Abstract: In order to study the effects of different freezing methods on the quality of freshwater fish, channel catfish, Lateolabrax japonicus and mandarin fish were frozen by plate freezing at –30 ℃ (treatment 1), in a refrigerator at –80 ℃ (treatment 2) or with liquid nitrogen at –80 ℃ (treatment 3). The thawing loss percentage of frozen fish was measured as well as pressure-induced water loss percentage, cooking loss percentage, pH, color, toughness, microstructure, water distribution after thawing. The results showed that the thawing loss rate, pressure-induced water loss percentage and cooking loss percentage as well as the space between cells in each fish species with freezing treatments 1, 2 and 3 decreased in that order, whereas the toughness increased. Nevertheless, there was no significant difference in pH among the three treatments. Treatment 1 showed the highest whiteness, yet there was no significant difference between the other two treatments (P > 0.05). Low-field nuclear magnetic resonance (NMR) showed that the content of bound water (T22) in each fish species with treatment 1 was decreased compared with the remaining treatments, whereas the content of free water (T23) was significantly increased (P < 0.05). For each treatment, there were no significant differences in all parameters investigated among fish species. From the viewpoint of both freezing rate and quality maintenance, freezing in a refrigerator at –80 ℃ and with liquid nitrogen at –80 ℃ were more suitable for freshwater.

Key words: different freezing methods, texture, microstructure, low-field nuclear magnetic resonance