How do I emulsify a sauce or dressing?

Oil and water do not mix; an emulsion suspends tiny droplets of one in the other. That suspension is fragile, and it needs two things: droplets small enough to stay put, and an emulsifier that stops them recombining.

What you'll need

• Whisk or blender
• Oil
• Acid — vinegar or lemon
• Mustard, egg yolk or honey

Step by step

1. Start with the water-based ingredients.Vinegar or lemon juice, seasoning, and the emulsifier. Oil goes into this, never the reverse.
2. Include an emulsifier.Mustard is the easiest and works in almost any dressing. Egg yolk for mayonnaise and hollandaise, honey for sweeter dressings.
3. Add oil drop by drop at first.Genuinely slowly at the start, whisking constantly. The initial emulsion is the fragile part; once it forms, you can speed up.
4. Whisk hard and continuously.The mechanical energy is what breaks the oil into droplets small enough to stay suspended.
5. Match temperatures for warm emulsions.For hollandaise or beurre blanc, keep everything warm but not hot. Heat above about 80°C will scramble egg yolk.
6. Use a blender for a shortcut.A stick blender in a narrow jug makes mayonnaise almost foolproof — it does the droplet work for you.
7. If it splits, rebuild it.Start with a fresh spoonful of the water-based part in a clean bowl and whisk the split mixture into it slowly, as though it were the oil.

Tips

• A dressing in a jar with a lid, shaken hard, emulsifies well enough for a salad and takes ten seconds.
• More emulsifier makes a more stable sauce. If yours keeps splitting, add another half teaspoon of mustard.
• Room-temperature ingredients emulsify more readily than fridge-cold ones, particularly egg yolks.

Common mistakes

• Adding the oil too fast — The droplets are too large to stay suspended and the mixture separates immediately.
• Leaving out the emulsifier — Whisking alone gives a temporary emulsion that separates within minutes.

Questions people ask