So‐called emulsifiers do many things. Although the addition of an emulsifier to some mixtures does emulsify them, the emulsification of other mixtures may require other ingredients as well. Some existing emulsions may actually be broken by the same emulsifier; others may be broken and converted to a new emulsion; and some materials may be changed in firmness, viscosity, wettability or other properties even when no emulsion is involved. To make matters even more complicated, the effect of a given emulsifier in a given system may vary dramatically with temperature. For example, the most familiar of emulsifiers, egg yolk, causes most of the difference seen between a pastry dough and a cake batter or between pastry and cake: at mixing temperature it causes the difference between a plastic solid and a pourable liquid, whereas at baking temperature it causes the difference between a rigid, friable solid and an elastic solid. Emulsifiers are used in a very wide range of industrial processes, from oil‐drilling to paint‐making and printing. However their use in the manufacture of food and beverages clearly requires the most stringent standards of identity. Under current UK legislation, food emulsifiers are regulated by the same statutory instrument as the so‐called stabilisers. The difficult problem of definition is approached on the basis of function :—