How an emulsion works
Oil and water repel each other because their molecules have different polarities. An emulsifier molecule has a water-loving end (hydrophilic) and a fat-loving end (hydrophobic). It sits at the interface between the two liquids, coating tiny droplets of one inside the other and preventing them from coalescing. The result is a stable, homogenous mixture. According to ScienceDirect, the stability of a food emulsion depends on the type and concentration of emulsifier, the ratio of water to oil, and how much mechanical energy (stirring, whisking) was used.
Kitchen emulsions
- Mayonnaise — oil dispersed in water, emulsified by egg yolk lecithin. Up to 80% oil by volume.
- Hollandaise / béarnaise — clarified butter emulsified by egg yolk into an aqueous base.
- Vinaigrette — temporary emulsion of oil and vinegar, stabilised (partially) by mustard.
- Beurre blanc / beurre rouge — whole butter whisked into a reduction; the butter solids act as the emulsifier.
- Pasta sauce — starchy pasta water helps emulsify olive oil or butter into a coating sauce.
Temporary vs stable emulsions
A temporary emulsion (basic vinaigrette) separates within minutes — no emulsifier holds it together beyond the initial mixing. A stable emulsion (mayonnaise, hollandaise) stays combined for hours or days because the emulsifier properly coats each oil droplet. Adding mustard to a vinaigrette creates a semi-stable emulsion that holds for 15–30 minutes.
Why emulsions break
An emulsion breaks when the emulsifier is overwhelmed or conditions change: too much oil added too fast, too high a temperature (hollandaise scrambles above 65 °C), or not enough water phase left to hold the droplets apart. Serious Eats advises adding oil drop by drop at first when making mayonnaise, then in a thin stream — this gives the lecithin time to coat each new droplet before the next arrives.