How Do Emulsions Work? Mayo, Vinaigrette and Beyond

An emulsion works by smashing one liquid into tiny droplets and suspending them in another liquid it would normally refuse to mix with, like oil in vinegar or lemon juice. On their own those droplets quickly find each other and merge back into a separate layer, so stable emulsions need help: an emulsifier coats each droplet so the droplets can't fuse back together. That one trick explains mayonnaise, vinaigrette, hollandaise, butter sauces and even the milk in your fridge.

What actually is an emulsion?

An emulsion is two immiscible liquids pretending to be one. One liquid becomes the dispersed phase, broken into droplets, and the other becomes the continuous phase, the liquid the droplets float in.

Kitchen emulsions come in two flavours:

• Oil-in-water: oil droplets floating in a watery liquid. Mayonnaise, vinaigrette, hollandaise, cream and milk all work this way.
• Water-in-oil: water droplets trapped in fat. Butter is the famous one, about 80% fat with roughly 16% water held in tiny pockets.

Which liquid is continuous matters more than the ratio. Mayonnaise can be close to 80% oil and still behave like a water-based sauce, because the oil is chopped into droplets and the small amount of lemon juice, vinegar and yolk water forms the connected network around them. That's why mayo spreads like a cream instead of pouring like oil.

The droplets are tiny. In a good mayonnaise most are just a few micrometres across, too small to see, which is why the sauce looks smooth and opaque. Light bounces off billions of droplet surfaces, and that scattering is what turns clear oil and clear vinegar into something pale and creamy.

Why don't oil and water mix on their own?

Water molecules are polar. Each has a slightly negative end and a slightly positive end, and they cling to each other through hydrogen bonds. Oil molecules are long, non-polar chains of carbon and hydrogen with nothing to offer that network. When you shake them together, water "prefers" to stick to water, and oil gets pushed into its own pockets.

Every droplet you create adds a huge amount of new boundary between oil and water, and that boundary costs energy (chemists call it interfacial tension). The system lowers its energy by shrinking that boundary, and the easiest way to do that is for droplets to bump, merge and grow. This merging is called coalescence. Meanwhile oil is lighter than water, so droplets also rise to the top, a process called creaming. Leave an unprotected oil-and-vinegar mixture for a few minutes and you'll watch both happen: big shiny blobs, then a clean layer of oil on top.

So making an emulsion takes two things. First, energy: whisking, shaking or blending to break the oil into small droplets. Second, protection: something that stops those droplets from merging the moment you stop.

What does an emulsifier do?

An emulsifier is a molecule with a split personality. One end is water-loving (hydrophilic), the other end is oil-loving (hydrophobic). It parks itself at the boundary, oily tail in the droplet and watery head facing out, so each droplet ends up wearing a protective coat. Coated droplets slide past each other instead of fusing, and many emulsifiers also give droplets a slight electrical charge, so they actively repel one another.

Egg yolk: the overachiever

Yolk is about one-third fat and is loaded with phospholipids, especially lecithin (phosphatidylcholine), plus lipoproteins that do much of the heavy lifting in mayonnaise. One yolk carries far more emulsifier than a typical recipe needs. The real limit is usually water, not emulsifier: if you keep adding oil without adding liquid, the droplets get packed so tightly there's nowhere left for them to sit, and the sauce turns stiff, greasy and eventually breaks.

Mustard, garlic and honey: the helpers

Mustard seed coats contain mucilage, a gummy mix of carbohydrates, plus proteins that cling to oil droplets. A teaspoon of Dijon won't make a vinaigrette permanent, but it can hold it together through a whole meal instead of a few minutes. Crushed garlic and honey help a little too, mostly by thickening the watery phase so droplets move more slowly and collide less often.

Proteins, gelatin and starch: the quiet stabilisers

Milk proteins (casein and whey), gelatin from a good stock and starch from pasta water don't have the neat two-ended shape of lecithin, but they coat droplets and thicken the liquid around them. That's how a splash of starchy pasta water turns butter and cheese into a glossy sauce instead of an oil slick.

Chef Fizz says: think of an emulsifier as a bouncer standing between every oil droplet. No bouncer, and the droplets crash the party together.

Why does mayonnaise turn thick while vinaigrette stays runny?

Same ingredients in spirit (oil, acid, something to help), totally different textures. The difference is how much oil is crammed in and how well it's protected.

