Abstract: 31P nuclear magnetic resonance (NMR) spectroscopy was used to compare the composition of egg yolk phospholipids before and after enzymatic oxidation. 1H NMR spectroscopy was used to monitor and compare the changes in the molar percentage of fatty acyl groups and the concentrations of primary and secondary oxidation compounds between the unoxidized control, lipoxygenase-catalyzed oxidation and thermal oxidation (in water bath at 95 ℃) groups. The results showed that phosphatidylcholine and phosphatidylethanolamine were the major components of egg yolk phospholipids. After oxidation of egg yolk phospholipids, the contents of phosphatidylcholine and phosphatidylethanolamine decreased, while the content of hemolytic phospholipids increased. The molar percentage of total unsaturated fatty acids in both oxidized samples decreased, while the molar percentage of saturated fatty acids increased. The degradation rate of linoleic acid was the fastest with the enzyme at pH 6, and more hydroperoxide was formed from the enzymatic oxidation of egg yolk phospholipids. (Z,E)-2,4-dienal was mainly produced after heating. At pH 9, hydroperoxide was produced and rapidly degraded and more n-alkaldehyde was generated after heating. Therefore, lipoxygenase can be used to catalyze the oxidation of egg yolk phospholipids at pH 6 to produce more flavor compounds such as 2,4-diene aldehydes.

Key words: egg yolk phospholipids; lipoxygenase; enzymatic oxidation; 31P nuclear magnetic resonance; 1H nuclear magnetic resonance; hydroperoxide; aldehyde