Role of emulsifier layer, antioxidants and radical initiators in the oxidation of olive oil-in-water emulsions

Lipid oxidation in oil-in-water (O/W) emulsions is largely affected by the properties of the interfacial layer surrounding the oil droplets. In this work, the effect of the emulsifier layer structure, presence of both hydrophilic and lipophilic antioxidants and radical initiators on the development of lipid oxidation in olive oil-in-water emulsions was investigated. The olive oil-in-water emulsion is a suitable model of low fat food emulsions. The rationale of the work was to study the role of the interfacial layer when both the antioxidants and the radical initiators came from the two different emulsion compartments. Emulsions were prepared by using food grade emulsifiers of the Tween series (polyoxyethylene sorbitan esters) in the water phase and Span 80 (sorbitan monoleate) in the oil phase. The properties of the interfacial layer were changed by using Tween 20, Tween 60, Tween 80, which have different hydrophobic tails. These systems were oxidized by means of both hydrophilic (AAPH (2,2'-azobis,2-methyl-propanimidamide dihydrochloride), AIPH (2,2'-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride) and lipophilic (AMVN (2,2'-azobis(2,4-dimethylvaleronitrile)) radical initiators at 40 °C. A continuous fluorescent method based on the front face technique allowed us to follow directly the development of oxidation in the whole emulsion. The combination Tween 80/Span 80 produced an interfacial layer more resistant to radical attack. Moreover, a polar paradox was verified also for radical initiators, while the results suggest that the best way to protect emulsions is to use a combination of antioxidants in both phases, to promote a synergy and the regeneration of antioxidants mediated by the interfacial layer.