Vol. X · Emulsions, Foams and Gels Plate 4 of 11mayonnaise, emulsion, egg yolk, oil, whisking, broken mayonnaise, aïoli, packing, dropletsMT

Mayonnaise.

A yolk, a spoon of water and lemon, and a cup of oil added drop by drop. Pour too fast at the start and you get greasy soup; patience gives a sauce stiff enough to hold a spoon.

Mayonnaise is about four-fifths oil, held as droplets of a few micrometres in a little water, lemon and egg yolk. At that much oil the droplets cannot stay round: past about 64 % of the volume, spheres packed at random touch, and above 74 % they must squeeze into polyhedra (X.1). Pressed together like that, they resist flowing, and the sauce stands up. Nothing thickens mayonnaise but crowded oil.

The danger is at the start. The whisk can break only so much oil into droplets each second; any oil it has not broken pools on the surface. While the emulsion is thin and there are few droplets, a big pool overwhelms the yolk and the droplets merge — the sauce "breaks". Once a thick emulsion has formed, it helps break new oil, and the oil can go in faster. At the other end, too little water leaves the droplets nothing to sit in and it breaks again. The workshop pours the oil.

Below: the oil dispersed as droplets and the oil pooled on top against time as you pour (colloid bench), whether the sauce holds, and how the droplets pack at the end.

What you will learn
Workshop
Colloid bench, a teaching model: the whisk breaks oil at a rate that rises as the emulsion thickens; the sauce breaks if, while under 60 mL of oil is dispersed, the pool of unbroken oil exceeds 6 mL plus half the dispersed oil, or if the oil passes 88 % of the volume. Packing descriptions from the random (64 %) and densest (74 %) packing of spheres.

Slow, then faster

Poured at 1 mL a second with a steady whisk, 200 mL of oil into 35 mL of yolk, lemon and water makes a sauce that is 85 % oil: crowded past the point where droplets touch, so it holds its shape. Pour at 4 mL a second from the first moment and in this model it breaks after 2.0 seconds, before a thick base has formed. Push 300 mL into only 25 mL of water and the oil reaches 92 % — too little water to hold the droplets apart, and it breaks the other way.

So start drop by drop, whisking hard, until the sauce thickens and turns pale; then pour in a thin stream. If it goes thick and greasy-looking, add a teaspoon of water or lemon to give the droplets room. If it breaks, start again: a fresh yolk (or a spoon of mustard) in a clean bowl, and whisk the broken sauce into it drop by drop as if it were oil. A stick blender in a narrow jar makes mayonnaise in seconds because it breaks oil so fast. Raw yolk carries a small risk of Salmonella: use pasteurised eggs for the very young, old, pregnant or ill (XVII.6).

At the stove

Make mayonnaise with one yolk, a teaspoon of mustard, a tablespoon of lemon juice and 200 mL of neutral oil, adding the first quarter of the oil a few drops at a time. Note the moment it thickens and turns pale: from then on you can pour faster.

Aïoli of Provence and the Catalan allioli are older than mayonnaise: garlic pounded with oil, the garlic itself acting as the emulsifier, with or without yolk. 沙拉酱 in Chinese shops is usually a sweetened mayonnaise.

Terms

Mayonnaise
An emulsion of about 80 % oil in a little yolk, water and acid.
Breaking
An emulsion separating into oil and liquid as droplets merge.
Random close packing
About 64 % of the volume: spheres packed at random touch.
Aïoli
A garlic and oil emulsion from Provence.
Mayonnaise is thick because crowded oil droplets press together. Add oil slowly at first; give it enough water; rescue it with a fresh yolk.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.