Effects of different freezing methods on the quality of Micropterus salmoides oriented to long-distance cold chain

A freezing mode has a major impact on the quality of the product in the long-distance cold chain. In this study, a series of tests were carried out on samples of Micropterus salmoides to reduce the deterioration of largemouth perch during the freezing process. A systematic investigation was also implemented to explore the effects of freezing at -20, -50, and -80 ℃ and freezing at -80 and -50 ℃ combined with storage at -20 ℃ on the quality of perch samples. These quality indicators included color, pH, ice crystals, texture, water-holding capacity, and moisture distribution. The freezing temperatures of both -50 and -80 °C were found to be effective in suppressing the deterioration of the quality of the perch during freezing. The test results showed that the phase transition time of the perch samples in the -80 °C group was 68.8% less than that of the bass samples in the -20 °C group; The phase transition time of the bass samples in the -80 °C group was reduced by 37.5%, compared with the perch samples in the -50°C group. All perch samples showed significant changes (P<0.05) in total color and chromatic aberration scores, compared with the control. However, the ΔE values were all less than 12 in the perch samples treated with the different freezing modes, which were the same color as that of fresh perch. Among them, the -80 °C treated samples showed the smallest change in ΔE values. Except for the perch samples frozen at -80°C, there were significant changes (P < 0.05) in pH in all test groups, compared with the control. Meanwhile, the ice crystal equivalent diameter and cross-sectional area of the perch samples frozen at -80 °C were significantly lower than those of the rest (P < 0.05). The perch samples treated with five freezing modes showed decreasing trends in color, pH, water-holding capacity, and textural properties. The smallest variation was found in the perch samples frozen at -80 °C, similar to the control group. In contrast, the freezing at -50 °C combined with -20 °C storage and freezing at -80 °C combined with -20 °C failed to improve the quality of perch during freezing storage. Correlation analysis between freezing rate and ice crystals showed that the equivalent diameter and cross-sectional area of ice crystals decreased with the increasing freezing rate and duration during the freezing of perch samples. At the same time, correlation analysis between the indicators showed that there were significant correlations between ice crystals, water distribution and color, texture, and water holding capacity. Low freezing temperatures reduced the size of ice crystals and water loss, thus better maintaining the quality characteristics of perch, such as color, pH, texture and water holding capacity. Freezing modes at -50 and -80 ℃better maintained the freezing quality of perch, in terms of the color of perch samples during freezing, pH change, the decreasing size of ice crystals, water migration, hardness, elasticity and water holding capacity. Ultra-low temperature freezing can be expected to reduce the quality deterioration of sea bass. The finding can provide a theoretical basis for reducing the quality deterioration of largemouth perch during cryopreservation and cold chain transport.