Effects of high-humidity hot-air impingement treatment on the conformational transformation of collagen of fish swim bladder

The purpose of this study is to investigate the effects of high-humidity hot-air impingement (HHAI) on the conformational transformation collagen in fish swim bladder. Fish swim bladder was subjected to HHAI treatment at different blanching temperatures (100, 110, 120 ℃), relative humidities (20%, 30%, 40%), and treatment durations (4, 5, 6 min). A multifaceted analytical approach was employed to capture the multifaceted effects of the treatment. Water distribution and migration were characterized by low-field nuclear magnetic resonance and nuclear magnetic resonance imaging. Protein structure changes were characterized by fourier transform infrared spectroscopy. Protein thermal stability was characterized by differential scanning calorimetry. Conformational shifts at the molecular level were further probed. Protein conformational changes were characterized by fluorescence spectroscopy and ultraviolet spectroscopy. Protein surface properties were characterized by surface hydrophobicity measurement (8-phenyl-1-naphthalenesulfonic acid fluorescence and contact angle). To link microstructural changes to macroscopic properties, the microscopic structure was characterized by scanning electron microscopy. Changes in apparent viscosity and dynamic modulus were characterized by rheological tests. Boiling water blanching was used as a control. The results showed that the optimal treatment conditions were 110 ℃, 30% humidity HHAI for 5 min. Under these optimized parameters, a profound enhancement in product quality was observed. Compared with the control group, the proportion of bound water reached 57.32% under this condition, increasing by 19.77%, and the moisture distribution was the most uniform. The water-holding capacity of fish swim bladder was enhanced. Structural analysis revealed a notable stabilization of the protein matrix. The content of β-sheet in the secondary structure of protein increased by 6.74%, and the thermal denaturation temperature increased by 8.12%, indicating that the thermal stability was improved. Surface property modifications were equally significant. The contact angle decreased by 34.97%, indicating that the hydrophilicity of protein was enhanced. Microstructural and rheological data provided compelling evidence of a strengthened protein network. The microstructure was dense and porous, and the apparent viscosity increased by 272.02%. Moreover, the storage modulus was significantly higher than the loss modulus (P < 0.05), indicating that a more stable protein elastic network was formed. Importantly, the benefits of HHAI were not exclusive to the optimal parameters. The HHAI non-optimal treatment group also outperformed the control group in all indicators. Specifically, compared with the control group, the proportion of bound water increased by 6.29% to 16.04%, the content of β-sheet increased by 0.39% to 5.95%, the thermal denaturation temperature increased by 0.31% to 7.89%, the contact angle decreased by 8.51% to 34.97%, and the apparent viscosity increased by 13.03% to 261.27%. This consistent outperformance underscores the inherent advantage of the HHAI method over traditional boiling. Overall, the HHAI treatment significantly improved the quality of fish swim bladder by optimizing water distribution, stabilizing protein secondary structure, enhancing thermal stability, and improving surface hydrophobicity. The technology effectively promoted the orderly rearrangement of collagen molecules into a more stable and functional network. This research provided theoretical reference and technical basis for the efficient processing of fish swim bladder and suggested potential applicability for other collagen-rich aquatic and food materials.