How do you make a vinaigrette that doesn't separate?

A vinaigrette is oil and vinegar persuaded to cooperate for a few minutes. Left alone they separate, because they do not mix: oil droplets drift together and float to the top. The trick is to make the droplets very small and give them something that helps them stay apart.

What is actually happening

A vinaigrette is an emulsion: one liquid spread through another as tiny droplets. As Wikipedia's emulsion entry explains, a plain vinaigrette is an unstable emulsion that quickly separates unless it is kept moving. Whisking or shaking breaks the oil into droplets; given time, those droplets merge and rise.

An emulsifier is something that sits between oil and water and slows that process down. The same source notes that mustard contains compounds around the seed that act as emulsifiers, and egg yolk works thanks to lecithin. That is why mustard turns up in so many dressings: it is there for structure as well as flavour.

The method that holds

1. Start with the acid. Put your vinegar or lemon juice in a bowl or jar with a good pinch of salt. Let the salt dissolve: it will not dissolve once the oil is in.
2. Add the emulsifier. A spoonful of Dijon or wholegrain mustard is the classic. Whisk it into the vinegar until smooth.
3. Add the oil slowly. Pour it in a thin stream while whisking, or add it in two or three goes to a jar and shake hard each time. Adding it gradually makes smaller droplets, which separate more slowly.
4. Taste on a leaf. Dressing always tastes sharper from a spoon than on salad.

The traditional ratio, according to Wikipedia's vinaigrette entry, is about three parts oil to one part vinegar. We treat that as a starting point: a mellow vinegar can take less oil, sharp lemon a bit more.

Other helpers

Mustard is not the only friend a dressing has:

• Honey or a little maple syrup thickens the water side and helps slow separation, and it softens a sharp vinegar.
• Crushed garlic or finely chopped shallot add flavour and a little body.
• A spoonful of yoghurt, tahini or grated Parmesan makes a thicker, creamier dressing that holds well.
• A blender or stick blender makes the smallest droplets of all, and a dressing made that way can hold for a surprisingly long time.

Why it splits

• The oil went in all at once. Big droplets separate fast.
• No emulsifier. Oil, vinegar and salt alone will always split quickly, and that is fine if you shake it just before serving.
• It is cold. Olive oil can thicken and cloud in the fridge. Let the jar come back to room temperature and shake it again.
• Time. Even a good vinaigrette separates eventually. That is normal.

Do you need it not to split?

Honestly, not always. An Italian-style salad is often dressed straight in the bowl: salt, then vinegar, then olive oil, tossed with the leaves. Nothing is emulsified, and it is delicious. A stable vinaigrette matters most when you want an even coating, when you dress a salad in advance, or when you drizzle it over roasted vegetables or beans. For more on dressing leaves well, see how to make a salad that isn't sad.