Emulsions

Emulsions are stable mixes of two immiscible liquids, mostly water and oil but sometimes air.

The two types of liquids are named phases:
- dispersed phase to the liquid in minor proportion
- continuous phase to the liquid in major proportion.

Water and oil can be eventually mixed during a short time, e.g. vinaigrette.
But, when cooking, we want a stable mix, at least for the period during we are eating it.

This can be done in two ways:
- With a substance, called emulsifier, which works like a stable frontier between both phases
- With a substance, called stabilizer, generally a thickener or gel, forming a three-dimensional web, which traps and retains dispersed phase microspheres into the continuous phase.

Depending of the relative amount of water and oil, we talk about either emulsions of water-in-oil (e.g mayonnaise) or oil-in-water (butter) or air-in-water (mousses or airs)

The most common emulsifier is lecithin.
The most common stabilizer has been gelatine but, nowadays, there are a lot of substances in use: methylcellulose, alginate, xanthane or guar gum.

In the practical side, the most of the sauces are emulsions: mayonnaise, hollandaise, mousseline, pil-pil, etc., where the aqueous ingredient (broth, egg white, vinegar, etc.) is mixed with the oily ingredient (oil, butter, egg yolk) thanks to an emulsifier or stabilizer (egg proteins, butter proteins, gelatine).

Mayonnaise: egg lecithin emulsifies water from egg and vinegar with oil.
Hollandaise: egg lecithin and butter proteins emulsify emulsifies water from egg and vinegar with fat from butter.
Mousseline: It is the same than hollandaise but adding milk cream which has more emulsifier proteins from milk. It produces a different final texture.
Pil-pil: gelatine and water from cod are emulsified with olive oil. Gelatine stays liquid while temperature is above jellifying temperature (e.g. 4 Celsius) giving a fantastic creamy texture.

Nowadays, commercial lecithin allows to prepare sauces with no naturally containing emulsifier ingredients (e.g. egg, butter) for instance stable vinaigrette.
Methylcellulose is specially interesting at this respect because it produces emulsions when in cold and it jellifies when heat it up, which can be eventually mashed off to become liquid gel.

In other hand, air (and some other pure gases as N2O) can be emulsified with water thanks to lecithin, making foams, mousses, or airs (read more about airs and foams)