Mayonnaise from a jar is fine, but making it yourself is fast, cheap, and teaches you one of cooking's most useful ideas: the emulsion. Once you understand how to force oil and water to combine into something creamy, a whole family of sauces opens up.
What an emulsion is
Oil and water normally refuse to mix. An emulsion is a stable blend of the two, held together by an emulsifier. In mayonnaise, the egg yolk is the emulsifier. It lets you whisk oil into a tiny bit of liquid, droplet by droplet, until the whole thing thickens into a smooth, glossy cream.
The method
- Whisk an egg yolk with a little acid, like lemon juice or vinegar, and a pinch of salt.
- Add oil drop by drop at first, whisking constantly, until it starts to thicken.
- Once it takes, pour the oil in a slow, steady stream while you keep whisking.
If it breaks
Adding oil too fast is the classic mistake. If the sauce turns greasy and separated, it has broken. Do not throw it out. Start a fresh yolk in a clean bowl and slowly whisk the broken mixture into it, and it usually comes back together. The same principle underlies hollandaise and vinaigrettes. Master mayo and you understand them all.