In this study, non-targeted metabolomics based on ultra-high performance liquid chromatography-mass spectrometry (UPLC-MS) was used to explore the change in the metabolite profile in giant salamander meat during cold storage at 4 ℃ (0, 2, 4 and 8 days). The differences within each group and between the 0- and 2-day storage groups were small, while the intra- and inter-group differences between days 4 and 8 of storage were large. As the storage time increased, the number of differential metabolites between adjacent groups increased gradually. Using the variable importance in the projection (VIP) value of the first principal component in the partial least squares discriminant analysis (PLS-DA) model greater than or equal to two, and the P-value of t-test less than or equal to 0.001 as criteria, a total of 125 differential metabolites were selected, including organic acids and their derivatives (17), esters and their derivatives (53), amino acids and their derivatives (25), nucleotides and their derivatives (13), alcohols (3) and other compounds (14). The abundance of most of the metabolites decreased significantly on 8 day of storage (P < 0.05). The cumulative change in the abundance of the organic acids and their derivatives (A1) had a similar trend to that of the amino acids and their derivatives (A3), that is, there was a small increase from days 0 to 2, a small decrease from days 2 to 4, and a rapid decrease from days 4 to 8. The cumulative changes in the abundance of the esters and their derivatives (A2) as well as the nucleotides and their derivatives (A4) showed a downward trend, but the abundance of the esters and their derivatives (A2) decreased slowly from days 0 to 4 and rapidly from days 4 to 8, while the abundance of the nucleotides and their derivatives (A4) showed a linear downward trend. The results of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis and Pearson correlation analysis showed that histidine metabolism, arginine and proline metabolism, arginine biosynthesis, lysine degradation, taurine and sub taurine metabolism, valine, leucine and isoleucine biosynthesis, and aminoacyl-tRNA biosynthesis and other metabolic pathways had a good correlation with the changes of giant salamander meat quality; at the same time, creatine, L-histidinol, L-glutamate, histidine, ornithine, L-arginine and phytosphingosine could be used as potential markers for evaluating the quality change of giant salamander meat during cold storage. The results of this study provide a theoretical basis for understanding postmortem metabolism in giant salamander muscle and for quality control of giant salamander meat during cold storage.

To explore the microbial community and special spoilage bacteria in pallet-packaged giant salamander (Andrias davidianus) meat during cold storage, the total volatile basic nitrogen (TVB-N) content and total bacterial count of giant salamander meat were evaluated after different refrigeration durations (0, 2, 4, 6 and 8 d) at 4 ℃ , and the changes and diversity of the microbial flora were explored by Illumina MiSeq sequencing. The results showed that the total count of bacteria and TVB-N levels in salamander meat showed an upward trend during refrigeration, and exceeded the safety limit on the 6th and 8th days, respectively. The results of high-throughput sequencing showed that the microbial abundance in giant salamander meat decreased with storage time. The dominant phyla of bacteria were Bacteroidetes and Firmicutes during the early storage period (0 and 2 d), and Proteobacteria during the middle (4 d) and late (6 and 8 d) storage periods. The dominant genera were Bacteroidetes and Faecalibacterium during the early storage period, and Pseudomonas, Aeromonas, Hafnia-Obesumbacterium and Serrella during the middle and late storage periods. Principal coordinate analysis showed that there were great differences in microbial community structure between the three storage periods, and the degree of superposed interpretation of the two principal coordinates was 80.92%. Linear discriminant analysis effect size (LEfSe) analysis showed that Proteobacteria, Actinobacteria, Bacteroides, Fibrinobacteria and Firmicutes were the major bacterial phyla that significantly changed during cold storage. The main bacterial genera that significantly changed during storage were Pseudoxanthomonas, Bauldia, Serrella, Acinetobacter, Aeromonas, Pseudomonas, ambiguous_taxa, Hafnia-Obesumbacterium, Rikenellaceae_RC9_gut_group, Prevotella_9, Bacteroides, Fibrobacter, Lachnospira, Faecalibacterium, Clostridium_sensu_stricto_1. Evolutionary analysis showed that there was a strong correlation between the succession of microflora and storage time. Overall, the dominant spoilage microorganisms in giant salamander meat are Pseudomonas, Aeromonas, Hafnia-Obesumbacterium and Serrella. This study provides a reference for the targeted bacteriostasis of spoilage bacteria in giant salamander meat during cold storage to extend its shelf life in the future.