Study Of Thermal Oxidation And Antioxidant Properties Of Egg Yolk Phospholipids

Egg yolk phospholipids are a kind of precursor that can produce frying and baking aroma,and can significantly increase the lipid-derived characteristic odor of egg yolk,chicken and fried foods.Egg yolk phospholipids were used as the research object to explore the thermal oxidation mechanism at frying temperature（160℃）in this paper.A small amount of egg yolk phospholipids was added to sunflower oil to study their effect on the thermal oxidation and flavor of sunflower oil,and synergistic antioxidant effect withα-tocopherol,which provided theoretical support for the development and utilization of egg yolk phospholipids.The main research contents and results are as follows:（1）Egg yolk phospholipids（EPLs）and egg yolk triglycerides（ETGs）extracted from fresh egg yolks by the ethanol/n-hexane/acetone method were heated at 160℃.The changes in the phospholipid composition of EPLs during heating at frying temperature were monitored by phosphorus-31 nuclear magnetic resonance(³¹P NMR)technology.It was found that fresh EPLs contained phosphatidylcholine（PC）of 76.74±1.48%,phosphatidylethanol-amine（PE）of 17.97±1.40%and a small amount of lysophosphatidylcholine（LPC）,phosphatidylserine（PS）and cardiolipin（CL）.During the heating process,the contents of PC,PE,PS and CL all decreased significantly,especially PE.（2）The changes of fatty acyl groups,primary and secondary oxidation products of EPLs during heating at 160℃were monitored by hydrogen-1nuclear magnetic resonance（¹H NMR）technology,and ETGs were used as a control.Meanwhile,the changes of conjugated diene（CD）,conjugated triene（CT）value and 2-thiobarbituric acid（TBA）value of EPLs and ETGs during thermal oxidation were determined.The differences of acyl groups and oxidation products of the two lipids under different heating times were analyzed by cluster heatmap.The results showed that during the heating process,the unsaturated acyl groups of EPLs decreased,and the saturated acyl groups increased,while the acyl groups of ETGs did not change significantly.The hydroperoxide content of EPLs was the largest at 5 min,while ETGs did not detect hydroperoxides until 40 min.The difference between the two conjugated dienic systems was similar to that of hydroperoxides.EPLs produced secondary oxidation compounds after 5 min,while ETGs were delayed until 10 min,where the production of 2,4-alkadienals in EPLs was higher than ETGs.In EPLs,the CD and CT values were consistent with the changes of hydroperoxides,and the TBA value was higher.The cluster analysis showed that the changes of acyl groups and hydroperoxides of EPLs were more significant during the heating process,and 2,4-alkadienals were produced more,which were significantly different from ETGs.The above results indicated that EPLs were more easily oxidized at 160℃and could generate more oxidation products than ETGs,especially the compounds with fried odor.（3）0.02%（w/w）and 0.05%（w/w）of egg yolk phospholipids were added to fresh sunflower oil（SO）（SO+0.02 and SO+0.05）,and the oil samples were heated under 160℃for 8 h.The acyl groups,primary and secondary oxidation compounds of the oil samples were measured every 1 or 2 h by ¹H NMR technology to monitor the oxidation changes of the oil samples.The results showed that the addition of egg yolk phospholipids slowed down the changes of fatty acyl groups and primary products in sunflower oil,indicating that egg yolk phospholipids played an antioxidant role.（4）The changes of iodine value（IV）and peroxide value（POV）of SO,SO+0.02 and SO+0.05 during heating and the acid value（AV）after heating for 8h were measured.Headspace solid-phase microextraction combined with gas chromatography-mass spectrometry（HS-SPME-GC-MS）was used to determine the generation of volatile compounds in the three oil samples after heating for 8h.The results showed that in the oil samples added with egg yolk phospholipids,the changes of IV and POV and the AV after 8 h of were smaller than those in SO,while the types and contents of volatile compounds,especially the characteristic compounds of frying aroma,were not significantly smaller than SO.It indicated that egg yolk phospholipids inhibited the oxidation of sunflower oil,and the addition of 0.05%（w/w）had better antioxidant effect,and maintained the fried odor of sunflower oil.（5）The polar components of fresh egg yolk phospholipids were determined by ¹H NMR,and various amino acids,two kinds of glucose and choline were found,indicating that there were Maillard reaction substrates in egg yolk phospholipids.At the same time,the results of GC-MS showed that the Maillard reaction products were detected in the samples with egg yolk phospholipids,proving that the Maillard reaction occurred in SO+0.02 and SO+0.05 under heating.（6）0.02%（w/w）α-tocopherol was added to SO+0.05（SO+0.05+T）,which was heated under 160℃for 8 h,and then also used ¹H NMR technology and chemical methods to determine the oxidation.The results showed that compared with SO and SO+0.05,the acyl groups in SO+0.05+T had the smallest change（0.22%/h）,the hydroperoxides were always increasing,and the production of secondary products was also significantly reduced.It was demonstrated thatα-tocopherol and egg yolk phospholipids played a synergistic antioxidant role during the heating process of sunflower oil.