Cooking science

Emulsion breakage

An emulsion is fat and water held together in a state neither of them wants to be in. Understanding why it is unstable explains every split mayonnaise, broken hollandaise and greasy cheese sauce, and it explains the fixes too.

Why fat and water separate

Water molecules attract each other strongly. Fat molecules do not participate in that attraction, so the water effectively squeezes them out, and the two settle into separate layers with the smallest possible surface between them. That is the low-energy state and it is where a mixture will go if left alone.

An emulsion works against that. Fat is broken into droplets small enough that they stay dispersed rather than rising and merging — which means creating an enormous amount of surface area between the two, exactly what the system was trying to avoid. It takes energy to make and it requires something to hold it there.

That something is an emulsifier: a molecule with a water-loving end and a fat-loving end, which sits at the boundary of each droplet and stops it merging with its neighbours. Lecithin in egg yolk is the classic; milk proteins, mustard, starch and gelatine all do versions of it.

The three things that break one

Heat

Droplets move faster and collide more, and above a certain point the emulsifier itself can be cooked — egg yolk sets, milk protein clumps. This is why hollandaise is made over gentle heat and why a cheese sauce splits when boiled.

Too much fat, too fast

The emulsifier can only coat so much surface. Adding oil faster than it can be broken into coated droplets gives you free fat, which merges with the next lot. Hence the slow start to a mayonnaise.

Not enough movement

Whisking is what breaks the fat into droplets and keeps them apart while the emulsifier does its work. Stop, and droplets sit together long enough to join.

A fourth, less obvious one — temperature shock

Cold fat into a warm emulsion, or cold cream into a hot pan, destabilises it even without heat being the issue.

Why rebuilding works

Once droplets have merged into visible pools, whisking the split sauce harder does not help — you are just moving two separated phases around.

The rescue works because you start a fresh, stable emulsion in a clean bowl, using a small amount of emulsifier and liquid, and then feed the broken sauce into it very slowly. Each small addition is broken into droplets and coated before the next arrives, exactly as if you were making the sauce from scratch — which, in effect, you are.

That is why every rescue on every sauce follows the same shape, and why going too fast makes it split again immediately.

The emulsifiers worth knowing

• Egg yolk — lecithin. The strongest common one. Mayonnaise, hollandaise, aioli, custard.
• Mustard — mucilage and lecithin. A teaspoon holds a vinaigrette together for hours.
• Milk and cream proteins — why cream sauces are more forgiving than butter ones, and why a cream sauce can be reduced hard.
• Starch — pasta water, a little cornflour, or the flour in a roux. Physically gets between droplets.
• Gelatine — why a well-made stock-based sauce is glossy and stable.
• Honey — mildly emulsifying, which is part of why it works in a dressing.

Common questions