Relationship between the Particle Size and the Oxidative Stability of Aqueous Emulsion Prepared from Triacylglycerol Rich in Docosahexaenoic Acid

The oxidative stability of polyunsaturated fatty acid (PUFA) generally decreases with increasing unsaturation, however, the opposite results have been reported for aqueous emulsions prepared from PUFA. One study showed that among PUFA (fish oil) emulsions with varying μm-sized particles, the smaller emulsions had better oxidation stability. Thus, particle size may be involved in the mechanism behind the improved oxidative stability of PUFA by emulsification, and PUFA emulsions with even smaller particle sizes (i.e., nm-sized) may exhibit higher stability. To evaluate these hypotheses, once optimizing conditions under which emulsions with different sizes (100-250 nm) can be prepared using docosahexaenoic acid (DHA)-rich algal oil (DAO), these emulsions were subjected to thermal oxidation (40°C, 5 days under light shielding). Based on peroxide value (POV) data, it is apparent that even DHA, which is particularly prone to oxidation, becomes less susceptible to thermal oxidation when emulsified, and that smaller emulsions lead to greater oxidative stability. The high oxidation stability in the smallest emulsion remained high after removal of the antioxidants (tocopherols) from this emulsion. Considering these results together with preliminary data on triacylglycerol (TG) mono-hydroperoxides measured by high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS), the physical properties of the emulsion might account for the greater oxidative stability in smaller emulsions, probably due to the rapid termination of the oxidation reaction within the smaller particle in a more substrate-dense environment, rather than to the action of antioxidant tocopherols. Further research will hopefully lead to a solution to the longstanding problem of oxidation of PUFA such as DHA, which has a variety of benefits.