Changes of Metabolites and Related Flavors in Mandarin Fish (Siniperca chuatsi) during 24 h of Refrigeration

The study aimed to investigate the changes of metabolites and related flavors in the muscle of fresh mandarin fish (Siniperca chuatsi) during 24 h of postmortem refrigeration at 4 ℃ by ultra performance liquid chromatography coupled with high resolution mass spectrometry (UPLC-HR-MS). Non-targeted metabolomics was applied for multivariate statistical analysis, metabolic pathway analysis and related flavor calculation of the differential metabolites after 0 versus 2, 4, 6, 9, 12 and 24 h. The results showed that 33 differential metabolites were identified, including 3 organic acids and derivatives (amino acids and organic acids); 8 organic heterocyclic compounds (purines, purine derivatives and lactones); 7 nucleosides, nucleotides and analogues (purine nucleosides, purine nucleotides and pyrimidine nucleotides); 9 organic oxygen compounds (carbohydrates and their conjugates, as well as a small amount of alcohols); 4 lipid and lipid molecules (fatty alcohols and fatty acid esters); 1 flavonoid glycoside; and 1 alkaloid or its derivatives. Through metabolomic pathway analysis (MetPA) and Kyoto Encyclopedia of Genes and Genomes analysis, 9 key differential metabolites were identified including succinic acid, L-aspartic acid, 5’-adenosine monophosphate (AMP), inosine, adenyl succinic acid, xanthine, 5’-guanosine monophosphate (GMP), ribose 5-phosphate, and hypoxanthine. The major metabolic pathway involving these key differential metabolites was purine metabolism, followed by L-aspartic acid metabolism, glutamic acid metabolism and the tricarboxylic acid (TCA) cycle. By calculating the relative taste activity value (RTAV) and relative equivalent umami concentration (REUC), it was found that umami amino acids such as L-aspartic acid together with AMP and GMP mainly contributed to the taste of fresh mandarin fish, while the major taste compounds were AMP and GMP after 2–6 h of refrigeration and GMP after 9–24 h. Overall, the taste of fresh mandarin fish was best. During 24 h of storage at 4 ℃, the umami of mandarin fish decreased due to a decrease in L-aspartic acid content, but it increased between 9 and 24 h due to the accumulation of GMP.