When two immiscible substances are in contact, the contact surface between two layers is called interface. Interfacial tension is a surface tension that acts on this interface. The higher interfacial tension, the greater two substances tend to split apart from each other.

Surfactant (Surface Active Agent) is a chemical that modifies the properties of the interface. By lowering interfacial tension, it binds together immiscible substances and conclusively establishes a homogenous mixture. Emulsion can be defined as a dispersion of one liquid in another with which it is immiscible. In other words, it is a stable blend of such liquids as water and oil that do not mix easily.

Interface is not only present between water and oil but also among various immiscible substances such as carbohydrates, protein, fats and oils, water, and air, all of which are the major components of foods. Surfactant specifically for food is called food emulsifier, which distinguishes it from other surfactants for industrial purpose, and it has diverse functions, something more than just a surface-active agent.