Comparison of chemical, microbiological and histological changes in fresh, frozen and double frozen rainbow trout (Oncorhynchus mykiss)

The final quality of fish meat depends on the chemical and microbiological quality of fish at the time of freezing as well as on other factors including storage temperature and freezing rate. Analysis of chemical composition (water, protein and fat content), expressible drip, total volatile nitrogen levels, microbiological analyses (total viable counts, Enterobacteriaceae, psychrotrophic bacteria) and histological examinations on dorsal skeletal muscles were carried out to distinguish fresh, frozen and double frozen rainbow trout (Oncorhynchus mykiss). Significantly higher expressible drip and total volatile base nitrogen concentrations (P less than 0.05) were observed in frozen and double frozen trout, whereas chemical composition of fresh fish muscles was not significantly affected by freezing. The highest total viable counts, counts of Enterobacteriaceae and psychrotrophic bacteria were determined in double frozen trout. The light microscopy of fresh trout muscles did not show any microstructural changes, whereas deformations of muscle fibres and optically empty areas were found in frozen trout. Remarkable defects of the muscle structure in double frozen trout were demonstrated and total disruption of muscle fibres was found. The freezing of trout resulted in various structural changes in the dorsal skeletal musculature. This is a first study comparing changes in fresh, frozen and repeatedly frozen trout. Chemical, microbiological and subsequent histological examinations can be used for revealing the foul practices confusing the consumer with offering thawed fish instead of fresh cooled fish.