Insights into HIU enhancement of salt-reduced surimi gelation properties: Dual regulation of endogenous transglutaminase activation and protease inhibition

Previous studies have demonstrated that high-intensity ultrasound (HIU) could enhance the gel properties of salt-reduced (1.5 % NaCl) surimi gels. Understanding its mechanism, particularly the role of key endogenous enzymes, is essential for HIU-assisted modulation of salt-reduced surimi gels with satisfactory textural properties. In this study, enzyme-specific additives were employed to investigate the enhancement effect of HIU on the gelation properties of salt-reduced (1.5 % NaCl) surimi in terms of puncture properties, protein degradation and crosslinking, dynamic rheological properties, water distribution and microstructures. Results showed that CaCl₂ and E-64 significantly increased the breaking force of surimi gels. This improvement occurred because Ca²⁺ activated endogenous transglutaminase (eTGase), as evidenced by the increased formation of non-disulfide covalent bonds, while E-64 reduced protein degradation by inhibiting cysteine protease. The combination of HIU and CaCl₂ lowered the setting temperature during heating and facilitated the formation of non-disulfide covalent bonds, contributing to a three-dimensional gel network that effectively trapped more water. Consequently, the puncture properties were improved. The combination of HIU and E-64 effectively inhibited protein degradation, as evidenced by decreased TCA-soluble peptides and increased G′ at the end of the modori stage. Notably, although the combination of HIU and NH₄Cl improved the gel properties, it still failed to form the typical three-dimensional gel network. Collectively, HIU enhanced the gelation properties of surimi through dual mechanisms of eTGase activation and cysteine protease inhibition, with eTGase activation contributing more substantially than protease inhibition, providing a theoretical foundation for HIU-assisted production of salt-reduced surimi gels.