Study On Flavor Formation Of Soup Based On The Interaction Between Structural Protein And Flavor Compounds During Stewing Yellow-feathered Chickens

The traditional recipe for Chinese chicken broth creates a popular taste of particular umami and aroma.Due to a long feeding period for yellow-feathered chicken,the muscles require a long cooking time to be tender and form the strong and rich meat aroma.However,the related research is limited,and meat quality change and flavor formation of chicken broth is not fully elucidated.Moreover,the key volatile compounds of meat product have a certain hydrophobic property and a low solubility in the aqueous phase.Thus,the key volatile compounds tend to be interacted with the main macro molecules of broth matrix and then affect the sensory evolution.Meanwhile,fats have also a capacity to bind flavor compounds,influencing the sensory evolution.The objectives of this thesis are to investigate the effect of a long stewing time on the meat quality and flavor formation of chicken broth made by stewing Chinese yellow-feather chickens,and to identify the structural protein in broth.On this basis,underlying mechanisms involved in an in vitro simulation of structural protein interacting with aroma conpounds was performed,and the role of fats in emulsion-flavor binding was assessed.Finally,the role of structural protein in flavor formation of chicken broth was verified through adding protein into chicken soup.The research content and results are as follows:1.Evaluation of the taste-active and volatile compounds in stewed meat from the Chinese yellow-feather chicken breedChicken meat is the main provider of flavor precursors of chicken soup,thus evaluating the changes in flavor of stewd chicken meat was to provides the basis for revealing the flavor formation of chicken soup.Effects of stewing time(1,2,and 3 h)on the levels of taste-active and volatile compounds of chicken meat were measured.The flavor characteristics of the stewed yellow-feather chicken meat were assessed with sensory evaluation and electronic nose.Results showed that increasing stewing time significantly decreased the contents of taste components such as free amino acids(FAAs),5'-nucleotides,minerals.Inosine 5'-monophosphate and chloride were the major umami-related compounds in stewed meat and decreased significantly during stewing.The taste active values(TAVs)of the equivalent umami concentration(EUC)decreased from 283.2 to 38.7 after 3 h of stewing.In contrast,increasing stewing time improved aroma levels.The volatile compounds mainly included pentanal,hexanal,heptanal,octanal,(E)-2-octenal,nonanal,(Z)-4-decenal,decanal,(E,E)-2,4-decadienal 1-pentanol,and 1-octen-3-ol.With increased stewing time,aldehydes significantly decreased(P<0.05),whereas alcohols significantly increased(P<0.05).The high intensity aroma after 2 h of stewing could be attributed to 1-pentanol and 1-octen-3-ol.The aroma scores of the chicken meat were at maximum after stewing for 3 h.The overall flavor characteristics tended to stabilize after 2 h of stewing.In general,stewing improved the aroma but decreased the taste components in the chicken meat,especially within the first 2 h.The data herein not only provides insight into the changes in odor and taste of chicken meat during cooking,but also guidelines for improving the stewing process.2.Influence of stewing time on the texture,ultrastructure,and in vitro digestibility of meat from the yellow-feathered chicken breedThis study assessed the influence of stewing(1,2 or 3 h)on the texture,ultrastructure,and in vitro digestibility of meat from the yellow-feathered chicken,which is a popular broiler chicken in Asia.Results indicated that longer stewing considerably increased cooking loss of the chicken carcass and tenderness of thigh meat After 3 h of stewing,protein digestibility decreased from 90.5%to 80.3%and fiber diameter decreased by 8.63?m.The shear force value of breast meat decreased from 32.34 N at 1 h to 10.29 N at 2 h,and then increased to 39.98 N at 3 h.The texture profile of breast meat remarkably decreased during stewing.Moreover,increased stewing time induced longitudinal and transversal shrinkage of muscle fibers and the degradation of the myosin heavy chain(MHC).These findings suggested that prolonged stewing(3 h)resulted in decreased meat qualities,based on the changes in cooking loss,digestibility and texture profile,but that stewing for 2 h increased thigh and breast tenderness.Producers could utilize this information to stew yellow-feathered chicken meat with desirable qualities.3.Characteristic flavor of traditional soup made by stewing Chinese yellow-feather chickensThe present study investigated the effects of stewing time(1,2,3 h)on the principal taste-active and volatile compounds and the overall flavor profile of traditional Chinese chicken soup by measuring the contents of Free Amino Acids(FAAs),5'-nucleotides,minerals and volatile compounds and by evaluating the taste and aroma profiles using an electronic nose,an electronic tongue and a human panel.Results showed that the major umami-related compounds in the chicken soup were inosine 5'-monophosphate(IMP)and chloride,both of which increased significantly(P<0.05)during stewing.The taste active values(TAVs)of the equivalent umami concentration(EUC)increased from 4.08 to 9.93(P<0.05)after stewing for 3 h.Although the FAA and mineral contents increased significantly(P<0.05),their TAVs were less than 1.The volatile compounds were mainly hexanal,heptanal octanal,nonanal,(E)-2-nonanal,(E)-2-decenal(E,E)-2,4-decadienal,1-hexanol and 2-pentyl furan.With the prolonged stewing time,the