The Vinaigrette

Measure 45 ml oil to 15 ml vinegar into a clear jar with salt. Shake hard for 20 seconds and immediately start a timer.

Record the time until a clean layer of oil reappears on top.

Expect well under a minute.

This is your baseline, and every later result is meaningless without it.

Repeat exactly, but whisk a small amount of egg yolk into the vinegar before adding the oil.

Shake identically and time the separation.

Expect it to last many times longer.

Egg yolk carries lecithin, whose two ends face opposite ways at the droplet surface. You changed nothing about the shaking — the persistence came entirely from a molecule that likes both liquids.

Make two more, both with emulsifier. In one, pour all the oil in at once and shake. In the other, whisk constantly and add the oil in a slow thin stream.

Time both, and look at them against the light.

Expect the slowly-added one to be paler, thicker and longer-lived.

Pale means small droplets — more, smaller droplets scatter more light and rise more slowly.

Make 4:1, 3:1 and 2:1 versions and taste each on a leaf, not from a spoon.

Expect 3:1 to sit closest to balanced with an ordinary 6 % vinegar, 4:1 to read as flat and oily, 2:1 as harsh.

Then remake the best one using lemon juice instead.

It will need more oil, because you changed both the acid strength and its character. The ratio is a response to the acid, not a law.

Take a well-made emulsion and break it deliberately — add oil far too fast, or chill it hard.

Then rebuild: start a fresh spoonful of vinegar and emulsifier in a clean bowl and whisk the broken mixture into that, slowly.

Expect it to come back.

You cannot fix a broken emulsion by adding more of anything to it — you have to start a new continuous phase and feed the broken one in as the dispersed phase.

What an emulsion actually is. Two liquids that do not mix, one broken into droplets suspended in the other. Which is which matters: a vinaigrette and mayonnaise are both oil-in-water — oil droplets dispersed through the vinegar — even though mayonnaise is mostly oil by mass. Butter is the reverse, water-in-oil. The continuous phase is what your tongue meets first, which is why mayonnaise tastes less greasy than its oil content suggests.

Why emulsions fail, in three distinct ways. Creaming is droplets rising because oil is less dense — reversible with a shake. Flocculation is droplets clustering while staying separate — also reversible. Coalescence is droplets actually merging, and that is irreversible; it is why a broken dressing has to be rebuilt rather than re-shaken. An emulsifier's real job is to prevent the third.

⚠ The common explanation that mustard "works because of the oil in mustard seed" is wrong. Dijon is the classic vinaigrette emulsifier, and it works chiefly because mustard seed contains mucilage — polysaccharides that both sit at the oil-water interface and thicken the continuous phase, slowing droplets down. Honey and garlic paste do something similar. Thickening the water phase is half of what a good emulsifier does, and it is the half most explanations omit.

Small droplets are the other half. Stokes' law says a droplet's rise speed scales with the square of its radius, so halving droplet size slows separation by roughly a factor of four. That is why the slow stream in step 3 works, why a blender makes a more stable dressing than a jar, and why commercial dressings are homogenised at high pressure.

Practical notes. Cold oil emulsifies poorly, so bring everything to room temperature. Salt goes in the vinegar first, where it dissolves. Extra-virgin olive oil can turn bitter under high-shear blending — its polyphenols are released by damaged droplets — so whisk rather than blitz it. And a dressing made with raw egg carries the ordinary raw-egg advice: keep it refrigerated, use it the same day, and use pasteurised egg for anyone who should avoid raw egg.