The Influence of Secondary Emulsifiers on Lipid Oxidation within Sodium Caseinate-Stabilized Oil-in-Water Emulsions

The effect of protein displacement at the interface by a secondary emulsifier on the oxidative stability of sodium caseinate-stabilized tuna oil-in-water emulsion systems was determined. Emulsions were prepared with a selection of anionic and non-ionic emulsifiers and stored at both 25 and 50 degrees C with no added prooxidant, and at 4 degrees C in the presence of ferrous sulfate. The progress of oxidation during storage was monitored through solid phase microextraction headspace analysis. Metal ion catalyzed oxidation was enhanced for the emulsions stabilized with an anionic emulsifier in comparison to emulsion systems stabilized with non-ionic emulsifiers and sodium caseinate alone. The increased oxidation observed for the emulsion with the anionic surfactant is due to electrostatic interactions between divalent metal ions and the negatively charged surfactant at the oil-water interface. The sodium caseinate interfacial layer had little prooxidant effect at the droplet surface, most likely due to the ability of free protein molecules in solution to sequester metal ions, which may have provided some protection against oxidative deterioration.