Emulsion
An emulsion is a mixture of two liquids that normally refuse to mix, such as oil and water, where one is broken into tiny droplets and spread evenly through the other. Mayonnaise, vinaigrette, hollandaise, milk and butter are all emulsions.
How it works
Oil and water separate because water molecules attract each other much more strongly than they attract oil. Whisking or blending smashes one liquid into droplets, but left alone those droplets bump into each other, merge and float apart.
Every emulsion has two parts:
- Dispersed phase: the droplets (oil in mayonnaise).
- Continuous phase: the liquid surrounding them (the water from yolk, lemon juice and vinegar in mayonnaise).
Two main types
- Oil-in-water: oil droplets in water. Mayonnaise, hollandaise, vinaigrette, milk and cream.
- Water-in-oil: water droplets in fat. Butter and margarine.
What keeps an emulsion together
- An emulsifier. Molecules like lecithin in egg yolk have one end that loves water and one that loves fat. They coat each droplet and stop them fusing.
- Small droplets. Vigorous whisking or a blender makes droplets so tiny they stay suspended longer.
- Thickness. A viscous continuous phase slows droplet movement. Mustard, garlic paste and starch all help.
- The right temperature. Warm butter sauces like beurre blanc hold best around 50–60 °C / 120–140 °F. Too hot and they split; too cold and the butterfat solidifies.
Temporary vs permanent
A shaken vinaigrette is a temporary emulsion that separates within minutes. Mayonnaise is a permanent emulsion that can hold for days in the fridge because yolk emulsifiers and a very high oil ratio (about 75–80% oil) pack the droplets tightly.
Why emulsions break
- Adding oil faster than it can be broken into droplets.
- Too much oil for the amount of water and emulsifier.
- Overheating, which can cook egg proteins or melt the structure.
- Chilling a butter sauce until the fat sets.
How to rescue one
Put a teaspoon of warm water, mustard or a fresh yolk in a clean bowl. Whisk the broken sauce in a few drops at a time. Once it thickens, add the rest in a thin stream.
Questions
What are examples of emulsions in food?
Mayonnaise, hollandaise, vinaigrette, milk, cream, butter, aioli and beurre blanc are all emulsions. Butter is the reverse type, water droplets held in fat.
Why did my emulsion break?
Usually the fat was added too fast, the sauce got too hot or too cold, or there was too much oil for the water and emulsifier available. The droplets merge and the fat separates.
How do you fix a broken emulsion?
Start fresh with a small amount of the water phase (a teaspoon of water, mustard or a new egg yolk) and whisk the broken sauce into it drop by drop until it thickens again.
What makes an emulsion stable?
An emulsifier coating each droplet, small droplet size from vigorous mixing, and a thick continuous phase that slows the droplets from moving and merging.