Study On Peptides Variation Of Chicken During Postmortem Aging And Refrigerated Storage

Chicken is favored by most people for its good texture,rich nutrition,and excellent meat quality.With the development of social economy and the popularization of healthy food concept,requirements of people for chicken quality are also increasing.However,at present,the research on chilled chicken mainly focuses on tenderness of meat and extension of product shelf life.Then it is not clear about changes of peptides during postmortem aging and refrigerated storage.Trichloroacetic acid(TCA)was used to precipitate proteins,then small molecular polypeptide fragments were left in the supernatant.Changes of polypeptides and free amino acids in the extract were determined to assess the quality of meat.The experiment was designed to start from the quality changes of chicken during postmortem and refrigerated storage.The conventional cooling and rapid cooling methods were used to change the pre-temperature conditions of samples.Then the changes of quality indicators,contents of polypeptides and free amino acids,and activities of myofibrillar-related degradation enzymes of chicken during postmortem were determined.Works and results are as followed.1.Changes of quality indicators during postmortem aging and refrigerated storage.Firstly,the changes of pH value,TVB-N value,total bacterial count,tenderness,and sensory evaluation score of chicken were studied.The results showed that at day 0.5 after slaughter,the pH value of conventional cooling group and rapid cooling group reached its minimum value for 5.65 and 5.72,respectively.And the carcass entered stiffness maximization stage of postmortem.Then,the pH value gradually increased with refrigeration time,and there were significantly differences(p<0.01 or p<0.05)between the two groups.During storage,the TVB-N values increased continuously,and the increase rate of conventional cooling group accelerated after day 7.The TVB-N content of conventional group was always higher than that in the rapid group,and the difference was extremely significant at the end of storage(p<0.01).The total bacterial counts of chicken during storage also showed an upward trend: conventional cooling group increased significantly after 7 days(p<0.05),and it was significantly higher than that in the rapid cooling group after 5 days of cold storage(p<0.01).The results of shear force value showed that both groups increased first and then decreased during storage.The shear force value reached its maximum at day 0.5,and then decreased rapidly(p<0.05).Sensory evaluation scores showed a downward trend,and the differences were not significant(p>0.05).Based on the changes of various indicators,it can be concluded that chicken entered the post-mortem stiffness period at day 0.5,and at that time the quality of meat was the worst.With the refrigerated time,samples in conventional cooling group were corrupted after 7 days,while the samples could be stored for 10 days in rapid cooling group.Thus,it’s indicated that rapid cooling after slaughter was conducive to maintaining chicken quality and extending the shelf life of meat.2.Changes of peptides and free amino acids during postmortem aging and refrigerated storage.The content and molecular weight distribution of chicken peptides,the total content and composition of free amino acids were determined.Then the correlation between peptide content and chicken meat freshness and tenderness was analyzed.The results showed that the peptide content decreased to the minimum at day 0.5,and then gradually increased.From day 3 to the end of storage,the peptide content of chicken in the conventional cooling group was significantly higher than that in the rapid cooling group(p <0.01).The total content of free amino acid reached changed in a manner opposite to peptide content.The components of peptides with molecular weight less than 1 kDa in chicken accounted for the highest proportion,followed by those with molecular weight between 1 kDa and 5 kDa and those with molecular weight greater than 5 kDa.The contents of Glutamate(Glu),Proline(Pro),Asparagine(Asp),Lysine(Lys)and Leucine(Leu)were higher than other free amino acids.Then,the peptide content in two groups was significantly or extremely significantly(p<0.05 or p<0.01)positively correlated with the total bacterial counts,pH value and TVB-N value.Therefore,the peptide content can be used to evaluate the freshness of chicken.3.Changes in myofibrillar and related enzyme activities during postmortem aging and refrigerated storage.The muscle fiber structure,MFI index,and the activities of calpain,collagen protease and trypsin during postmortem aging and refrigerated storage of chicken were determined.The results showed that,the muscle fibers of chicken samples were straight from the early stage through the observation of scanning electron microscope,and it was gradually transformed into destruction,as the water holding capacity of muscle was reduced.At the same time,the MFI values in two groups increased by 66.23% and 66.43%,respectively.The MFI value of the conventional cooling group was generally higher than that of the rapid cooling group,and it was negatively correlated with the shear stress value.From the activities results of three endogenous proteases,calcium activated enzyme activity decreased significantly(p<0.05),collagen protease activity and trypsin activity showed a slow upward trend.In the early stage of postmortem,calpain was mainly involved in the early age of protein degradation,while collagen protease and trypsin played a role in the late stage of preservation.