Differentiation of frozen-thawed fish from unfrozen fish by determination of neutral beta-N-acetylglucosaminidase activity in the blood

A method for distinguishing frozen-thawed from unfrozen fish was developed. The principle of this method is based on the findings that in the blood of most species of fish, particularly in the cytosol of the red blood cells, the presence of a neutral beta -N-acetylglucosaminidase which does not express activity in the intact cells unless the cells are damaged, e.g. by freezing and thawing. The differentiation was performed by fluorometric determination of the neutral beta -N-acetylglucosaminidase activity in fish blood. For the fresh and for the chilled fish kept for one to three days at 4 degree C, the blood showed little fluorescence, while for the frozen fish stored at -20 degree C or -40 degree C even for one day, the blood showed a strong fluorescence. The method is applicable to most commercial fish species, both marine and freshwater. The entire procedure can be done in about 20 min.