Oxidation of linoleic acid in emulsions: Effect of substrate, emulsifier, and sugar concentration

Linoleic acid oxidation in oil-in-water emulsions stabilized by a nonionic surfactant (Tween-20) was studied. The emulsion composition was varied at a constant oil droplet size. Lipid oxidation was measured as a function of t ime in the presence of a catalyst (FeSO4/ascorbic acid) by two methods: gas chromatographic determination of residual substrate and ultraviolet-visibl e spectrophotometric determination of conjugated dienes. Rate of oxidation was influenced by the emulsion composition (relative concentrations of subs trate and emulsifier) and especially by the partition of the emulsifier bet ween the interface and water phase. Concentrations of emulsifier exceeding the critical micelle concentration protected the fatty acid against oxidati on. Excess surfactant formed micelles and mixed micelles with linoleic acid , which retarded oxidation by diluting the substrate or perhaps by replacin g linoleic acid at the interface, making it less accessible to radical atta ck. The addition of sucrose also had a protective effect, but only up to a certain concentration, indicating the effect may involve factors other than viscosity.