Identification And Formation Mechanism Of Off-flavor From Duck Egg Yolk During Heat Treatment

Egg is an important ingredient in many food products as it combines nutritional and functional characteristics.An unpleasant off-flavor produces during heat treatment and has brought an undesirable effect on the sensory properties of the products.Research investigating the volatile compounds in many egg products have been reported.However,fewer studies have attempted to characterize the key aroma compounds of off-odor in the heat-treated eggs.The oxidation of lipid or protein result in the formation of volatiles(such as aldehydes,alcohols,ketones and so on),which produce unpleasant flavor in food products.Lipids and proteins of egg yolk are mainly found in low density lipoprotein(LDLs)particles.LDLs are spherical nanoparticles with a lipid core of triglycerides and cholesterol esters in a liquid state surrounded by a monofilm of phospholipids and proteins.These compositions and the special structure can prevent the unsaturated fatty acid from being oxidized when the structure of LDLs particles is not destructed.However,the sability of LDL particles,the effect of oxidation on off-flavor in thermal egg yolk,and the relationship between heating temperature and the formation of off-flavor compounds are still not clear.Therefore,the duck egg yolk were used to investigate the stability of LDLs spherical particles and the role of oxidation of lipids and proteins in yolk on the formation of off-odor.The data from this study revealed formation mechanism of off-flavor.The results were stated as follows:First of all,the volatile compounds of thermal egg yolk gels were analyzed by sensory evaluation,gas chromatography-mass spectrometry(GC-MS),GC-olfactometry-MS(GC-O-MS).Hexanal,(E)-2-octenal,(E,E)-2,4-decadienal,1-octen-3-ol and 2-pentylfuran were identified as the important components for the off-odor.There were no remarkable differences between the recombination model with five aroma compounds(4.38±0.35)and EY gel(4.97±0.05).The data confirm that these five substances were the key off-odor ingredients of egg yolk.The major aroma profile of the off-flavor from egg EY gels included strong fatty,nutty,and mushroom aromas.Secondly,in order to investigate the effect of oxidation of lipid and protein on the information of off-flavor compounds in egg yolk,the oil-off ratio(%)and oxidation indicators of fatty acid and protein were analyzed.The concentrations of five key off-flavor compounds in egg yolk with different heating temperature were detected by GC-MS.When the temperature increased to about 65?,the sensory off-flavor score and the key volatile compounds improved significantly.The content of oxidation products of fatty acid and protein was highest when temperature reached65?-70?.The oil-off ratio was 3.16±0.01% when heating at 65?.In addition,the stability of LDL particles and change of the structure of LDL lipoprotein and phospholipids were studied.When the temperature increased to 60?,LDL lipoprotein aggregated and the structure of phospholipids changed.The content of ?-sheet increased and the ?-helix structures of LDL lipoprotein decreased at 60?.Also,the LDL spherical nanoparticles structure disrupted and free lipids leaked out at 65?.Finally,addition of the succinic anhydride led to modification of yolk protein.The relationship between the succinylation and the content of off-flavor volatiles by sensory evaluation and GC-MS was analyzed.Succinic anhydride treatment decreased the off-odor of heated egg yolk,which resulted from inhibition of the thermal aggregation of LDL lipoprotein,decrease of the leakage of lipids and the oxidation of lipids,and maintenance of stability of LDL particles.