Study On Quality Changes And Mechanism Of Lateolabrax Japonicas During Ice Storage

During storage and transportation,Lateolabrax Japonicas are susceptible to be deteriorated due to the combined effects of microorganisms and endogenous enzymes,while with protein denaturation and degradation occur,which affects its edible value.Therefore,it is particularly important to ensure its quality and safety by clarifying the mechanism of fish quality changes during storage and transportation,and finding the appropriate regulation methods.The research contents are divided into the following three parts: the effect of epigallocatechin gallate(EGCG)and agar oligosaccharide on the quality of sea bass fillets during ice storage;RNA-seq technology was used to transcribe the sea bass muscle tissue sequencing;the molecular mechanism of quality changes of sea bass fillets during ice storage was explored by proteomics.The results are as follows:Firstly,the hardness,color and other freshness related indicators(sensory score,thiobarbituric acid value,volatile base nitrogen,total number of colonies)and the protein biochemical characteristics(myofibrillar protein content,secondary structure,sulfhydryl group and carbonyl group content)were measured in order to explore the preservation effect of EGCG and agar oligosaccharides on the quality of sea bass fillets during ice storage.The results showed that the total number of colonies and K values of the control group at the 16 th day were as high as 6.78±0.15 lg(CFU/g)and 67.37±1.75% respectively,and the sensory score was lower than 8,indicating the fillets were inedible at this time.While the samples of EGCG group(EG),the agar oligosaccharide group(AO)and the compound group can maintain value of the hardness,color,and other freshness related index.The TBA value of the control group were as high as 0.81±0.03 mg/ kg after post mortem 20 days.However,the TBA values of the EG group,AO group,and composite group did not increase significantly during storage,and were always lower than the control group.In addition,the carbonyl content of the control group was significantly higher than that of other treatment group from 12 days(P<0.05),and increased to 1.63±0.04 nmol/mg at the end period of storage,while the carbonyl content of the EG group,AO group and composite group was only 1.14±0.03,1.28±0.08 and 0.99±0.09 nmol/mg respectively.The content of protein,?-helix and sulfhydryl of the composite group samples were higher than those in the control group and other two treated group at 20 days.EGCG combined with agar oligosaccharide had better preservative effect than the single preservative,and could significantly inhibit protein denaturation and oxidation,maintain the freshness of fish fillets,and extend the shelf life to more than 20 days.This research provides a theoretical basis for the preservation mechanism of EGCG and agar oligosaccharide and a reference for the development the novel efficient biological preservatives.Secondly,the transcripts of L.Japonicas muscle tissue were obtained by RNA-seq technology to conduct a transcriptomic sequencing and bioinformatics analysis.A total of 20163 unigenes were generated by de novo assembly with an average sequence length of 1151.11 bp and N50 of 1954 bp.The unigenes were assessed by length distribution and GC content,which showed the sequencing data were of high quality and reliability.A total of 13885,13298,12111,10358,9978 and 9309 unigenes were annotated from the Nr,COG,Swiss-Prot,Pfam,KEGG and GO databases,respectively,among which 5897 were successfully annotated in all the databases.Blasted with KEGG pathway,9978 unigenes were annotated,belonging to 43 categories.Of all the categories,the number of unigenes joining in the signal transduction was the most,with 1579.The results provide rich data to understand transcriptome information of L.Japonicas and lay the foundation for further research on genetic variation,growth and development,immune response,aquaculture improvement,genomic and proteomic analysis in L.Japonicas.Thirdly,the different abundant proteins(DAP)in three groups of sea bass fillets samples-control(CK0),post-mortem storage of 8 days(CK8)and complex preservation solution treated groups(T8)-were qualitatively and quantitatively analyzed by label-free proteomics technique.Among the 708 proteins identified from the muscle samples,156 proteins were significantly differentially expressed between CK8 vs CK0 comparison group,109 proteins for T8 vs CK0 comparison group and 133 proteins for T8 vs CK8 comparison group(P<0.05).The DAP were mainly structural proteins,followed by metabolic enzyme,and some proteins related with transcription,translation regulation and protein turnover.Our results indicate that the structural proteins of fish fillets were degraded,and the expression of metabolic enzymes,transcription and translation proteins related to glucose metabolism were significantly increased(P<0.05)after 8 days of ice storage;while the expression of NADH dehydrogenase and proteins related to protein turnover were significantly decreased(P<0.05).The composite preservative treatment group may inhibit the degradation of structural proteins and the expression of glycolytic enzymes and calpain,and maintain a high level of transcription and translation regulation to achieve the preservite of fillets.Western blotting analysis results showed the expression of pgk1 protein in CK8 group was significantly higher than other two groups.The results reveal the molecular mechanism of quality changes and composite preservation of sea bass during ice storage,and provide a theoretical basis for selecting the fresh indicator proteins and novel preservatives.