What Is an Emulsion? Getting Oil and Water to Stay Mixed

Water molecules attract each other strongly through hydrogen bonding, and oil molecules do not participate in that network. Forcing oil into water requires breaking water to water attractions to make room, which costs energy, and the system reduces that cost by minimising the contact area between the two, which means the oil gathers into the fewest, largest droplets possible and eventually a single layer. Shaking a dressing supplies energy that breaks the oil into small droplets and increases the contact area enormously, and the system immediately begins reversing that by having droplets collide and merge. The separation is therefore not a failure of mixing but the system returning to a lower-energy state, and any stable emulsion requires something that makes the mixed arrangement stable rather than merely temporary. That something is almost always a molecule that can sit at the boundary.

An emulsifier has one part attracted to water and one attracted to oil, and that dual nature does the work:

• •It sits at the droplet surface with each end in the phase it prefers, which lowers the energy cost of the boundary
• •It forms a physical layer around each droplet that resists merging on contact
• •It frequently carries an electric charge, so droplets repel each other and collide less
• •Lecithin from egg yolk is the classic culinary example and is why yolks emulsify so effectively
• •Proteins in milk, mustard and cream perform the same role, which is why mustard is in most vinaigrette recipes
• •Thickening the water phase slows droplets down and buys stability without any emulsifier, which is why starch and gums appear in commercial dressings

Most classic sauces are emulsions and the differences between them are mostly about which phase is continuous and what stabilises it. Mayonnaise is oil dispersed in a small amount of water from the yolk and acid, stabilised by lecithin and yolk proteins, and it holds an extraordinary proportion of oil, which is why it is thick. Hollandaise is similar with butter and the added complication of heat, which can denature the proteins and break the sauce. Vinaigrette is a temporary emulsion that separates within minutes unless mustard or another stabiliser is added. Butter is the reverse arrangement, with water droplets dispersed in fat, which is why it behaves so differently from cream, itself fat dispersed in water, and why churning cream is a phase inversion rather than a simple separation. Sauces finished with butter, and cream sauces generally, depend on the same physics and break for the same reasons.

How finely the dispersed phase is broken up decides much of how an emulsion behaves. Smaller droplets are more stable, since they rise or settle more slowly and present a curved surface that resists merging, and they also scatter light differently, which is why a finely worked emulsion looks opaque and white while a coarse one looks translucent. Texture follows too, since a sauce with very small droplets feels smoother and thicker at the same fat content, which is why mayonnaise made in a blender differs from one made by hand. Industrial production exploits this with homogenisers that force the mixture through narrow openings at high pressure, which is exactly what is done to milk to stop the cream separating and why homogenised milk never forms a layer. In a kitchen the equivalent tools are a whisk, a blender and patience, and the slow addition of oil at the start exists precisely to keep the droplets small while there is enough emulsifier to coat them.

Emulsions fail in characteristic ways and each has a corresponding repair. Adding oil too quickly at the start overwhelms the available emulsifier, leaving oil with nothing to coat it, and the fix is to start again with fresh yolk and add the broken mixture slowly as though it were oil. Heat breaks protein-stabilised sauces by denaturing the proteins, which is why hollandaise is made over gentle heat and why an overheated sauce curdles. Cold breaks some emulsions by crystallising the fat, which is why mayonnaise can separate in a very cold fridge. Excess water thins the continuous phase until droplets move freely and merge. Mechanical disruption after the fact rarely helps on its own, since whisking a broken sauce simply redistributes droplets that will merge again. The general principle for repair is to rebuild the emulsion from a stable base rather than to attack the broken one.

Oil gathers into a layer because minimising contact with water costs less energy, so separation is the system relaxing rather than a mixing failure. An emulsifier has a water-loving end and an oil-loving end, sits at each droplet surface and keeps droplets from merging. Butter is water in fat and cream is fat in water, which is why churning is an inversion. Broken sauces are rebuilt from a fresh base, not whisked harder.