Formation Mechanism Of Key Odorants Derived From Lipid Oxidation And Degradation In Chinese Mitten Crab (Eriocheir Sinensis)

Chinese mitten crab(Eriocheir sinensis)is one of the popular and delicious aquatic foods due to its unique odor.Aldehydes,derived from lipid thermal oxidation and degradation,are the major odorant sources during the crab steaming.However,up to date,it is largely unclear for the formation mechanism of the key odorants for E.sinensis.This study was firstly designed to compare the content changes of lipid class and fatty acid composition in different edible parts of E.sinensis before and after steaming.The relationship between the formation of key odorants and the changes of lipids will be found for the different edible parts,and the major edible parts,lipid source and fatty acid compositions of key odorants would be preliminarily identified.Based on this,the lipid thermal oxidation and degradation conditions were optimized,then,the possible pathways of key odorants formation by the lipid oxidation and degradation will be validated by the comprehensive analysis of the thermal oxidation and degradation for the following lipids,including different purified lipid fractions from crabs,artificially modified lipids(PE with the specific fatty acids)and pure fatty acids.The research findings were mainly as follows:1.The major edible parts of E.sinensis and their key odorants were researched.(1)The edible proportion of gonad and hepatopancreas parts of female and male E.sinensis were approximately 35~50%.Gonad with high protein and hepatopancreas with high lipid,which were regarded as the main flavor precursors of E.sinensis.(2)The odor profiles of gonad and hepatopancreas of E.sinensis were evaluated by sensory analysis and electronic nose,the results indicated that the hepatopancreas and gonad of female E.sinensis both contributed to the overall aroma of “XieHuang”,while only the hepatopancreas of male E.sinensis played an important part in the overall aroma of “XieGao”,so gonad part of male E.sinensis was not considered as main research object for the further studies.(3)Key odorants of steamed female E.sinensis gonad were trimethylamine,3-methylbutanal,heptanal and benzaldehyde.Key odorants of steamed female E.sinensis hepatopancreas were trimethylamine,3-methylbutanal,pentanal,hexanal,heptanal,nonanal,furfural,1-octen-3-one and 2-pentylfuran.Key odorants of steamed male E.sinensis hepatopancreas were trimethylamine,2-methylbutanal,3-methylbutanal,hexanal,heptanal,nonanal,(E,E)-2,4-heptadienal,benzaldehyde,2-nonanone and 2-pentylfuran.(4)Key odorants of gonad and hepatopancreas of both female and male E.sinensis were mainly including aldehydes and ketones,which were repoted as the thermal oxidation and degradation products of lipids,so it is important for lipids or fatty acids to the formation of odorants in E.sinensis.Above all,the major edible parts of E.sinensis were gonad and hepatopancreas od female E.sinensis,and hepatopancreas of male E.sinensis.Based on the results of GC-O,hepatopancreas of male E.sinensis with the highest overall odor intensity,were considered as the main research object for the further studies.2.Important lipids and fatty acids in main edible parts of stesmed E.sinensis were determined.(1)The total lipid contents of gonad and hepatopancreas of both female and male E.sinensis were decreased after steaming,compared with raw samples,steamed female E.sinensis gonad showed decrease in contents of TG,DG and FFA,while hepatopancreas of both female and male E.sinensis showed decrease of PE and FFA contents.(2)The important fatty acids to the key odorants in female E.sinensis gonad were C18:1n-9c,C18:2n-6c,C18:3n-3,C20:5n-3,C22:6n-3 of TG type,C14:1,C16:1,C17:1n-7,C18:1n-9t,C18:1n-9c,C24:1n-9,C18:3n-6,C20:3n-6 of DG type,and C14:1,C16:1,C18:1n-9t,C18:1n-9c,C18:2n-6c of FFA type.(3)The important fatty acids to the key odorants in female E.sinensis hepatopancreas were C16:1,C18:1n-9t,C18:2n-6c,C20:2n-6,C18:3n-6,C20:3n-6,C20:4n-6,C20:5n-3,C22:6n-3 of PE type,and C14:1,C16:1,C18:1n-9t,C18:1n-9c,C18:2n-6c,C18:3n-3,C20:3n-6,C20:5n-3 of FFA type.(4)The important fatty acids to the key odorants in male E.sinensis hepatopancreas were C16:1,C22:1n-9,C18:2n-6c,C20:4n-6,C20:5n-3,C22:6n-3 of PE type and C17:1n-7,C18:1n-9c,C22:1n-9,C18:2n-6c of FFA type.3.Lipid thermal oxidation and degradation conditions were optimized based on the PE and FFA separated form male E.sinensis hepatopancreas.