| Summary: | Due to the limited number of approved antioxidants for food applications, several alternative strategies to improve antioxidant performance have been developed by focusing on synergistic antioxidant interactions. Susceptibility to lipid oxidation in food systems is the result of the summation of antioxidative and prooxidative mechanisms. Understanding the sometimes paradoxical behavior of antioxidants and prooxidants is a vital key to design synergistic antioxidant systems suitable for particular foods. This research focused on 3 main strategies to improve the performance of antioxidant activity in oil-in-water emulsions. The first part of this research has been focused on inhibition of lipid oxidation by a combination of the modification of liposomal surfaces by chitosan-coating techniques along with addition of rosmarinic acid esters of varying polarity. Repelling metal ions away from the interface of positively charged liposomes can inhibit lipid oxidation (induced by Fe2+), and also reduce antioxidant loss by Fe3+ reduction. As a result, lipid oxidation can be inhibited synergistically because of a reduction in the prooxidant activity of iron. Second, understanding non-linear antioxidant behavior (the cut-off effect) of antioxidant esters in oil-in-water (O/W) emulsions was also studied to determine how the distributions and locations of antioxidants impacted their antioxidant activity. Antioxidant activity of rosmarinic acid was improved by esterification with alkyl chain lengths between 4 to 12 carbons due to increased ability to partitioning at the interface in oil-in-water emulsions. Surfactant micelles which could increase or decreased the concentration of the antioxidants at the emulsion droplet interface altered antioxidant activity. In the last part of this research, rosmarinic acid and its esters were found to be an excellent tool for studying how antioxidant location could impact its ability to interact with α-tocopherol in O/W emulsions. Synergistic, additive, and antagonistic effects were observed in the combinations between the rosmarinate esters with α-tocopherol. Increases in alkyl chain lengths of rosmarinic acid have influenced both the partitioning of the rosmarinate esters as well as their ability to they interact with α-tocopherol at the interface of oil-in-water emulsions. Fluorescence quenching and EPR studies showed that water soluble rosmarinic acid (R0) exhibited more interactions with á-tocopherol than any of the esters (R4-R20). Synergistic antioxidant interactions between rosmarinic acid and α-tocopherol could not be explained by electron transfer mechanisms, but formation of caffeic acid from rosmarinic acid. Due to the thermodynamic infeasibility and the fact that increases in α-tocopherol degradation rates, α-tocopherol could not be regenerated efficiently by rosmarinic acid. This formation of caffeic acid was proposed to be responsible of the synergistic activity of R0 and α-tocopherol since the formation of an additional antioxidant could further increase the oxidative stability of the emulsion.
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