Effects of frozen storage on physicochemical characteristics of bighead carp (Aristichthys nobilis) fillets

To evaluate the effects of frozen storage on physicochemical characteristics of bighead carp (Aristichthys nobilis) fillets, we measured the changes of cathepsin activity in myofibrils and sarcoplasm; degradation of myofibrillar and sarcoplasmic protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; muscle texture and myofibrillar microstructure using scanning electron microscope and transmission electron microscope, water holding capacity (WHC) by drip loss and low-field nuclear magnetic resonance for fillets stored at -12 and -28 degrees C for 16 weeks. Higher cathepsin B and B + L activity in myofibrils and sarcoplasm, greater degradation of myofibrillar and sarcoplasmic protein, greater destruction of fillets texture, and sarcomere integrity for myofibrils, as well as a greater increase in drip loss and T2 of samples at -12 degrees C were observed than that of -28 degrees C. These results implied that ice crystal expansion during frozen storage led to an increase in cathepsin activity, which would help promoting destruction of texture and decline of WHC for bighead carp fillets under frozen storage due to their proteolysis effect. Practical applications This research illustrated changes of cathepsin activity, structure destruction and WHC deterioration of fillets during frozen storage, thus implied lower temperature of frozen preservation and appropriate storage time could better maintain fillets texture and WHC, for example, -28 degrees C for 16 weeks. Also, controlling the cathepsin activity of frozen fish products by adding some allowed cathepsin inhibitors was equally helpful in maintaining fillets texture and WHC.