Morphological changes in tilapia muscle following freezing by airblast and liquid nitrogen methods

The cross-section spacing between the muscle fibre bundles of fresh tilapia chunks was ≈3.06 μm. After freezing by airblast at −20 and −36°C, and by liquid nitrogen at −87 and −128°C at freezing rates of 0.25, 1.53, 9.74 and 19.4 cm h−1, respectively, the spacing increased to 3.21–7.69 μm, which was 5 to 151% greater than that in the fresh samples. The spacing further increased with storage time. Liquid nitrogen freezing resulted in smaller increases in spacing than airblast freezing. On freezing at constant temperatures of −20 to −128°C followed by storage at −20°C for 1 month, the extracellular spacings were 7.38–13.8 μm, and increased to 22.16–29.38 μm after 2 months. After storage at −20°C or −40°C for 6 months, the muscle fibre bundles showed fragmentation in both the airblast and the liquid nitrogen frozen tilapia chunks. The integrity of muscle structure was maintained better with liquid nitrogen freezing than with airblast freezing. All the differences resulting from freezing methods or freezing rates disappeared upon prolonged frozen storage at −20°C or −40°C. The correlations between the freezing temperature and extracellular spacing, and the activation energy (Ea) was calculated. The time required for freezing-temperature-induced differences in crystal growth, or in the extracellular spacing of muscle fibre bundles to disappear when Ea= 0 can be considered as the high-ultrastructural quality shelf-life, which is predicted to be 2.7 months at −20°C for tilapia frozen with liquid nitrogen.