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.

Lipid oxidation is a significant issue in the food industry, which can cause severe food quality deteriorations and nutrition losses. Transition metals, especially iron, are a main factor to accelerate lipid oxidative reactions in food systems. Non-migratory active packaging techniques provide an alternative strategy to food preservations without the need for food additives. The objective of this work was to develop non-migratory metal chelating active packaging films to control the iron-promoted lipid oxidation in food products. Both "grafting to" and "grafting from" techniques were used to introduce metal chelating polymer poly(acrylic acid) (PAA) onto inert packaging film surfaces. Compared to the "grafting to" approach, graft polymerization of PAA yielded a higher available carboxylic acid density on the packaging film surface, with a stronger chelating activity toward both Fe2+ and Fe 3+. Compared to the native polypropylene (PP) film, the PP- g-PAA film prepared by the "grafting from" approach significantly delayed the lipid oxidation in an oil-in-water emulsion system by a factor of 4~5 times at pH 7.0. PP-g-PAA films equally inhibited lipid oxidation throughout the range of surface area-to-product volume (SA/V) ratios tested (2 to 8 cm2/mL), and were most capable of preventing lipid oxidation in foods at pH values of 5.0 and higher. A siderophore-mimetic poly(hydroxamic acid) (PHA) was grafted from PP film surface to mimic the chelating property of siderophores, naturally occurring small molecules with high specificity and affinity toward Fe3+. The PHA showed a broad active pH range, which retained 50% ability to chelate iron at pH 3.0 compared to pH 5.0, almost double the retention of low-pH chelating ability of PAA. In an accelerated lipid oxidation study at pH 3.0, PP-g-PHA films performed even better than EDTA in preventing the formation of volatile oxidation products in emulsion systems. The particle size and zeta potential of emulsions indicated that both PP-g-PAA and PP- g-PHA films would unlikely affect the physical and chemical stability of the emulsion system. The results of this work suggest that the application of non-migratory active packaging films represents a promising approach to reduce additive use while maintaining food quality.