Effects of different cold chain logistics situations on quality and microstructure of tuna (Thunnus obesus) fillets

Abstract: The process of cold chain logistics including storage, transportation, selling, and other steps, requires the temperature to be kept at an ultra-low degree of -55℃ at all times-. However, because there is a current lack of mechanical cryogenic transportation vehicles in China that meet this requirement, ordinary refrigerated trucks with a temperature of only -18℃ are widely used for tuna transport. The aim of this study is to investigate the effects of variations in storing temperature and selling models on the quality and microstructure of tuna (Thunnus obesus). In this paper, four diverse situations of transportation and storage were performed to simulate: complete ultra-low temperature cold chain (-55℃), normal cold chain (-18℃), and tuna stored at -55℃ but transported afterwards at -18℃ once and twice. Three different sales models were also designed: ultra-low temperature refrigerator sale (-55℃), refrigerated display case sale (2℃) and freezer sale (-18℃). In these situations and models, changes in sensory evaluation, color value, water holding capacity, and texture were assayed periodically, and the gap between muscle fibers was measured by making paraffin sections at the end of frozen storage. The results showed that the quality of bigeye tuna stored at ultra-low temperature was significantly affected by temperature changes in the logistics process. Whereas the quality of tuna can be kept well under a complete ultra-low temperature cold chain of -55℃ during transportation and sale, temperature changes led to a decrease of sensory score, meat color (a* value), water holding capacity, hardness, and chewiness. The more the frequency and range of temperature changes, the greater the deterioration would be. When stored at -55℃ but transported at -18℃ twice before being stored at 4℃ in domestic refrigerators for 12 hours, sensory score, meat color (a* value), water holding capacity, hardness and chewiness of tuna were -0.81, 5.2, 53.08%, 1.128N and 58.03, respectively. And, the quality of tuna in the above situation was lower than when it was maintained at -18 °C all of the time. For microstructure, tuna which was kept in a complete ultra-low temperature cold chain had a smaller muscle fiber gap, which was only 7.81 μm, and the muscle bundles were relatively tight. However, tuna which was kept under a completely normal cold chain and transported afterwards at -18℃ twice, had more loose organizational structures, and the gap between muscle fibers were 10.37 μm and 17.20 μm, respectively. Experimental results showed that the lack of ultra-low temperature refrigerated trucks would result in a lower quality of tuna. All in all, temperature variation in transportation and storage should be avoided. Storage temperature should be appropriately increased when ordinary refrigerated trucks are applied for transportation. As compared with those displayed under -18℃, tuna sales under the condition of 2℃ could be favored by the consumers in the first 12 hours due to its color after thawing. However, the quality of the meat color (a* value), water holding capacity, hardness and chewiness was not better. In general, this study provided references for retaining the freshness of tuna in different stages of storage, transportation and sales.