A classic vinaigrette is about 3 parts oil to 1 part vinegar. The droplets are fairly large (shaking a jar doesn't make them very small) and there's plenty of room between them, so the dressing pours easily. With little or no emulsifier, it's a temporary emulsion that separates as soon as you stop shaking.

Mayonnaise pushes the oil fraction to around 75–80%. Spheres of equal size can only fill about 74% of a space before they touch, so in a properly made mayo the oil droplets are squeezed together so tightly that they deform against one another, like a crowd of balloons pressed into a box. That crowding is what makes mayo stand up on a spoon. Add more oil and it gets thicker still; add a teaspoon of water or lemon juice and it loosens, because you've given the droplets more room.

This is also why the first few tablespoons of oil in mayo must go in slowly. At the start there are no droplets yet, only a small puddle of yolk and acid. Pour in a big glug and the whisk can't break it up fast enough, so the oil stays as one continuous mass. Once a stable base forms, the existing droplets help shear the new oil, and you can pour faster.

What's the best ratio for a vinaigrette?

The classic French starting point is 3:1 oil to acid, but that's a preference, not a law. Sharper vinegars and lemon juice often taste better at 3:1 or even 4:1; milder rice vinegar or a sweet balsamic can handle 2:1.

A few practical notes:

• Dissolve salt in the vinegar first. Salt dissolves in water, not oil, so adding it to finished dressing leaves crunchy grains.
• Add the oil to the acid, not the other way round, whisking as you pour. You're building oil droplets in a watery base.
• A blender makes smaller droplets than a whisk, and smaller droplets separate more slowly. A blended vinaigrette can look almost creamy.
• Cold makes extra-virgin olive oil cloudy and thick in the fridge. That's fat crystallising, not spoilage. Let it warm up and shake again.

Want to watch droplets merge and split in real time? The Cooking Scientist app has a quick lesson on emulsions and vinaigrette ratios, with a mini game where you play the emulsifier.

Which everyday foods are secretly emulsions?

Once you know what to look for, emulsions are everywhere:

Hot emulsions add a twist: temperature. Butter sauces fall apart if they get too hot or too cold, and egg-based ones scramble if overheated. That's a big enough topic for its own guide on why sauces split and how to rescue them. Chocolate has its own quirks too, which we cover in why chocolate seizes.

Try it at home

Run a three-jar race to see an emulsifier at work.

1. Grab three small clean jars with lids. Into each, pour 45 mL (3 tbsp) neutral oil and 15 mL (1 tbsp) vinegar.
2. Leave jar one plain. Add ½ teaspoon Dijon mustard to jar two. Add ½ teaspoon mustard and ½ teaspoon honey to jar three.
3. Put the lids on and shake each one hard for 15 seconds, then set them side by side and start a timer.

What you'll see: Jar one turns cloudy for a moment, then clear oil starts rising within a minute or two. Jar two stays cloudy and pale far longer, and when it does separate, the line is fuzzy rather than sharp. Jar three usually lasts longest of all.

Why: Mustard's mucilage and proteins coat the oil droplets and slow coalescence. Honey thickens the vinegar, so droplets drift more slowly and bump into each other less. Look closely at jar one from the side and you can actually spot large droplets merging, the exact process an emulsifier is there to stop.

Common mistakes

• Adding oil too fast at the start of mayonnaise. Without an established base of droplets, a big pour stays as one oily mass and the sauce never thickens. Add the first 60 mL (¼ cup) drop by drop, then speed up once it's thick and pale.
• Using ingredients straight from the fridge. Cold yolks and very cold oil are harder to break into droplets and emulsify sluggishly. Let eggs sit out for 20–30 minutes, or warm them briefly in a bowl of warm tap water.
• Running out of water. If your mayo turns stiff, shiny and greasy, the droplets have no room left. Whisk in a teaspoon of water or lemon juice before adding more oil.
• Expecting a plain vinaigrette to stay mixed. Oil and vinegar alone always separate. Add mustard, blend it, or just shake it again right before serving.
• Salting the oil instead of the acid. Salt won't dissolve in oil, so you get gritty bits and uneven seasoning. Dissolve salt in the vinegar or lemon juice first.
• Serving raw-yolk sauces to vulnerable people. Homemade mayo and aioli use uncooked yolk. For anyone pregnant, very young, elderly or immunocompromised, use pasteurised eggs.

Master the droplet, and a dozen sauces stop being mysterious.