What you need
1 egg yolk
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1 tbsp lemon juice
1 tsp Dijon mustard 250 ml vegetable oil
OIL and water famously don’t play well together. Water is a polar molecule, with a negative charge concentrated around the oxygen atom and a positive charge at the two hydrogen atoms. This means that water molecules attract each other, the hydrogen atoms forming bonds with the oxygen atoms of nearby molecules. Oil, on the other hand, is made from non-polar molecules, which aren’t attracted by the water molecules, so it is hard for them to mingle.
If you shake oil and vinegar vigorously enough, you can get the vinegar to be dispersed as tiny droplets in the oil, making what is called an emulsion. But they are likely to separate before long.
Chemicals called emulsifiers can make the emulsion more stable because an emulsifier molecule has one pole (or part) that loves oil and one that loves water so it can form an interface between oil and water. Mustard plays this role well in a vinaigrette, helping ensure the dressing won’t separate so readily.
A vinaigrette that isn’t well emulsified will make salad leaves wilt quickly, because the oil can get through the waxy cuticle of the leaves. But if the oil droplets are properly dispersed in vinegar, with the help of mustard, the leaves remain crisp for longer.
Egg yolks contain compounds called lecithins that are very effective as emulsifiers. These allow large volumes of oil to be suspended in a relatively small amount of lemon juice to make mayonnaise. Even though mayonnaise is mostly oil, it doesn’t feel greasy because your mouth senses only the so-called continuous phase of the emulsifying process – which is water-based.
Proteins can also work as emulsifiers, because they have polar and non-polar parts too. Vegan mayonnaise can be made using a few mashed chickpeas and some of the protein-rich liquid from a tin of chickpeas, known as aquafaba, instead of egg yolk.
In sauces such as gravy, thickening agents like flour help to create a stable emulsion. They work differently: by increasing the viscosity of water, they prevent fat droplets from moving through the liquid and coalescing.
The traditional way to make mayonnaise starts by whisking egg yolks with a bit of mustard and lemon juice. The oil must be added very slowly and whisked in vigorously so it becomes dispersed as droplets in the water. If oil is added too quickly, it forms a continuous phase on the surface instead of an emulsion.
You can think of mayonnaise as a template for making emulsion sauces from any flavour you like. Add an egg yolk to pureed vegetables, garlic, herbs, miso paste or anchovies, for example, then add oil slowly in the same way. For a fancy finishing touch to your presentation, use a squeezy bottle to put blobs of emulsion sauce on the plate.
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