Vol. X · Emulsions, Foams and Gels Plate 2 of 11emulsifiers, lecithin, egg yolk, mustard, proteins, surface, droplets, mayonnaise, coverageM

Emulsifiers.

One egg yolk can hold a cup of oil in a mayonnaise. Its molecules have two ends, and each droplet needs a coat of them.

Oil and water separate because water molecules attract each other more than they attract oil, and squeeze the oil out into as small a surface as possible — one big blob. An emulsifier is a molecule with a water-loving (hydrophilic) end and an oil-loving (hydrophobic) end. At the surface of a droplet it lines up with one end in each liquid, lowering the cost of the surface and forming a coat that keeps droplets from merging when they touch.

Cooks use three families. Lecithins and other phospholipids, abundant in egg yolk and soy. Proteins: egg-yolk and milk proteins unfold at the surface and form a tough film, which is why milk, cream and cheese sauces hold their fat. Fine particles and mucilages: the seed coat of mustard, crushed garlic, ground spices and starch, which thicken and armour the droplets. Each has a limited amount of surface it can cover, and the workshop compares that with the surface the oil creates.

Below: the surface created by 200 mL of oil at the droplet size you choose, against what your yolks, lecithin and mustard can cover, and the share of the surface coated (colloid bench; coverage per gram is a teaching value).

What you will learn

Describe how an emulsifier works.
Name the cook's emulsifiers.
Relate droplet size to how much emulsifier is needed.
Explain why one yolk goes a long way.

Workshop

Colloid bench: the oil's surface is 6V/d; one yolk is taken to coat about 300 m² of droplet surface, a gram of lecithin 400 m², a teaspoon of mustard 40 m² (teaching values chosen so that one yolk emulsifies a mayonnaise of a few hundred millilitres with droplets of a few micrometres, as kitchens find).

A coat for every droplet

Two hundred millilitres of oil whisked into 3 µm droplets has 400 m² of surface — more than a tennis court. In this model one yolk covers about 300 m²: enough for the oil at 4 µm, not at 2 µm. Turned round, one yolk can coat about 150 mL of oil in 3 µm droplets, which is why one yolk is enough for a cup of mayonnaise, and why larger batches use more yolk or mustard.

Mustard in a vinaigrette, garlic in aïoli, a spoonful of miso or tahini in a dressing, a little cream or cheese in a pan sauce, egg yolk in a carbonara: each is an emulsifier quietly doing this job. Commercial products use purified ones — soy or sunflower lecithin, mono- and diglycerides — which a cook can buy and use by the gram. The water phase matters too: an emulsifier needs some water to work in, and an emulsion with too little water becomes so crowded that its droplets merge (X.4).

At the stove

Shake oil and vinegar in two jars, one with a teaspoon of mustard. Set both down and time how long each takes to separate. The mustard one lasts many times longer.

Lecithin was first isolated from egg yolk in 1846 by the French chemist Théodore Gobley, who named it from the Greek lekithos, egg yolk.

Terms

Emulsifier: A molecule with a water-loving and an oil-loving end that coats droplets.
Lecithin: A phospholipid emulsifier from egg yolk and soy.
Hydrophilic: Water-loving.
Hydrophobic: Water-fearing; oil-loving.

Emulsifiers have two ends, one for oil and one for water; they coat each droplet, and smaller droplets need more of them.

Sources

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