MORPHOLOGICAL-CHANGES IN TILAPIA MUSCLE FOLLOWING FREEZING BY AIRBLAST AND LIQUID-NITROGEN METHODS

The cross-section spacing between the muscle fibre bundles of fresh ti lapia chunks was approximate to 3.06 mu m. After freezing by airblast at -20 and -36 degrees C, and by liquid nitrogen at -87 and -128 degre es C at freezing rates of 0.25, 1.53, 9.74 and 19.4 cm h(-1), respecti vely, the spacing increased to 3.21-7.69 mu m, which was 5 to 151% gre ater than that in the fresh samples. The spacing further increased wit h storage time. Liquid nitrogen freezing resulted in smaller increases in spacing than ah-blast freezing. On freezing at constant temperatur es of -20 to -128 degrees C followed by storage at -20 degrees C for 1 month, the extracellular spacings were 7.38-13.8 mu m, and increased to 22.16-29.38 mu m after 2 months. After storage at -20 degrees C or -40 degrees C for 6 months, the muscle fibre bundles showed fragmentat ion 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 res ulting from freezing methods or freezing rates disappeared upon prolon ged frozen storage at -20 degrees C or -40 degrees C. The correlations between the freezing temperature and extracellular spacing, and the a ctivation energy (E-a) was calculated. The time required for freezing- temperature-induced differences in crystal growth, or in the extracell ular spacing of muscle fibre bundles to disappear when E-a = 0 can be considered as the high-ultrastructural quality shelf-life, which is pr edicted to be 2.7 months at -20 degrees C for tilapia frozen with liqu id nitrogen.