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Kitchen science · Heat and transformation

Emulsions: mayonnaise, tahini and making peace between oil and water

Oil and water don't mix, yet mayonnaise is a thick cream that is mostly oil, and tahini sauce is perfectly smooth. Learn how emulsifiers keep oil in tiny droplets, why mayonnaise splits, why tahini thickens before it loosens, and how to stay safe with raw eggs.

  • 17 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

Fill a jar half with water and half with sunflower oil, screw on the lid and shake it hard. The contents turn cloudy, but within a few minutes the oil has gathered at the top again. Mayonnaise, on the other hand, is mostly oil, yet it stays thick enough to stand up on a spoon, and it keeps for days in the fridge without separating.

How do a few spoonfuls of egg yolk "tame" so much oil? And why does tahini, when you add a little water, first seize up as hard as stone and then turn into a silky sauce?

Try it yourself

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Matching

Phases and emulsifiers

Choose a role for each component (radio buttons; the arrow keys on the keyboard work too). When you've matched them all, press Check.

Prediction

Predict first

Most of mayonnaise is oil. Which liquid do you think is spread through the other as droplets in mayonnaise?

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Number line

How much of mayonnaise is oil?

Drag the marker or move it in steps of 1 with the arrow keys, then press Check.

Prediction

Predict first

Which do you think the share of oil in this mayonnaise is closest to?

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Frequently asked

5 questions

Is mayonnaise mostly egg?

No, it is mostly oil: roughly 65–80% of the total mass in most recipes. Egg yolk is a small part. Its job is to coat the oil droplets with its lecithin and lipoproteins and keep them suspended in the water.

If oil is the majority, is the water spread through the oil in mayonnaise?

No. What matters is not which liquid there is more of, but which one surrounds the droplets; in mayonnaise the continuous phase is water. Stir a teaspoon of mayonnaise into a glass of water and it disperses. Butter does not, because its continuous phase is fat.

Why does mayonnaise split, and how can I rescue it?

The usual causes are adding the oil too fast or adding too much for the water phase, and ingredients that are too cold or too warm. Put a teaspoon of mustard and a teaspoon of water in a clean bowl and whisk the split mixture in drop by drop; the emulsion builds up again from scratch.

Does lemon juice make raw egg safe?

No. The acid changes the taste and texture but does not remove the risk of Salmonella. Use pasteurised eggs for homemade mayonnaise, keep it in the fridge and eat it soon. Raw-egg mayonnaise is not recommended for young children, pregnant women, older people or anyone with weak immunity.

Why does tahini first stiffen when I add water?

A little water clings to the surface of the sesame particles and forms small bridges that glue them together, so the mixture thickens. As you keep adding water, at some point water becomes the continuous phase, the particles and oil spread through it, and the sauce loosens and turns smooth.