The aim of this thesis is to explore how phospholipids at concentrations similar as in refined vegetable oils impact bulk oil lipid oxidation. The possible formation of association colloids and synergism with primary antioxidants are considered. The results provided a better understanding of the pro- and antioxidant activities of phospholipids.
Lipid oxidation leads to quality deterioration by generating off-flavor, nutrient loss, color alteration, texture changes, and even generation of potential toxic products. Phospholipids are important minor components in edible oil that play a role in lipid oxidation. Surface active phospholipids have an intermediate hydrophilic–lipophilic balance value, which allows them to form association colloids such as reverse micelles in bulk oil. These association colloids can influence lipid oxidation since they create lipid–water interfaces where prooxidants and antioxidants can interact with triacylglycerols. In this study, we examined the formation of reverse micelles in a stripped oil system by dioleoyl phosphoethanolamine (DOPE) and the effect of these physical structures on lipidoxidation kinetics. The critical micelle concentration (CMC) of DOPE was approximately 200 μmol/kg oil at 45 °C. Oxidation kinetics studies showed that DOPE was prooxidative when it was above its CMC (400 and 1,000 μM), reducing the lag phase from 14 days (control) to 8 days. The addition of combinations of DOPE and dioleoyl phosphocholine (DOPC) resulted in formation of mixed micelles with a CMC of 80 μmol/kg oil at 45 °C. These mixed micelles were also prooxidative when concentrations (100 and 500 μM) were above the CMC, decreasing the lag phase from 14 to 8 days.
DOPC and DOPE reverse micelles were examined on their impacts on the activity of primary antioxidants such as the nonpolar α-tocopherol and the polar trolox in stripped and commercial soybean oils. The results showed that DOPC reverse micelles decreased the activity of 100 μM α-tocopherol or trolox. On the other hand, DOPE increased the antioxidant activity of both α-tocopherol and trolox. The polar trolox exhibited better antioxidant activity than the nonpolar α- tocopherol in the presence of both DOPC and DOPE reverse micelles because trolox partitioned more at the water-lipid interface, which was confirmed by a fluorescence steady state spectroscopy. Different ratios of DOPE to DOPC were added to oil containing 100 μM α-tocopherol, and antioxidant activity increased with increasing DOPE/DOPC ratio. Addition of DOPE to commercial oil inhibited lipid oxidation, where as DOPC was ineffective. HPLC showed that DOPE regenerated α-tocopherol.
Overall, these findings provide a better understanding of the role of phospholipids reverse micelles in lipid oxidation in edible oil and indicating that the antioxidant activity of tocopherols could be improved by utilizing phosphatidylethanolamine (PE) to engineer the properties of reverse micelles in bulk oil
