Oxidation Behavior of Docosahexaenoic Acid-Enriched Algal Oil and Its Oil-in-Water Emulsion

In this study, low-internal-phase oil-in-water (O/W) emulsion consisting of 3% algal oil rich in docosahexaenoic acid (DHA) as the oil phase, 1% Tween-60 as the emulsifier and 96% water phase was prepared using high-pressure micro-fluidization. The oxidation behavior of DHA-rich algae oil and its emulsion system at ambient temperature under sealed conditions was investigated. The levels of primary and secondary oxidation products in algae oil were determined by measuring peroxide value (PV) and thiobarbituric acid reactive substances (TBARS) value. Gas chromatography (GC), ¹H nuclear magnetic resonance (¹H NMR) spectroscopy, and headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) were employed to analyze the loss of fatty acids and changes in the composition of oxidation products. Results indicated that the PV, TBARS value, and fatty acid loss of the emulsion system were higher than those of algae oil, suggesting a faster oxidation rate of DHA-algae oil in the low-internal-phase emulsion. In the ¹H NMR spectra, resonance signals of terminal monohydroperoxide (DHA-TMHP) and inner monohydroperoxide (DHA-IMHP) were observed, with the inner hydroperoxides containing cyclic hydroperoxides and intermediate oxidation products, which produced α,β-unsaturated oxygenated aldehydes by secondary cleavage. The content of conjugated diene (CD) was the highest, followed by hydroperoxides (OOH), and the content of non-volatile aldehyde oxidation products (NAD) was the lowest, reflecting the generation of high levels of intermediate oxidation products from DHA. In addition, it was found that the proportion of DHA-TMHP in the emulsion system was significantly higher than that in algae oil, indicating that the emulsion system favors the formation pathway of DHA-TMHP. DHA-NAD were generated at the initial oxidation stage, the content of DHA-NAD declining in the order: saturated aldehydes > (E,Z)-2,4-dienals > oxygenated α,β-unsaturated aldehydes > (E)-2-enals. GC-MS showed that volatile secondary oxidation products (VSOPs) were produced in the emulsion from the beginning of storage. The major components of VSOPs in both the emulsion and algae oil included enal, enone and enol, the relative levels of volatile compounds decreasing in the order: aldehydes > ketones > alcohols. (E,E)-2,4-heptadienal, 1-octen-3-ol, 3,5-octadien-2-one, 2,4-nonadienal, 2-pentenylfuran, 1-penten-3-ol, heptane, 1-nonene, and octanal were the major contributors to VSOPs, probably generated through the α and β cleavage of DHA-IMHP.