The polar paradox states that polar antioxidants are more active in bulk lipids than their nonpolar counterparts, whereas nonpolar antioxidants are more effective in oil-in-water emulsion than their polar homologs. However, recent results, showing that not all antioxidants behave in a manner proposed by this hypothesis in oil and emulsion, lead us to revisit the polar paradox and to put forward new concepts, hypotheses, and theories. In bulk oil, new evidences have been brought to demonstrate that the crucial site of oxidation is not the air-oil interface, as postulated by the polar paradox, but association colloids formed with traces of water and surface active molecules such as phospholipids. The role of these association colloids on lipid oxidation and its inhibition by antioxidant is also addressed as well as the complex influence of the hydrophobicity on the ability of antioxidants to protect lipids from oxidation. In oil-in water emulsion, we have covered the recently discovered non linear (or cut-off) influence of the hydrophobicity on antioxidant capacity. For the first time, different mechanisms of action are formulated in details to try to account for this nonlinear effect. As suggested by the great amount of biological studies showing a cut-off effect, this phenomenon could be widespread in dispersed lipid systems including emulsions and liposomes as well as in living systems such as cultured cells. Works on the cut-off effect paves the way for the determination of the critical chain length which corresponds to the threshold beyond which antioxidant capacity suddenly collapses. The systematic search for this new physico-chemical parameter will allow designing novel phenolipids and other amphiphilic antioxidants in a rational fashion. Finally, in both bulk oils and emulsions, we feel that it is now time for a paradigm shift from the polar paradox to the next theories.

The antioxidant polar paradox postulates that nonpolar antioxidants are more effective in oil-in-water emulsions than polar antioxidants. However, this trend is often not observed with antioxidants esterified with acyl chains to vary their polarity. In this study, the nonpolar eicosyl rosmarinate (20 carbons, R20) was less effective at inhibiting lipid oxidation in oil-in-water emulsions than esters with shorter fatty acyl chains such as butyl (R4), octyl (R8), and dodecyl (R12) esters. Interestingly, in the presence of surfactant micelles, the antioxidant activity of R20 was significantly increased while the antioxidant activity of R4 and R12 was slightly decreased. The presence of surfactant micelles increased the concentration of R20 at the interface of the surfactant micelles and/or emulsion droplets as determined by partitioning studies, front-face fluorescence properties, and the ability of R20 to interact with the interfacial probe, 4-hexadecylbenzenediazonium. A possible explanation for why the antioxidant activity of R20 was so dramatically increased by surfactant micelles is that a portion of the nonpolar R20 localizes in the emulsion droplet core and the surfactant micelles are able to increase the interfacial concentrations of R20 and thus its ability to scavenge free radicals produced from the decomposition of interfacial lipid hydroperoxides.

Liposomes have substantial potential to deliver bioactive compounds in foods. However, the oxidative degradation and physical instability of liposomes limit their utilization. This research evaluated the ability of chitosan and rosmarinic acid and its esters to increase the physical and oxidative stability of liposomes. Particle size analysis studies showed that the physical stability of liposomes was enhanced by depositing a layer of cationic chitosan onto the negatively charged liposomes. The combination of octadecyl rosmarinate (40 microM) and chitosan coating resulted in significantly greater inhibition of lipid oxidation in the liposomes compared to chitoson or octadecyl rosmarinate alone. Increasing the concentrations of octadecyl rosmarinate to a concentration of 40 microM in the chitosan-coated liposomes decreased lipid oxidation. Only butyl rosmarinate exhibited stronger antioxidant activity than free rosmarinic acid. Eicosyl rosmarinate (20 carbons) had lower antioxidant activity than all other rosmarinic acid derivatives. These results suggest that by combining the inclusion of appropriate antioxidants such as rosmarinic acid and the deposition of a chitosan coating onto the surface of liposomes may significantly increase the oxidative and physical stability of liposomes.

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Many antioxidants can interact to produce synergistic interactions that can more effectively inhibit lipid oxidation in foods. Esterification of rosmarinic acid produces a variety of compounds with different antioxidant activity due to differences in polarity and thus differences in partitioning in oil, water, and interfacial regions of oil-in-water emulsions (O/W). Therefore, rosmarinic acid and rosmarinate esters provide an interesting tool to study the ability of antioxidant to interact in O/W emulsions. In O/W emulsions, rosmarinic acid (R0) exhibited the strongest synergistic interaction with α-tocopherol while butyl (R4) and dodecyl (R12) rosmarinate esters exhibited small synergistic interaction and eicosyl rosmarinate esters (R20) exhibited slightly antagonistic interaction. Fluorescence quenching and electron paramagnetic resonance (EPR) studies showed that water-soluble rosmarinic acid (R0) exhibited more interactions with α-tocopherol than any of the tested esters (R4, R12, R20). This was also confirmed in O/W emulsions where R0 altered the formation of α-tocopherol quinone and α-tocopherol increased the formation of caffeic acid from R0. This formation of caffeic acid was proposed to be responsible for the synergistic activity of R0 and α-tocopherol since the formation of an additional antioxidant could further increase the oxidative stability of the emulsion.