Effect Of Postmortem Aging On Flavor Formation And Meat Quality Of Mutton And Its Underlying Mechanism

The series of physical and biochemical reactions that occur in muscles are the main reasons for the production of flavor precursor substances,the formation of flavor and meat quality during the postmortem aging process.AMPK is an energy receptor that responds to various reactions in the intracellular environment during this process,regulating muscle metabolism and affecting the flavor and meat quality.In this study,Dorper × Small-tailed Han sheep were taken as the experimental objects,and the M.longissimus thoracis was assigned to six aging groups at 4℃ for aging,including 0 h,24 h,48 h,72 h,96 h,and 120 h.These samples were used to analyze the changes in flavor substances and meat quality during the postmortem aging process.And,widely targeted lipidomics and metabolomics were used to comprehensively understand the evolution and transformation pathways of flavor precursors(lipid-soluble metabolites and water-soluble metabolites)during aging.Then,AMPK was used as an entry point to explore the potential mechanism of postmortem aging affecting muscle metabolism and flavor formation.And,we conducted the technical research on improving the flavor and meat quality of mutton.The main findings are as follows:The pH value of mutton decreased significantly within 24 h after slaughter(P <0.05).The shear force reached its maximum at 24 h after slaughter and then decreased continuously(P < 0.05).The cooking loss rate and L* of mutton increased significantly with the aging time(P < 0.05).a* and b* were significantly increased within 24 h(P <0.05).The results indicated that postmortem aging could effectively improve the tenderness and color of mutton.Volatile metabolomics was used to detect the volatile flavor substances of sheep during aging.A total of 206 volatile flavor substances were identified,which were divided into 14 subclasses.Hydrocarbon compounds accounted for the largest proportion,but contributed little to the flavor of mutton.Esters,heterocycles,alcohols,ketones and aldehydes followed,and most of them increased significantly during aging(P < 0.05),the maximum value was reached in the middle and late aging period(72 h ～ 120 h),indicating that postmortem aging could give mutton more intense flavor.A total of 812 lipid-soluble metabolites were identified in mutton during postmortem aging using widely targeted lipidomics,which were divided into 6subclasses,among which glycerolphospholipids accounted for the largest proportion.After screening 99 differential lipids,it was found that postmortem aging had the greatest effect on glycerolphospholipids and fatty acyl.Except that some phospholipids(PC and PI)increased significantly with the aging time,the levels of phospholipids(PE,PG,etc.)and acylcarnitine decreased significantly from 0 h to 24 h.And free fatty acids increased significantly during aging.Pathway analysis showed that differential lipids during postmortem aging of mutton were mainly involved in thermogenesis,unsaturated fatty acid biosynthesis,phospholipase D signaling pathway,glycerol phospholipid metabolism,linolenic acid metabolism and arachidonic acid metabolism,which could be used as a target pathway to study the transformation mechanism of lipid molecules during postmortem aging.A total of 891 water-soluble metabolites were identified during postmortem aging of mutton by widely targeted metabolomics,which were classified into 13 subclasses,among which amino acids and their metabolites accounted for the largest proportion.354 differential water-soluble metabolites were screened,including 12 subclasses,most of which increased significantly with aging time and reached their maximum value in the middle and late stages of aging(72 h ～ 120 h).Postmortem aging had the greatest effect on amino acids and their metabolites,followed by nucleotides and their metabolites and organic acids and their derivatives.Pathway analysis showed that the differential water-soluble metabolites during postmortem aging were mainly involved in aminoacyl t-RNA biosynthesis,amino acid biosynthesis,nucleotide metabolism,protein digestion and absorption,carbon metabolism,thiamine metabolism,etc.These pathways might be the target pathways to study the transformation mechanism of water-soluble metabolites during postmortem aging.The effect of postmortem aging on AMPK signaling pathway and muscle metabolism was analyzed with AMPK as the entry point.The results showed that the upstream factor,which regulates AMPK,Ca MKKβ decreased significantly after 24 h(P< 0.05)and LKB1 fluctuated continuously during aging and reached the maximum value at 24 h.AMPK increased significantly at 0 h ～ 24 h postmortem and then decreased significantly(P < 0.05).The downstream factor of AMPK,ACC reached the lowest value at 24 h and then increased significantly(P < 0.05);CPT1 decreased significantly with aging time(P < 0.05).The change trend of ATGL was consistent with AMPK,while HSL had no significant change during aging(P > 0.05).In addition,the expression of PPARγ at 24 h ～ 72 h was significantly lower than that at 0 h and 96 h ～ 120 h(P <0.05),while Akt was significantly higher at 0 h ～ 24 h than 48 h ～ 120 h(P < 0.05).These results indicated that the change of energy supply mode after slaughter could cause the change of AMPK through the upstream factors of AMPK,thus promoting the catabolism and releasing muscle metabolites(lipid-soluble metabolites and water-soluble metabolites)continuously during aging.Based on the above results,the regulation of AMPK by probiotics to improve the quality and flavor of mutton after slaughter showed that dietary supplementation of probiotics significantly decreased the expression of AMPK protein during the aging of sheep after slaughter(P < 0.05),and changed the composition of flavor precursors:decreased the SFA content(P < 0.05)and increased the contents of MUFA,PUFA and free amino acids(P < 0.05).Accordingly,the volatile flavor components of mutton also changed significantly,including aldehydes,alcohols,esters and hydrocarbons.It was proved that it was feasible to regulate the flavor formation and meat quality of mutton after slaughter by supplementing probiotics.