An emulsion may be perceived as a system containing two immiscible liquid phases, one of which is dispersed in the other as droplets varying between 0.1 and 50 mm in diameter. The phase present in the form of droplets is said to be the internal or dispersed phase, and the matrix in which the droplets are dispersed is known as the external or continuous phase. The importance of mesomorphic or liquid crystalline phases to the properties of emulsions has been discovered only recently and is reflected in the 1972 definition of an emulsion by the IUPAC: "In an emulsion, liquid droplets and/or liquid crystals are dispersed in a liquid". The abbreviations O/W and W/O are frequently used to indicate the type of emulsion, that is, oil-in-water and water-in-oil, respectively.
The formation of small dispersed droplets is associated with a growth in the interfacial area between both liquids. This enlargement occurs exponentially with a decrease in droplet diameter. In specific situations the interfacial area may become unbelievably large. For example, if 1 ml of oil is dispersed as 1 mm diameter particles in water, 1.9 x 1012 globules are created and the total interfacial area is 6 m2.
The volume percentage of the dispersed phase can vary from 2-3%, in milk, to a larger value such as 65-80%, in a mayonnaise, or even to the extent of 99% in certain experimental emulsions. It is although interesting to note that perfect spheres, when packed to a maximum density, only occupy 75% of the sample volume. Consequently emulsion of dispersed-phase volumes larger than 75% only exist due to the diversity in globule size and/or the ability of the globules to deform.
The work, W, necessary to increase the interfacial area by an amount A may be represented by the relationship W = a D A, where a is the interfacial tension. As a consequence of the large positive free energy at the interface of the two liquids, emulsions are thermodynamically unstable. Many emulsions tend to destabilise by one or more of the following three mechanisms:
Creaming or sedimentation can result from the action of gravitational force on phases that differ in density. The rate of this occurrence obeys Stokes law.
where V is the velocity of the globule, r is its radius, g is the acceleration of gravity, Dr is the difference in density between the two phases and m is the viscosity of the continuous phase. When clustering occurs, the radius of the cluster must be used and not that of the individual components.
Flocculation or clustering, which is the result of closer aggregation of globules, albeit without breach of the individual surface film, may also be the main culprit for emulsion destabilisation, as happens in non-homogenised milk.
Coalescence involves rupture of the individual globular membrane and can be the consequence of flocculation. The decrease of interfacial area makes it a very probable phenomenon as stability increases.
To produce stable emulsions, the tendency to minimise interfacial area through coalescence must be counteracted, and this is normally executed by adding emulsifiers. These are usually surface-active compounds that adsorb at the interface to lower interfacial tension to produce a physical resistance to coalescence and, occasionally, to increase surface charge.
Emulsions and emulsifiers are fundamental to the food industry. O/W emulsions come in the form of milk, cream, mayonnaise, salad dressings, ice cream mix and cake batters. On the other hand butter and margarine are W/O emulsions. "Meat emulsion" is a more complex system in which the dispersed phase is solid fat in the form of fine particles and the continuous phase is an aqueous matrix containing salts, soluble and insoluble proteins and particles of muscle fibres and connective tissues.
The ever increasing introduction of new food products and the continuing mechanisation of food processes have promoted the use of food emulsions and the need for further understanding of their properties and benefits. Emulsifying agents are nowadays commercially available, being produced to satisfy a variety of specific applications.
B. The HLB System for Selecting Emulsifiers
C. PIT as a Basis for Selecting Emulsifiers
