An emulsion occurs when two substances that do not normally mix are forced into a mixture in which one of the substances is evenly dispersed in the form of small droplets throughout the other substance. Under normal conditions, fat (either liquid or solid fat) and water do not mix, but these two substances are the most common ingredients in culinary emulsions.
An emulsion consists of two phases, the dispersed phase and the continuous phase. An emulsified vinaigrette is an example of an oil-in-vinegar emulsion, meaning that the oil (the dispersed phase) has been broken up int very small droplets suspended through the vinegar (the continuous phase).
Temporary emulsions, such as vinaigrette form quickly and require only the mechanical action of whipping, shaking, or stirring. To make an emulsion stable enough to keep the oil in suspension, additional ingredients, know as emulsifiers, are necessary to attract and hold together both the oil and the liquid. Commonly used emulsifiers include egg yolks (which contain the emulsifier lecithin), mustard, and glace de viande. Natural starches, such as those in garlic, or modified starches, such as cornstarch or arrowroot, are also used.