The word “emulsion” comes from the Latin word for “to milk”, as milk is an emulsion of fat and water, among other components. An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both the dispersed and the continuous phase are liquids. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions include vinaigrettes, milk, mayonnaise, and etc.
Emulsifiers are used in ice cream because they contribute greatly to smooth and creamy texture by promoting fat destabilisation. Fat destabilisation refers to the process of clustering and clumping (known as partial coalescence) of the fat in an ice cream mix when it is churned in a machine. Because it is the proteins that stabilise the fat emulsion in an ice cream mix, emulsifiers are actually added to ice cream to reduce the stability of this emulsion and encourage some of the fat globules to come together, or partially coalesce. When a mix is churned in the ice cream machine, air bubbles that are beaten into the mix are stabilised by this partially coalesced fat, giving a smooth texture to the ice cream. If emulsifiers were not added, the air bubbles would not be properly stabilised and the ice cream would not have the same smooth texture. The balance between protein and emulsifier is important to ice cream making because it controls the stability of the emulsion and of the air bubbles. If an ice cream mix contains too much emulsifier, the formation of objectionable butter particles can occur. However, if there is too much protein, the emulsion may be too stable so that not enough fat is destabilised. This produces an unstable foam, and the ice cream is coarse and wet.