Mayonnaise Is Mostly Oil, and One Egg Yolk Keeps It From Falling Apart

Pour oil and vinegar into a jar, screw on the lid and shake it hard. For a moment you get a cloudy, golden dressing. Leave it on the kitchen table while you slice the tomatoes, and by the time you come back the jar has sorted itself into two neat layers, oil on top and vinegar underneath, as if nothing had happened. Yet a jar of mayonnaise, made from almost the same oil and the same vinegar, sits in the fridge for weeks and stays thick, pale and smooth.

The difference is a spoonful of egg yolk, and the way the oil goes in. Mayonnaise is what chemists call an emulsion: one liquid broken into tiny droplets and spread through another liquid it would normally refuse to mix with. In mayonnaise the oil is the droplets, and the watery part, vinegar or lemon juice plus the water in the yolk, is the liquid around them.

What people often get wrong

Most people think of mayonnaise as an egg sauce with a little oil in it. It is the other way round. In the United States, a food sold as mayonnaise must by law contain at least 65 per cent vegetable oil by weight, and a typical full-fat mayonnaise is closer to 80 per cent oil. A student thesis at Lund University in Sweden puts it plainly: the main ingredient is oil. The egg is a small helper, and the surprise is that so much oil can hide inside so little water.

That is also why mayonnaise is thick, although none of its ingredients is thick on its own. The oil droplets are packed tightly together in that small amount of water, and a crowd that dense does not flow easily, which is why mayonnaise holds its shape on a knife. The same thesis notes that more oil gives a firmer texture.

• 65% the least oil, by weight, in a food sold as mayonnaise in the US
• ≈ 80% oil in a typical full-fat mayonnaise
• pH 3.5–4.5 how acidic mayonnaise usually is

How the yolk keeps the peace

Left alone, oil droplets bump into each other and merge, which is exactly what happens in the shaken dressing. Egg yolk stops this. It contains lecithin, a molecule with one end that likes water and one end that likes fat, as researchers at the University of Lodz in Poland explain. The fat-loving end sinks into the oil and the water-loving end stays outside, so every droplet ends up wearing a thin coat. Proteins from the yolk join that coat too. Two coated droplets that touch simply do not join.

The vinegar or lemon juice helps as well. Mayonnaise is acidic, usually somewhere between pH 3.5 and 4.5, and the yolk proteins hold the droplet surfaces better in that mildly sour setting. The acid also slows the growth of many microbes, which is part of the reason the jar keeps.

Why mayonnaise sometimes breaks

Anyone who has made mayonnaise by hand knows the sad moment when it suddenly turns thin and greasy. Food scientists call this phase inversion. If the oil is poured in too fast, or the mixing goes on far too long, the emulsion flips: the oil stops being the droplets and becomes the liquid around them, and the sauce slumps. That is why recipes ask you to add the oil almost drop by drop at first. With a stick blender, the Lodz researchers suggest keeping the blade at the bottom of a tall jar for the first few seconds, so that a small, stable emulsion forms there and slowly takes in the oil above it.

So the next time a vinaigrette separates on the table, do not call it lazy. It simply has nobody holding the droplets apart. Give it a yolk and a little patience, and the same oil and the same vinegar will stand up on a spoon.