| Summary: | Factors affecting lipid oxidation in oil-in-water emulsions were studied. Sodium dodecyl sulfate (SDS), polyoxyethylene (10 or 23) lauryl (Brij), and dodecyltrimethylammonium bromide (DTAB) were used to make anionic, non-ionic, and cationic emulsion droplets, respectively. Iron accelerated lipid oxidation in the emulsions with the oxidation rates of SDS $>$ Brij $>$ DTAB. Oxidation of SDS emulsion increased with decrease in pH in iron-added systems. Iron associated strongly with SDS-stabilized emulsion droplets but not with Brij and DTAB. EDTA ($\geq$ 1000 $\mu$M) decreased lipid oxidation to levels lower than no-added iron controls. This was due to the ability of the EDTA to remove both added and contaminating metals from the emulsion droplet surface as determined by changes in zeta potential. Gallic acid, methyl gallate, and gallamide were tested as anionic, nonionic, and cationic antioxidants, respectively. These galloyl derivatives exhibited both antioxidative free radical scavenging and prooxidative Fe$\sp{3+}$-reducing activity. Initial metal-reduction rate by the galloyl derivatives was higher at pH 3 than 7. Galloyl derivatives did not associate with SDS-stabilized emulsion but partition into Brij-stabilized emulsion droplets. Galloyl derivatives did not alter iron-droplet association by chelation. Charge status of the galloyl derivatives influenced their ability to partition into Brij-stabilized emulsion. Strong association of iron with emulsion droplets is responsible for the low oxidative stability of SDS-stabilized emulsions. The amount of iron commonly found as a contaminant in emulsion can significantly accelerate oxidation. The prooxidant activity of contaminating iron can be controlled by use of chelators and nonionic or cationic emulsifiers. The net antioxidant/prooxidant effect of galloyl derivatives in emulsions is a balance between their free radical scavenging and metal-reducing activity. This balance is influenced by factors including pH, emulsifier type, galloyl derivative concentration and physical location. This study shows that surface property of the emulsion droplets and interactions among emulsion droplets, antioxidants, prooxidants and other co-existing substances are critical to oxidative stability of food emulsions.
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