How does an egg yolk hold oil and water together?

Oil and water do not mix because merging into one large blob is the lowest-energy state available to the oil. An emulsifier changes that arithmetic by putting a coat around every droplet. Understanding this one mechanism explains mayonnaise, hollandaise, custard-based ice cream, and every sauce that has ever broken on you.

Step by step

1. Picture what an emulsion physically is.Millions of tiny oil droplets suspended in a small amount of water — the water from the yolk, plus any lemon juice or vinegar. The mixture looks thick and creamy because light scatters off all those droplet surfaces, not because anything has thickened in the ordinary sense.
2. Understand the emulsifier's two ends.Lecithin is a phospholipid: one end is attracted to water, the other to fat. Thousands of them arrange themselves around each oil droplet with their fat-loving ends inward and their water-loving ends outward. The droplet is now wearing a water-compatible jacket and cannot merge with its neighbours.
3. See why droplet size decides thickness.Smaller droplets mean more of them, more surface area and more crowding. A tightly packed crowd of tiny droplets cannot flow past one another, which is why mayonnaise stands up in a spoon. Whisking harder makes the droplets smaller and the sauce thicker with no extra ingredients.
4. Add the oil slowly at the start and faster later.At the beginning there is very little water phase and the droplets are easily forced together. Drop by drop until it takes, then a thin stream. Once the emulsion is established, it will accept oil surprisingly fast.
5. Know the real capacity of a yolk.A single yolk carries enough emulsifier for well over a litre of oil in principle. In practice you run out of water phase long before you run out of lecithin, which is why a mayonnaise that gets stiff and greasy is fixed by a teaspoon of water or lemon juice, not by another yolk.
6. Recognise the three ways it breaks.Oil added too fast, so droplets never got coated and pooled. Not enough water phase, so the droplets are jammed together with nothing between them. Or heat, in a hot emulsion like hollandaise, which coagulates the yolk protein and destroys the structure — the only one of the three that cannot be rescued by whisking.
7. Rebuild rather than repair.A split cold emulsion is not stirred back together. Start again with a fresh yolk or a teaspoon of water in a clean bowl, and whisk the broken mixture into it drop by drop as though it were oil. You are building a new emulsion out of the wreckage of the old one.

Tips

• Mustard is a second emulsifier, not just a flavour. A teaspoon in a mayonnaise makes it substantially more stable and more forgiving of fast pouring.
• Everything at the same temperature. Cold oil into a room-temperature yolk is one of the most common causes of a mayonnaise that will not take.
• A stick blender emulsifies far more aggressively than a whisk and makes mayonnaise nearly foolproof — all the oil in at once, blender at the bottom, and lift it slowly as it takes.
• Egg-free emulsions work on the same principle with different emulsifiers: mustard, garlic, aquafaba proteins and soy lecithin all do the same job less powerfully.
• Too thick is a water problem. Too thin is an oil or agitation problem. Diagnosing which way it is wrong tells you which to add.

Common mistakes

• Adding another yolk to a split mayonnaise in the bowl — You are adding emulsifier to a mixture that has already separated. Nothing brings the coalesced oil back apart. Start a new base and whisk the broken sauce into it.
• Blaming the yolk when the sauce is stiff and greasy — That is not enough water phase, not too little emulsifier. A teaspoon of water, vinegar or lemon juice loosens it immediately.
• Overheating a warm emulsion to thicken it — Hollandaise thickens by droplet crowding, not by cooking. Push it past about 70°C and the yolk protein sets into scrambled specks, which is unrecoverable.

Questions people ask