An emulsion consists of two liquids – such as oil and water – where one is dispersed in the other in the form of small droplets, e.g. an oil-in-water emulsion (o/w) or a water-in-oil emulsion (w/o).
The dispersion of oil in water is achieved by vigorous stirring or homogenization. Make emulsions that are physically stable requires emulsifiers. These form a film, or membrane, over the droplets to prevent them from aggregating (see figure below).
Lipid oxidation in o/w emulsions is strongly influenced by the size of the oil droplets and the nature of the interface, which is affected in turn by the composition of the oil and water phases. The membrane surrounding the droplets consists of surface active substances (e.g. emulsifiers, proteins). This membrane affects not only physical stability, but also lipid oxidation in many ways.
The activity of antioxidants varies in different multiphase systems, which differ in the distribution of the lipid phase. In o/w emulsions, hydrophilic antioxidants are generally less effective than lipophillic ones. In bulk oil systems, hydrophilic antioxidants are more effective (polar paradox). This can be explained in relation to the affinities of the antioxidants to air-oil interfaces in bulk oil and water-oil interfaces in emulsions.
As a result, careful consideration should be given to the nature of the system when selecting an appropriate antioxidant system. Prevention of lipid oxidation in food lipid emulsions is inherently a more complex subject, because of the many factors involved in bulk systems.
 | Figure: o/w-emulsion together with proteins, emulsifiers and antioxidants (hydrophilic and lipophillic)
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