Oil and water don't mix, yet mayonnaise is thick and smooth. It's an emulsion: tiny drops of one liquid spread through another, held in place.
Key idea
An emulsion has tiny drops of fat suspended in a water-based liquid (or the other way round). An emulsifier such as the lecithin in egg yolk, or mustard, coats the drops and keeps them apart. Permanent: mayonnaise, hollandaise. Temporary: a plain vinaigrette, which separates as it stands. Beurre blanc is butter whisked into a wine and shallot reduction.
- Add the fat slowly at first, drop by drop, then in a thin stream.
- Keep the temperature right: hollandaise and beurre blanc stay warm (too hot breaks them; too cold sets the butter).
- Vinaigrette: about 3 parts oil to 1 part acid, with salt and a little mustard to help it hold.
- A broken mayonnaise or hollandaise can often be saved: start with a fresh yolk (or a spoon of warm water) and whisk the broken sauce into it slowly.
Why it works
A yolk can hold only so much oil at once. Pour fast, and the drops merge into an oily layer: the sauce breaks. Slow addition gives the emulsifier time to coat each new drop.
Worked example: Vinaigrette ratio
You want 1 cup of vinaigrette at 3 : 1. How much oil and vinegar?
- 4 parts in all; 1 cup = 16 tablespoons.
- Oil: $\tfrac34 \times 16 = 12$ tablespoons; vinegar 4 tablespoons.
Answer: ¾ cup oil and ¼ cup vinegar.
Safety
- Uncooked egg yolk sauces (homemade mayonnaise) use raw eggs. The Food Code requires pasteurized eggs for such dishes when serving highly susceptible people, and many kitchens use pasteurized eggs for them always.
Common mistake
Letting hollandaise sit over boiling water. The yolks overheat and it splits. Keep it warm, not hot.
Practice at home
- Make mayonnaise by hand with a whisk. When it is thick, make a second batch and pour the oil fast on purpose; then rescue it with a new yolk.
In Meal Prep
- Mayonnaise: the recipe
- Italian-style vinaigrette: a temporary emulsion
- Hollandaise Sauce: Escoffier's version, adapted