DROPLET COMPOSITION AFFECTS THE RATE OF OXIDATION OF EMULSIFIED ETHYLLINOLEATE

Our objective was to study the influence of droplet composition on the rate of lipid oxidation in emulsions. A series of oil-in-water emulsi ons stabilized by a nonionic surfactant (Tween 20) was studied. These emulsions had the same total oil concentration (5 wt%) and initial dro plet diameter (0.3 mu m), but contained droplets with different ratios of ethyl linoleate (substrate) and n-tetradecane (inert diluent). Lip id oxidation was measured as a function of time by three different met hods: gas-chromatographic determination of residual substrate; ultravi olet-visible spectrophotometric determination of conjugated dienes; an d measurement of aqueous thiobarbituric acid-reactive substances. All three methods showed similar trends for emulsions of similar compositi on. The progress of lipid oxidation in the emulsions was dependent on the concentration of ethyl linoleate in the emulsion droplets. At low concentrations (1% oil as substrate), oxidation proceeded at a relativ ely slow and constant rate. At intermediate concentrations (20%), the oxidation rate was rapid initially and then slowed down with time. At high concentrations (100%), the oxidation rate was slow at first, and then increased with time. An explanation of our results is proposed in terms of the distribution of substrate molecules between the droplet interior and interface, and the ingress of aqueous radicals into the e mulsion droplets.