Classic vinaigrette runs three parts oil to one part vinegar, and that number is about taste, not structure. Oil coats the tongue and mutes vinegar's sharpness, and three-to-one is roughly where a straight mouthful of acid turns into something a fork can carry. The two liquids never dissolve into each other, no matter what ratio you use. Whisking just scatters the oil into droplets suspended in the vinegar, and within minutes those droplets find each other again and the dressing splits back into two layers.
Why three to one, and not some other number
Vinegar is mostly water carrying around 5% acetic acid, which is sharp enough that a mouthful of it straight is unpleasant. Oil has almost no flavor of its own, so it doesn't compete with the acid — it dilutes it and coats the tongue in a thin fat layer that blunts how aggressively the acid registers. Three parts oil is close to the point where that blunting balances the sharpness without erasing it entirely.
It's a starting point, not a law. A sherry or malt vinegar with more bite than a mild rice vinegar wants more oil to match it, and a peppery, assertive extra-virgin olive oil can carry a vinaigrette at a lower ratio than a neutral one because the oil itself is contributing flavor instead of just muting the acid.
Why it separates in minutes when mayonnaise doesn't
Oil and vinegar are immiscible — whisking doesn't dissolve one into the other, it just tears the oil into droplets small enough to stay suspended for a while. Nothing in a plain vinaigrette coats those droplets, so the moment you stop whisking, they start colliding and merging into bigger and bigger drops. That's coalescence, and it's driven by the same thing that pulls a puddle into a single blob: liquid minimizes its own surface area whenever it can. Once the droplets are large enough, the density difference between oil and vinegar takes over and the oil floats to the top.
Mayonnaise skips that step because egg yolk is full of lecithin and proteins that adsorb onto every oil droplet and form a tough, elastic film around it. That film is a physical barrier — droplets can bump into each other all day and the film keeps them from merging. Mayonnaise also starts from oil added in a thin stream while whisking continuously, which breaks it into droplets small enough for the yolk's lecithin to fully coat. A vinaigrette shaken in a jar has none of that: no real emulsifier and much bigger, less uniform droplets.
What mustard is actually doing
Mustard's seed coat contains mucilage, a soluble fiber that dissolves into the liquid and thickens it slightly, plus a small amount of other surface-active compounds. Stirred into a vinaigrette, that mucilage coats a share of the oil droplets and gets in the way of them recombining — not a strong film like lecithin, just enough drag to slow the collisions down. A mustard vinaigrette still separates. It just takes thirty minutes to an hour instead of one.
| Emulsifier | Droplet size | Stays combined | |
|---|---|---|---|
| Vinaigrette, shaken | None | Large, uneven | Under a minute |
| Vinaigrette with mustard | Mucilage | Smaller | 30–60 minutes |
| Mayonnaise | Egg yolk lecithin | Very small, uniform | Indefinitely |
Fixing one that's already separated
A split vinaigrette isn't broken the way a split mayonnaise is. Nothing failed chemically — the droplets just merged back into two layers because nothing was holding them apart. Shake or whisk it again and it re-emulsifies exactly as well as it did the first time, as many times as you want. Mayonnaise can't do that. Once its protein film tears, the fix is a fresh yolk whisked in to build a new one, not more whisking of the same batch.