Emulsions Explained: Why Oil and Water Sometimes Mix

Pour oil into vinegar and shake the jar. It looks mixed for a second. Then it separates. Every time. This is one of chemistry’s most reliable behaviors: nonpolar molecules (fats and oils) don’t mix with polar molecules (water).

But your kitchen is full of successful exceptions. Mayonnaise. Hollandaise. Milk. Butter. Even bread dough. All of these are emulsions: stable mixtures of oil and water that shouldn’t work but do.

The reason they work comes down to one type of molecule: the emulsifier.

What an Emulsion Actually Is

An emulsion is a mixture of two liquids that don’t naturally dissolve in each other, where one liquid is dispersed as tiny droplets throughout the other.

There are two basic types:

Oil-in-water (O/W): Tiny oil droplets suspended in a water-based liquid. Mayonnaise, hollandaise, and milk are all oil-in-water emulsions. You can thin them with water. They rinse off with water.

Water-in-oil (W/O): Tiny water droplets suspended in fat. Butter and margarine are water-in-oil emulsions. They feel greasy because the outer phase is fat. They don’t mix easily with water-based liquids.

The distinction matters for cooking. When you make a butter sauce, you’re creating an oil-in-water emulsion by melting butter (which is already a water-in-oil emulsion) into a liquid base. Get the ratio wrong, or add butter too fast, and you invert the emulsion or break it entirely.

How Emulsifiers Work

Here’s the key concept: emulsifiers are amphiphilic molecules. “Amphi” means “both” in Greek. These molecules have two distinct ends.

One end is hydrophilic (water-loving). It’s attracted to water molecules and wants to be near them.

The other end is hydrophobic (water-fearing, or fat-loving). It wants nothing to do with water and gravitates toward oil and fat molecules instead.

Think of an emulsifier molecule as a tiny diplomat with one foot in each world. When you have oil droplets suspended in water, emulsifier molecules line up at the interface between each droplet and the surrounding water. The hydrophobic tails point inward toward the oil. The hydrophilic heads point outward toward the water.

This creates a protective coat around every oil droplet. The droplets can’t merge back together because their coats repel each other. The emulsion stays stable.

The more complete that coating, the more stable the emulsion. This is why technique matters. If you add oil faster than the emulsifier can coat the new droplets, the emulsion breaks.

Natural Emulsifiers in Food

You don’t need lab chemicals to make an emulsion. Some of the best emulsifiers exist in everyday ingredients.

Lecithin in egg yolk: This is the workhorse of kitchen emulsification. Egg yolk contains lecithin, a phospholipid with a strong hydrophilic phosphate head and two hydrophobic fatty acid tails. One egg yolk contains enough lecithin to emulsify about a cup of oil into mayonnaise.

Casein and whey proteins in milk: These proteins have hydrophilic and hydrophobic regions. They’re why milk is a stable emulsion at room temperature and why it behaves so differently from cream when heated.

Mucilage compounds in mustard: Prepared mustard contains complex polysaccharides that act as emulsifiers. This is why a teaspoon of Dijon makes vinaigrette stay mixed longer than plain vinegar and oil. Mustard’s emulsifying power is modest compared to egg yolk, but it’s real and useful.

Mono- and diglycerides: These are modified fats with one or two (rather than the usual three) fatty acid chains attached to a glycerol backbone. The glycerol end is relatively water-soluble. They occur naturally in small amounts and are added to many processed foods. Soy lecithin works on the same principle and is one of the most common food additives in the world.

How Mayonnaise Works

Mayonnaise is the classic demonstration of emulsification in action. The ratio seems impossible: a typical mayo recipe takes two egg yolks and uses them to hold up to a cup of oil in suspension.

Start with egg yolks and a small amount of acid (lemon juice or vinegar). The acid helps the lecithin disperse more effectively and adds flavor. Then you add oil, slowly, drop by drop at first.

Each drop of oil gets surrounded by lecithin molecules before the next drop arrives. As you add more oil, the droplets get packed more densely, but each one stays coated. After enough oil is incorporated, the mayo thickens. The droplets are so numerous and tightly packed that they can’t move freely. The result is a thick, stable emulsion.

The slow addition at the start is critical. If you pour oil in too fast, you overwhelm the lecithin before it can coat the new droplets. The droplets merge. The emulsion breaks. You end up with a greasy, separated mess.

Once you’ve got enough oil incorporated (about a quarter of the total), you can add the rest more quickly. The existing emulsion structure helps incorporate new oil more efficiently.

Why Emulsions Break

An emulsion breaks when the oil droplets find each other and merge. Several things cause this:

Too little emulsifier: If you don’t have enough lecithin or other emulsifier relative to the amount of oil, you can’t coat all the droplets. They coalesce.

Temperature extremes: Cold temperatures can cause fats to crystallize, puncturing the emulsifier coating. This is why mayo breaks when you freeze it. High temperatures can denature proteins that are acting as emulsifiers. Hollandaise breaks above about 160°F (71°C) because the egg proteins start to coagulate rather than emulsify.

Mechanical disruption: Aggressive stirring after an emulsion forms can force droplets together. This is why you gently reheat hollandaise rather than whisking it over high heat.

Oil added too fast: The classic beginner mistake. More oil than the emulsifier can handle at one time.

Fixing a Broken Emulsion

For mayo or aioli that has broken: Whisk a fresh egg yolk in a clean bowl. Then very slowly whisk the broken sauce into that yolk, one teaspoon at a time at first. The fresh lecithin in the new yolk re-coats the oil droplets and pulls the emulsion back together. This works reliably if the sauce hasn’t been contaminated or overheated.

For a broken butter sauce (beurre blanc, hollandaise, béarnaise): Remove from heat immediately. Whisk vigorously. If that doesn’t work, start with a spoonful of cream in a clean pan and slowly whisk the broken sauce into it. The cream provides fresh protein and some water base to rebuild around.

Vinaigrette: A Temporary Emulsion

Standard vinaigrette (just oil and vinegar) forms a temporary emulsion. You shake it, the droplets disperse, and within minutes they separate again. This is fine for dressing a salad because you toss the salad immediately.

Add a teaspoon of Dijon mustard and the emulsion holds longer, sometimes for hours in the refrigerator. The mustard compounds extend the stability without changing the fundamental chemistry.

For a truly stable vinaigrette, blend it with a very small amount of egg yolk. Now you’ve got something more like a thin mayo. It’ll hold for days.

Butter Sauces: Emulsification in Disguise

Beurre blanc, beurre rouge, and pan sauces all depend on emulsification that most cooks don’t consciously think about.

Butter is about 80% fat, 18% water, and 2% milk solids. The milk proteins and phospholipids in the water phase act as emulsifiers. When you whisk cold butter into a hot reduction, those proteins emulsify the butterfat into the water-based sauce.

The keys: keep the temperature between 160-180°F (71-82°C). Too cool and the butter won’t melt properly. Too hot and the proteins denature and the sauce breaks, leaving you with greasy liquid. Add butter in small pieces, whisking constantly. Each piece adds more emulsifier along with more fat.

Finished beurre blanc is an oil-in-water emulsion with a satiny, glossy texture. It’s genuinely impressive chemistry, and it happens in a sauce pan in about 10 minutes.