Role of association colloids in bulk oils on lipid oxidation
Lipid oxidation leads to quality deterioration of foods high in unsaturated fatty acids. Edible oils contain surface active compounds and water that can form physical structures known as association colloids. To better understand the influence of physical structures on the oxidative stability of bulk oils the role of association colloids on lipid oxidation was investigated. The effectiveness of chain-breaking antioxidants at retarding lipid oxidation depends on their chemical properties and physical location within a food. The first study showed that the surface activity and/or polarity of lipid-soluble antioxidant were not the only determinant of their effectiveness in food lipids. In the second study of model association colloids in oils, we found that the size of the reverse micelles increased with increased water or phosphatidylcholine concentration, but decreased upon addition of cumene hydroperoxide or oleic acid. Iron catalyzed oxidation of methyl linolenate in the reverse micelle system decreased with increasing water concentration. Phosphatidylcholine decreased methyl linolenate oxidation compared to control and reverse micelles with added oleic acid. These results indicate that water, cumene hydroperoxide, oleic acid, and phosphatidylcholine can alter reverse micelle size and lipid oxidation rates. The influence of these compounds on physical structures of bulk oil was also confirmed in a study using surface active fluorescence probe. The fluorescence intensity of 5-dodecanoylaminofluorescein (DAF) increased with increasing water concentration in the edible oil. Addition of oleic acid decreased DAF fluorescence due to the ability of the free fatty acid to decrease the pH of the aqueous phase of the bulk oil. Phosphatidylcholine increased DAF fluorescence due to its ability to increase DAF exposure to the aqueous phase. Oleic acid had no impact in the interactions between DAF and water soluble peroxyl radicals while phosphatidylcholine decreased peroxyl radical degradation of DAF. This research established the significance of physical structures of bulk oils on lipid oxidation. Understanding how the physical properties of bulk oils impact lipid oxidation could lead to development of novel antioxidant technologies that help improve the oxidative stability of oils containing increased concentrations of polyunsaturated fatty acids.