| Large yellow croaker is a commercially important marine-cultured fish species in China. It’s one of the most famous aquaculture products in China with high economic value and development potential. The products of large yellow croaker are marketed in China and abroad because of its delicious taste and high nutritional value. However, due to its high level of nutrient content and high activities of endogenous proteases, fish is an easily perishable product. Postmortem changes including lipid oxidation, protein degradation as well as the growth of microorganisms. Thus, the goal to search a new biological preservative to delay the deterioration and keep a good quality of large yellow croaker during storage is important and interesting. Lysozyme and Nisin are kinds of safe and efficient preservatives have being frequently used in fresh keeping in aquatic product. In view of the above-mentioned facts, the large yellow croaker was studied under refrigerated storage. On the one hand, the objective of our study was to evaluate large yellow croaker qualities by the microbial, physicochemical and sensory analyses as well as the change of myofibrillar proteins. We aimed to provide important theoretical basis for the Lysozyme and Nisin preservation mechanism. On the other hand, the combined effects of Lysozyme and Nisin on volatile compounds and protein degradation of large yellow croaker were studied during the refrigerated storage. The main results were showed as following:1. The macronutrient composition of large yellow croaker was72.24%moisture,18.82%protein,4.63%fat and1.26%ash. Interestingly, large yellow grouper is a fish with relative low-fat content and relative high-protein content, which is of great interest to the consumer. The quality changes of the fillet of large yellow croaker during refrigerated storage were observed. The results indicated that Lysozyme and Nisin could inhibited the increase of TVC, kept at the relative low TBA, K values and pH values, delayed the degradation of mRNA of Cathepsin L gene, enhances the quality and delayed the other physical and chemical parameters. In addition, the results also showed that Lysozyme and Nisin could slow down the decrease of emulsion, emulsification stability, total contents of sulfhydryl and Ca2+-ATPase activities of the myofibrillar protein significantly, thus, delaying the degradation and degeneration of myofibrillar protein.2. The volatile flavor compounds in fresh large yellow croaker flesh were extracted by head space solid phase microextraction (HS-SPME) technique and determined by gas chromatography-mass spectrometry (GC-MS). The extraction conditions were optimized: extraction head (CAR-PDMS,75μm), sample volume8mL, ionic strength (NaCl concentration of0.256g/mL), extraction time50min at60℃. Totally39kinds of volatile flavor components were identified successfully. They were alkanes, aldehydes, alcohols, ester, ketones, N-, S-, O-containing compounds and heterocyclic compounds with relative concentration of19.91%,16.5%,18.01%,38.2%,0.6%, and6.78%, respectively. The flavor characteristics of the different volatile compounds were analyzed. Eight volatile compounds were considered as good indicators for evaluating spoilage in large yellow croaker fillet during refrigerated storage. Next, we assessed the effect of Lysozyme and Nisin on the fillets freshness by comparing the difference of the relative concentration of these potential spoilage markers between the control group and the treated group. Significant difference was obtained between them. The results suggested that Lysozyme and Nisin can suppress the lipid oxidation and affect the changes of volatile components at a certain extent.3. The changes in the integrity of muscle protein in large yellow croaker were studied by the proteomics technologies during refrigerated storage. The altered protein spots were excised and analyzed by MALDI-TOF-MS/MS. Seven spots were successfully identified by Mascot search with reliable protein scores in the control group, including keratin (type I cytoskeletal10), keratin (type I cytoskeletal9), keratin (type Ⅱ cytoskeletal1), sarcalumenin precursor、immunoglobulin heavy chain constant alpha1membrane bound isoform1, titin isofonn1, F-actin-capping protein subunit alpha-1. The changes in the abundance of these identified spots were correlated well with the storage time, sensory score and K value. Moreover, the identified proteins might play an important role in body functions. It is expected that these protein spots can be the biochemical markers evaluating the postmortem changes in freshness during refrigerated storage. Coincidentally, six proteins of these successfully identified spots were also detected in the treated group. The effect of Lysozyme and Nisin on the fillet quality was studied by comparing the abundance of these common identified proteins. The result demonstrated that Lysozyme and Nisin might delay the degradation of certain proteins and delay the decline in fillet quality during refrigerated storage. |
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