What holds a vinaigrette together

An emulsion is tiny droplets of one liquid suspended in another that would not normally mix. In a vinaigrette, that means fine droplets of oil held inside the acid and water. Without help, those droplets find each other, merge into bigger drops, and rise back to the top.

An emulsifier is a molecule with one end that likes fat and one end that likes water. It sits at the edge of each oil droplet and keeps the droplets from merging, which is what lets a shaken dressing stay cloudy and combined for longer than plain oil and vinegar ever would.

Dijon mustard is the most common emulsifier in the catalog. It shows up in 454 recipes overall, 17 percent of the whole catalog, and its share climbs to 29 percent once you look at vinaigrettes specifically. Mustard carries its own sharp flavor too, so it earns its place twice: once for taste, once for structure.

Mustard is not the only option. Egg yolk appears in 281 recipes across the catalog and is a stronger emulsifier than mustard, which is why it shows up more in mayonnaise and aioli style sauces than in a quick dressing. Honey, garlic paste, and shallot all contribute some emulsifying power too, usually alongside mustard rather than instead of it. Across the catalog, 37 percent of vinaigrette recipes carry some emulsifying ingredient, whether that is mustard, egg yolk, or another binder.

Not every combined dressing is a true emulsion. Shake oil and vinegar together with nothing else and you get a temporary emulsion: it looks combined for a few seconds, then splits again. That is fine for a dressing you shake right before pouring.

Add a real emulsifier and the mix becomes far more stable. It still separates eventually, oil is heavier and less polar than the acid and water, but it holds together for minutes to hours instead of seconds, long enough to dress a salad, coat a piece of fish, or sit in a jar in the fridge between uses.

Even a well emulsified dressing needs a shake before you pour it. Refrigeration slows the droplets down and lets any given emulsion drift back toward its stable, separated state faster than it would at room temperature. A jar that has been sitting for a day is not broken, it is just resting.

A firm shake, capped and inverted a few times, is usually enough to bring a mustard based vinaigrette back to a uniform, pourable state. If it looks separated after shaking, the emulsifier ratio was probably too light to begin with.

A cold, thick vinaigrette straight from the fridge can also feel harder to bring back together than the same one at room temperature, since chilled oil is more viscous. Letting a jar sit out for a few minutes before shaking often makes the difference between a dressing that recombines easily and one that needs a longer shake.