| Skipjack tuna(Katsuwonus pelamis) is the most important target species of tuna industry. Seine fishing operation has become an economic and highly efficient fishing method for catching tuna. And immersion freezing preservation has been widely used to keep the quality of skipjack tuna on board. However, due to the actual situation and limited preservation conditions on board, the quality of skipjack tuna could be easily deteriorated. According to the actual conditions on board, our research focused on the quality changes of skipjack tuna during immersion freezing preservation in cold brine. The main results and conclusions are as follows:(1) The basic characteristics of ordinary and dark muscles of skipjack tuna were analyzed. Skipjack tuna has relatively higher protein content, but lower lipid content in fish muscle. Significant differences in color, texture, and myofibrillar morphology between ordinary and dark muscles could be observed (p<0.05). Alcohols and aldehydes were the prominent compounds produced in ordinary muscle, of which the total contents were30.41%and25.56%, respectively. In contrast, the main volatile compounds detected in dark muscle were hydrocarbons and ketones, of which the totals were39.51%and21.81%, respectively.(2) Physical, chemical, and biological changes of ordinary and dark muscles of skipjack tuna during immersion freezing storage (IFS) compared with air freezing storage (AFS) were analyzed. The results showed that IFS of skipjack tuna displayed a higher freezing rate than that AFS at freezing stage (p<0.05). IFS provided a significant benefit (p<0.05) in inhibiting the increase of pH value, total volatile basic nitrogen (TVB-N), K-value, and histamine content in ordinary muscle of skipjack tuna compared with AFS, and this benefit was further confirmed by textural, colour and sensory evaluations. Due to the consistent salt uptake during the IFS process, IFS also showed a positive effect in inhibiting the proliferation of bacteria during storage compared with AFS (p<0.05). Significant quality differences (p<0.05) between ordinary and dark muscles were also found during both IFS and AFS processes. These results indicated that compared with the AFS method, IFS showed a much better effect in maintaining the quality of skipjack tuna, and could be recommended for storage of skipjack tuna on board.(3) The different catching time on quality changes of skipjack tuna during immersion freezing and preservation on board in cold brine were analyzed. The result revealed that salt uptake of skipjack tuna by long-time catching was higher than that by short-time catching. The initial values of TVB-N, TBA, and metmyoglobin percentage of skipjack tuna by short-time catching were lower than that by long-time catching (p<0.05). During later freezing and storage period, the quality of skipjack tuna by long-time catching was better than that by short-time catching.(4) The differences between fast-freezing and slow-freezing of skipjack tuna in cold brine on board were analyzed. The results showed that fast-freezing could shorten freezing time. The salt uptake of skipjack by fast-freezing process was significantly (p<0.05) less than that by slow-freezing process. During freezing and later storage period, changes of TVB-N value, pH value, TBA value, metmyoglobin percentage of skipjack tuna by fast-freezing were significantly (p<0.05) slower than these by slow-freezing process, which was in accordance with the result of sensory assessment on color, flavor and texture of skipjack.(5) The different quality changes of skipjack tuna in different areas of cold brine well on board were analyzed. From top to bottom of central axis of cold brine well, the quality indices including salt content, TVB-N value, pH value, TBA value, metmyoglobin percentage and MFI value first increased then decreased. And the quality of skipjack tuna near the wall of cold brine well was better than that in the central axis of cold brine well. |
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