A lipid is one of major ingredients of foods and plays important roles in nutrition and taste. However, a lipid containing unsaturated fatty acids is prone to oxidation.Lipid oxidation is one of the major causes of the deterioration of foods containing lipids, and is affected by many factors. The state and structure of lipids in foods are also considered to be important factors as well as the conditions of how the foods are stored. An emulsion and solubilization in micelles, which is a dispersion system, are common structures in lipid-containing foods for both home cooking and industry.Rapid developments in nanotechnology have allowed us to focus on its application to foods. A decrease in the particle size of an emulsion produces an increase in the specific surface area of the particles. The high surface area would elicit an effect at the interface between the oil and aqueous phases on the reactions occurring in the emulsion or micelle. Because the lipid in the emulsion and micelle is oxidized by oxygen which is supplied through the interface, an increase in the specific surface area could possibly facilitate the oxidation.In this context, the lipid oxidation in different systems was measured in order to examine the effect of the droplet size in the system on the oxidation kinetics. The kinetics of the lipid oxidation in a bulk system is also described because it provides basic information for analysis of the kinetics in dispersion systems.
Oxidation in bulk systemLipid oxidation is a complicated process that proceeds through initiation, propagation, and termination steps.Labuza [1] reviewed the kinetics of the lipid oxidation in foods. Brimberg [2] discussed the oxidation mechanism based on kinetics. We also reported the autoxidation The oxidation kinetics of unsaturated lipids was examined in bulk, emulsion and micelle systems, and the related literature was also summarized. A kinetic equation of the autocatalytic type was applicable for the lipid oxidation in all these systems. The oxidation of a lipid is affected by many factors. In a dispersion system, such as an emulsion and micelle, the size of the dispersion phase is one of these factors. In spite of a higher specific surface area, the lipid was more slowly oxidized in both the emulsion and micelle systems containing smaller particles in the dispersion phase. This fact could be explained by dilution of the substrate by the hydrophobic moiety of a surfactant for both systems.