(1)Combining with the results of MMSE-GC-MS-O,as for PE,most of the key odorants reached their maximum at 175 °C for 60 min,while for FFA,most of the key odorants reached their maximum at 175 °C for 90 min.(2)Under the optimized lipid thermal oxidation and degradation condition,a total of 22 odorants were detected in PE,which including 7 key odorants(hexanal,heptanal,nonanal,(E,E)-2,4-heptadienal,benzaldehyde,2-nonanone and 2-pentylfuran)in male E.sinensis hepatopancreas,and 19 odorants were detected in FFA,which including 6 key odortants(hexanal,heptanal,nonanal,(E,E)-2,4-heptadienal,benzaldehyde and 2-pentylfuran).(3)Odorants of both PE and FFA under the optimized condition showed higher contents than under steaming condition,especially for the contents of aldehydes and furans,which was mainly due to the thermal oxidation and degradation of unsaturated fatty acids oxidation during thermal processing.4.Key lipid molecules of PE and FFA separated for male E.sinensis hepatopancreas were examined under the optimized lipid thermal oxidation and degradation conditions.(1)Under the optimized lipid thermal oxidation and degradation conditions,the fatty acid contents of both PE and FFA showed a significant reduction after thermal processing,which showed a consistent tendency with PE and FFA of male E.sinensis hepatopancreas under steaming condition.It illustrated that PE and FFA play an important part in the odorants formation of male E.sinensis hepatopancreas.(2)Under the optimized lipid thermal oxidation and degradation condition,the content of PE(18:2/20:4),PE(18:0/22:6),PE(16:0/20:5),PE(18:0/20:4),PE(16:0/22:6),PE(16:0/20:4),PE(16:0/18:2),PE(20:5/18:2)and PE(18:0/18:2)decreased significantly,and 4 acyl chain unsaturated fatty acids of PE molecular showed remarkable changes,which consistent with previous results,so these PE molecules could regarded as the important flavor precursors of key odorants in male E.sinensis hepatopancreas.(3)The contents of C17:1n-7?C18:1n9-c?C22:1n-9 ? C18:2n-6c decreased under the optimized lipid thermal oxidation and degradation condition,which also could regarded as the important flavor precursors of key odorants in male E.sinensis hepatopancreas.5.The key odorants derived from key lipid molecules standards under optimized lipid thermal oxidation and degradation conditions were determined.Under optimized lipid thermal oxidation and degradation conditions,hexanal was derived from PE(16:0/18:2),PE(18:0/18:2),PE(18:0/20:4),PE(16:0/22:6),PE(18:0/22:6),FFA(C17:1),FFA(C18:1)and FFA(C18:2).Heptanal was derived from PE(16:0/18:2),PE(18:0/18:2),PE(18:0/20:4),PE(16:0/22:6),FFA(C17:1),FFA(C18:1),FFA(22:1)and FFA(C18:2).Nonanal was derived from PE(16:0/18:2),PE(18:0/18:2),PE(18:0/20:4),PE(16:0/22:6),PE(18:0/22:6)and FFA(22:1).(E,E)-2,4-heptadienal was derived from PE(16:0/22:6)and PE(18:0/22:6).Benzaldehyde was derived from PE(16:0/18:2),PE(18:0/18:2),PE(18:0/20:4),PE(16:0/22:6)and PE(18:0/22:6).2-Nonanone was derived from PE(16:0/18:2)and PE(18:0/18:2).And 2-pentylfuran was derived from PE(16:0/18:2),PE(18:0/18:2),PE(18:0/20:4),FFA(22:1)and FFA(18:2).6.The contribution of key lipid molecules standards to the key odorants in male E.sinensis hepatopancreas under steaming condition was measured.(1)The addition of five PE molecules in male E.sinensis hepatopancreas could significantly increase the contents of key odorants(hexanal,heptanal,nonanal,benzaldehyde,(E,E)-2,4-heptadienal,2-nonanone and 2-pentylfuran),especially for PE(18:0/18:2),PE(18:0/22:6),and then PE(16:0/18:2),PE(18:0/20:4).While the addition of four FFA molecules in male E.sinensis hepatopancreas could significantly increase the contents of above key odorants except for benzaldehyde and 2-nonanone.(2)The content of aldehydes and ketones derived from FFA were higher than PE,it was mainly due to that,during thermal processing,part of fatty acids combined with PE molecules degraded into odorants directly,while others firstly degraded into FFA,and then into odorants.So it was easier for FFA to degraded into odorants than PE.(3)When Sn-2 of PE combined with C18:2,the PE molecules with C16:0 in Sn-1 showed stronger thermal oxidation and degradation ability than C18:0 in Sn-1,while when Sn-2 of PE combined with C22:6,there showed adverse tendency.However,when the fatty acid in Sn-1 were the same,the lipid thermal oxidation and degradation ability increased with the increase of fatty acid chain and their degree of unsaturation,especially for the formation of aldehydes,ketones,alcohols and furans,which were reported as the thermal oxidation and degradation products of unsaturated fatty acids.