Emulsions

An emulsion is a stable mixture of two liquids that normally refuse to combine — almost always oil and water. Emulsions are everywhere in cooking: milk, cream, butter, mayonnaise, hollandaise, vinaigrettes, and most pan sauces.

How emulsions work

Every emulsion has two phases:

• Continuous phase — the liquid that forms the background. In cream and mayonnaise, this is water. In butter, it’s fat.
• Dispersed phase — tiny droplets (0.1–10 micrometers) suspended within the continuous phase.

Left alone, oil and water separate because oil droplets coalesce — they merge into larger and larger pools until the two liquids are fully separated. Emulsions prevent this through emulsifiers: molecules that are amphipathic (one end loves water, the other loves fat). They arrange themselves at the oil-water interface, coating each droplet in a protective shell that prevents coalescence.