Characterization of iron-promoted lipid oxidation in emulsions
Lipids in most foods exist as emulsions, therefore it is important to understand the factors affecting oxidative rancidity in emulsions. The ability of iron to promote lipid oxidation in emulsions can be affected by iron's ability to interact with lipids at the emulsion droplet interface. This research was designed to better understand the factors affecting iron interactions with components of emulsions including lipid and surfactant peroxides. Salmon oil-in-water emulsions stabilized by SDS exhibited greater lipid oxidation rates than Tween 20- and DTAB-stabilized emulsions, especially at low pH. Lipid oxidation in salmon oil-in-water emulsions was inhibited by EDTA and the iron-specific chelator, apo-transferrin, indicating that oxidation was promoted by iron. Furthermore, Tween 20-stabilized salmon oil emulsions oxidized faster at pH 7.0 than 3.0. An explanation for this oxidation trend may be due to Fe3+ associating more with Tween 20 emulsion droplets at higher pH's due to its lower solubility. The ability of iron to decompose cumene hydroperoxide in emulsions stabilized by SDS (anionic), Tween 20 (nonionic), and DTAB (cationic) was determined. Peroxide decomposition was greater in SDS-stabilized emulsions due to increased iron-peroxide interactions at the emulsion droplet interface. Factors that decrease iron-emulsion droplet interactions including pH, EDTA, and NaCl inhibited Fe2+ promoted decomposition of cumene hydroperoxide in the SDS-stabilized emulsions. Cumene hydroperoxide in Tween 20 was more susceptible to decomposition by iron at pH 7.0 than 3.0.
Polyether surfactants, such as Tween and Brij will accumulate peroxides. Peroxides originating from Tween 20 micelles exhibited changes in peroxide formation and decomposition, the balance of which depended on the concentration and reactivity of iron. In the presence or absence of added iron or copper, peroxides originating from Tween 20 were capable of oxidizing α-tocopherol. Oxidation of α-tocopherol by Fe2+ in Tween 20 micelles was inhibited by EDTA. In conclusion, lipid oxidation rates in emulsions could be lowered by using techniques that decreased the interaction of transition metals at the interfacial region of the droplet.