Effects of ultrasound-assisted freezing at different power levels on the structure and thermal stability of common carp (Cyprinus carpio) proteins

•Ultrasound immersion freezing (UIF) at 175 W was better to maintain the fish quality.•Ultrasonic power less than 175 W minimized the change in protein primary structure.•UIF at 175 W had no obvious influence on protein secondary and tertiary structures.•Samples frozen by UIF at 175 W had high protein thermal stability. This study investigated the effect of ultrasound-assisted immersion freezing (UIF) at different ultrasonic power levels on the myofibrillar protein primary, secondary and tertiary structures of common carp (Cyprinus carpio). Furthermore, protein thermal stability, electrophoresis pattern, and microstructure of the muscle tissue were also studied. Compared with a control, an ultrasonic power of less than 175 W had no significant negative effect on protein primary structure (P > 0.05), including total sulfhydryl, reactive sulfhydryl, carbonyl groups, free amino groups, dityrosine content, and surface hydrophobicity. UIF at 175 W (UIF-175) minimized the changes in protein secondary and tertiary structures. There were no obvious changes in the SDS-PAGE patterns of the control and frozen sample proteins. Microstructure analysis showed that an appropriate ultrasonic power (UIF-175) promoted the formation of smaller and more uniform ice crystals, reduced the damage of muscle tissue by ice crystals, and maintained the sarcomere integrity. In addition, UIF-175 samples had higher protein thermal stability. Overall, ultrasound treatment at a proper power (UIF-175) effectively minimized the changes in protein structure and protected the protein thermal stability during freezing process.