aldehydes first increased and then decreased significantly(P<0.05),while 1-hexanol and 2-pentyl furan increased steadily(P<0.05).The aroma scores of the chicken soup reached the maximum after stewing for 3 h.The discrepancy in overall flavor characteristics tended to stabilize after 2 h of stewing.In general,stewing time has a positive effect on improving the flavor profiles of chicken soup,especially within the first 2 h.In addition,flavor substances were migarated from chicken meat to soup during stewing,and changes in the structure of the chicken influenced the magaration of taste substances.4.Identificatiron and characterization of the proteins in broth of stewed traditional Chinese yellow-feathered chickensSoups and broths are popular in the world due to their nutrition and flavor,and flavor compounds tend to be bound by the proteins in the soups and broth,influencing the flavor perception.Thus,identification of the major proteins in meat-based broth may present a basis for understanding protein adsorption of flavor compounds.The present study aimed to identify the major proteins in traditional Chinese chicken broth and to describe the structural changes of proteins during stewing(1,2,or 3 h).As stewing time increased,protein content in the broth significantly increased.SDS-PAGE indicated that the macro-molecule proteins(>10 KDa)in the broth were mainly gelatin and actin and that the micro-molecule proteins fractions(<10 KDa)increased substantially.The gelatin had an ordered structure even after 3 h of stewing,as seen by circular dichroism(CD)spectroscopy.The presence of reactive sulfhydryl grops increased remarkably with stewing time.The surface hydrophobicity of the proteins significantly increased withrin 2 h then deceased slightly after 3 h.The intermolecular crosslinks,as indicated by dispersion index increased remarkably,consistent with the result of atomic force microscopy(AFM,5 which together suggested that protein aggregation increased during stewing.These findings suggested that gelatin was the structural protein in the broth system and that intermolecular crosslinks functioned to maintain the broth system5.Thermal degradation of gelatin enhances its ability to bind aroma compounds:investigation of underlying mechanismsThe objective of this study was to investigate the ability of thermally degraded gelatin to bind aroma compounds such as nonanal,2-heptanone,1-octen-3-ol,hexyl acetate and 2-pentyl furan.Binding ability was strongly affected by changes in thermal-degradation products,the structure and surface hydrophobicity ofproteins during heating(1-4 h),through analyzing the results of GC-MS,SDS-PAGE,LC-MS/MS,free amino acid,circular dichroism spectra,Fourier transform infrared(FT-IR)spectroscopy,zeta potential,and surface hydrophobicity.Binding ability of gelatin to aroma compounds increased from 12-14%to 29-35%after heating for 4 h During the first 1 h of heating,there was no significant increase in surface hydrophobicity due to the effects of degradation and unfo lding of protein and protein aggregation,thus the increase in zeta potential and free pro line caused by gelatin degradation was likely responsible for the increase in binding ability through electrostatic interactions and hydrogen bonding,respectively.After heating for 2-3 h,unfolding and degradation of gelatin played a leading role in exposure of more hydrophobic amino acids on the protein surface,suggesting an enhancement of binding ability due to hydrophobic interaction Moreover,effects of zeta potential,exposure of more pro line on the interface,and increase in free pro line caused by protein degradation could also enhance the affinity of gelatin to flavor co mpounds.After heating for 4 h exposure of more hydrophobic amino acids and free proline continued to increase flavor-binding ability,although zeta potential decreased.The results provided insight into improving flavor profile of meat matrix.6.Effect of thermal degradation of gelatin on binding capacity of gelatin oil-in-water emulsion to flavor compoundsFats have the potential to interact with flavor compounds.Effect of fat content on flavor binding to gelatin emulsion was investigated,to identify the role of fat in emulsion-flavor binding.Moreover,the effect of thermal degradation of gelatin on binding capacity of oil-in-water emulsionto flavor compounds was evaluated through analyzing the results of particle size and confocal laser scanning microscopy.Then,identification of the role of gelatin in the retention of chicken soup flavor during stewing was performed by adding 0.1%gelatin into chicken soup.The results showed that with fat content increased(0.25%-20%),flavor binding capacity of emulsion increased from approximately 10%to more than 90%.With increasing heating time,the particle size of gelatin emulsion significantly decreased.,which was consistent with the result of confocal laser scanning microscopy.With the extension of heating time,the binding ability of gelatin emulsion to flavor compound increased when hydrophobicity of compounds is more than 3,and decreased whenhydrophobicity of compounds is less than 3.After adding gelatin,the binding ability of chicken soup to flavor compounds can be improved.Thu,gelatin,as a major protein in chicken soup,have the potential to improve soup flavor.In summary,gelatin,as a structural protein in chicken broth,can improve the flavor profile of chicken broth due to its binding ability with flavor compounds.In addition,change in the emulsification property of gelatin caused the change in fat in chicken broth during the heating process,which in turn affected the sensory evaluation of chicken broth.