| Large yellow croaker is one of main aquacultured seafood in China and the main aquatic product for export. It is popular due to its delicious nutritions and high medical value. However, on the one hand, the oxidative rancidity and protein degradation of fish will occue during storage time, due to higher fat, especially high unsaturated fat content. On the other hand, the major export countries of large yellow croaker, such as Japan, the USA and the European Union, have taken more stringent measures of trade in recent years. The problems concerning fish quality and safety have become one of important obstacles in international trade. Therefore, there are significant economic value and energy efficiency in studying preservation, storage and processing technique of large yellow croaker.In view of the above-mentioned facts, the large yellow croaker was studied under refrigerated storage to evaluate the qualities by the microbial, physicochemical and sensory parameter as well as texture index. The key factors such as the concentration of biological preservative, processing time and refrigerated temperature were optimized systematically for extending the shelf-life of the refrigerated large yellow croaker. Finally, the optimized parameters were found to keep the large yellow croaker good quality in refrigerated temperature with biological preservation technology. Further more, the microbiological changes during preservation course was researched. The spoilage reason caused by dominant putrefying bacteria was discussed, and the mechanism of biological preservation technology was pointed out preliminarily. The main conclusions were shown as follows.1-, The results showed that, the moisture, ash, crude fat and protein content of fish were66.82%,1.17%,13.15%and18.01%respectively, unsaturated fatty acids content of fish lipid were up to55.24%, which ratio of (n-6) polyunsaturated fatty acids and (n-3) polyunsaturated fatty acid was0.52, the ratio is close to the normal levels of cells. The essential amino acids of muscle are more reasonable, its content was about80.15mg/g, the content of the total amino acid was35.02%, the essential amino acid score was101, which was an ideal source of protein for body.During the storage, the growth of microbiological populations was retarded with the lower temperature, thiobarbituric acid (TBA) value, total volatile basic nitrogen (TVB-N), k value and the other physical and chemical parameters were also delayed, sensory quality was better. However, the changes of texture in adhesion and chewing, gum and resilience were not obvious during refrigerated storage, it might not be the suitable indicator for evaluating quality changes of large yellow croaker during the storage. The shelf-life of large yellow croaker stored were10-12d and19-21d at4℃and-2℃respectively2, In this paper, we observed that when concentration of tea polyphenols solution reached0.2%(2g/L), free radical scavenging ability of tea polyphenols solution was the best. Then, we used the tea polyphenols solution concentrations of0.1%,0.2%and0.3%to deal with large yellow croaker, and quality changes of large yellow croaker stored at4℃were observed. The results showed that tea polyphenols solution had a good effect on inhibiting the increase of TVC, at the same time, TBA, TVB-N, k value, hardness, and the other physical and chemical parameters were delayed. The higher the concentration was, and the more obvious preservation effects were. But when the concentration of tea polyphenols exceeded0.2%, the preservation effects reached saturation, and the sensory acceptance levels of fish were very poor by high concentrations of tea polyphenols. Therefore, the tea polyphenols solution of0.2%was the best preservation, it also economy and safety, the shelf life of large yellow croaker was extended by7-8days during4℃. 3,Effects of different chitosan concentrations on large yellow croaker were studied. The results showed that the change of physical and chemical parameters were delayed. The treatment group maintained a good sensory quality. And the effect was not significant when the chitosan concentration was higher. When the chitosan concentration was too high, the surface of fish remain more slime. It provided a good condition for microbial growth and reproduction during the storage time, ultimately leading to deterioration of fish quality. The concentration of1.5%chitosan solution kept the better preservation effect, the shelf life of large yellow croaker was extend of6-7days during4℃.4,The large yellow croaker was treated by0.2%tea polyphenols solution firstly and coated by1.5%chitosan solution, then was stored at4℃, the quality of large yellow croaker were research by5-days intervals. The results showed that initial sensory acceptable level of large yellow croaker treated by chitosan and tea polyphenols close to control group, the sensory side impact of tea polyphenols was counteracted by the coationg chitosan solution. TVC growth of treatment group in fish were inhibited during storage, the TVC were4.82log CFU/g at20days, it was still belong to second levels of freshness. During the storage, the conjunct preservation inhibited the increases of TVC, the changes of TBA value, pH, TVB-N, k value as well as hardness, and maintained good sensory quality. The shelf-life of large yellow croaker treated by composite preservation was extended12d comparing the control group.5,Selective culture methods and PCR-DGGE technology were applied in large yellow croaker by complex preservation. In refrigerated temperature, these dominant spoil bacteria mainly were Bacteria producing H2S, Photobacterium phosphoreum and Lactic acid bacteria, the next were Pseudomonas sp. and Enterobacter. They were inhibited by compound preservative, compound preservative extended the shelf life of large yellow croaker by inhibiting the growth of the dominant putrefying bacteria during refrigerated time. Meanwhile, the dominant putrefying bacteria of large yellow croaker under compound preservative treatment were mainly Lactic acid bacteria, Bacteria producing H2S and Photobacterium phosphoreum. Bacterias in fish were analysised by PCR-DGGE method during storage phase change, DGGE profiles showed that Aeromonas, Pseudomonas sp and Shewanella bacteri and was the dominant putrefying bacteria of large yellow croaker, which was similar to selective culture results. |
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