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 the high-humidity hot-air impingement (HHAI) on the conformational transformation of collagen in the fish swim bladder. The HHAI treatment was also carried out at the different blanching temperatures (100, 110, and 120 ℃), relative humidities (20%, 30%, and 40%), and durations (4, 5, and 6 min). Some parameters were then captured to evaluate the performance of the treatment using an analytical approach. Water distribution and migration were characterized by the low-field nuclear magnetic resonance and nuclear magnetic resonance imaging. Protein structure and thermal stability were characterized by the Fourier transform infrared spectroscopy and the differential scanning calorimetry, respectively. Conformational shifts at the molecular level were further probed after the test. Protein conformation was characterized by fluorescence spectroscopy and ultraviolet spectroscopy. Protein surface properties were characterized by the surface hydrophobicity measurement (8-phenyl-1-naphthalenesulfonic acid fluorescence and contact angle). The microscopic structure was characterized by scanning electron microscopy, in order to link the microstructure to the macroscopic properties. The apparent viscosity and dynamic modulus were characterized by the rheological tests. Boiling water blanching was used as the control. The results showed that the optimal treatment conditions were 110 ℃, 30% humidity, and HHAI for 5 min. The results show that the product quality was enhanced under the optimal parameters. Compared with the control group, the proportion of the bound water reached 57.32%, thereby increasing by 19.77%. There was the most uniform distribution of the moisture. The water-holding capacity of the fish's swim bladder was also enhanced under the optimal parameters. Structural analysis revealed that there was a notable stabilization of the protein matrix. The content of β-sheet in the secondary structure of protein increased by 28.50%, and the thermal denaturation temperature increased by 8.12%, indicating the improved thermal stability. Surface property modifications were equally significant. The contact angle decreased by 34.97%, indicating the enhanced hydrophilicity of the protein. Microstructural and rheological data were provided for the strengthened protein network. The dense and porous microstructure was observed, where the apparent viscosity increased by 272.02%. Moreover, the storage modulus was significantly higher than the loss modulus (P < 0.05), indicating a more stable elastic network of the protein. Importantly, the HHAI non-optimal treatment group also outperformed the control group in all indicators. Specifically, 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%, and the apparent viscosity increased by 13.03% to 261.27%, whereas, the contact angle decreased by 8.51% to 34.97%, compared with the control group. Overall, the HHAI treatment significantly improved the quality of the fish swim bladder, compared with the conventional boiling. The water distribution was optimized to stabilize the protein secondary structure, and then enhance the thermal stability and the surface hydrophobicity. The technology effectively promoted the orderly rearrangement of the collagen molecules into a more stable and functional network. This finding can provide the theoretical reference and technical basis for the efficient processing of the fish swim bladder. The potential applicability can also be offered for the collagen-rich aquatic and food materials.