Some liquids cannot mix by themselves. When combined they tend to separate. We call these liquids immiscible, for example oil and water. Being immiscible makes it difficult to produce food products containing these two liquids without affecting quality and appearance. However we can fix the problem by using an emulsifier to create an emulsion. An emulsion is formed by first breaking up one of the liquids and dispersing it homogeneously into the other. After that, an emulsifier is added to prevent the liquids from separating. A common emulsifier used for this purpose is lecithin. Lecithin is found abundantly in egg yolks and soybeans. It has a unique structure that allows it to bind both water and oil. This is called an amphiphilic property, i.e., it is both hydrophilic (water loving) and lipophilic (fat loving).
There are two main type of emulsions. Water-in-oil and oil-in-water. Oil-in-water emulsions are more common. Examples are mayonnaise, salad dressing and milk, where tiny droplets of oil is dispersed in water. The oil is called the dispersed phase and the water is called the continuous phase. It is interesting that the tongue senses the continuous phase and not the dispersed phase. That is why although mayonnaise have up to four times the amount of oil to vinegar, it does not taste oily. An example of a water-in-oil emulsion is butter and margarine, where the water is dispersed in the oil. The oil is the continuous phase, so it produces a greasy texture on the tongue. The figures below illustrates an example of an oil in water and a water in oil emulsion.
Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.
