as you add oil.

How to use it

• In a vinaigrette, a spoon of mustard keeps the dressing creamy longer.
• In pasta sauces, starchy pasta water helps oil or butter and cheese emulsify into a smooth coating.
• In pan sauces, swirling in cold butter off the heat, called monter au beurre, gives a glossy finish.

Common mistakes

• Adding fat too fast at the start, before enough droplets have formed.
• Overheating butter or egg sauces, so the emulsion breaks.
• Letting a cold sauce warm up or a warm one get too cold.

When an emulsion separates, it becomes a split sauce. Most can be rescued by whisking the broken sauce slowly into a little fresh liquid or a new egg yolk.