Effects of precision feedback microwave heating on the physicochemical properties of myofibrillar proteins in Nemipterus virgatus

This study aims to explore the effects of precision feedback microwave heating (PFMH) on the physicochemical properties of myofibrillar proteins (MPs) of Nemipterus virgatus. The MPs were collected from the Nemipterus virgatus as the research subject. Traditional water bath heating (40 ℃ for 30 min, and 90 ℃ for 20 min) was taken as the control. A systematic investigation was also made on the impacts of PFMH on the protein's turbidity, surface hydrophobicity, fluorescence intensity, ultraviolet (UV) absorbance thermal stability, gel electrophoresis patterns, surface morphology, and surimi gel chemical interactions under various temperatures and heating durations. Results showed that the turbidity and surface hydrophobicity of MPs significantly increased after PFMH treatment, compared with the control group. Both increases also indicated protein aggregation and denaturation, which were critical to understanding the variations in the internal structure of the protein. Specifically, the turbidity of MPs reached 0.78 when heated under S₈₅A conditions, which was an increase of 129.41%, compared with the control group (0.34). Meanwhile, the surface hydrophobicity increased to 177.45 μg, representing a 52.51% increase, compared with the control group (116.35 μg). As such, the PFMH treatment altered the tertiary structure of the protein. The lowest intensity of fluorescence was at S₈₅A, indicating that the PFMH treatment enhanced intermolecular interactions within the protein. Specifically, the PFMH treatment led to the dynamic fluorescence quenching of protein oxidative aggregates in the excited state, resulting in reduced fluorescence intensity. There was a higher fluorescence intensity of myofibrillar protein treated with PFMH at low temperatures (80 °C for 2 and 3 min), compared with the control. The reason was possibly that the rapid PFMH treatment caused some tryptophan residues to be buried within the MPs molecules. Conversely, the UV absorbance exhibited an upward trend, indicating that the PFMH heating induced the conformational changes in the protein structure. The exposure of more aromatic residues led to more effective UV light. Thermal stability was assessed using differential scanning calorimetry (DSC). The results showed that the PFMH treatment significantly enhanced the thermal stability of MPs. Furthermore, the degradation temperature (T_d) of the MPs under S₈₅A conditions reached the maximum of 62.20 ℃, an increase of 13.65%, compared with the control group (54.73 ℃). The enthalpy change (ΔH) decreased from 0.43 J/g in the control group to 0.03 J/g, representing a 93.02% reduction. As such, the PFMH treatment effectively prevented the thermal denaturation and degradation of MPs at elevated temperatures. Gel electrophoresis patterns and atomic force microscopy (AFM) images further revealed the structural changes induced by PFMH treatment. The MPs also unfolded to form a dense gel network structure under the S₈₅A condition. Additionally, the AFM images indicated that the size of MPs aggregates increased, while the quantities decreased after PFMH treatment at 90 °C. Chemical interactions revealed that the maximum content of hydrogen bond was 3.06 g/L after heating at S₈₅A conditions, which was 14.61% higher than that of the control group (2.67 g/L). Meanwhile, the hydrophobic interactions and disulfide bonds increased by 105.60% and 97.80%, respectively. However, the ionic bonds decreased after PFMH treatment, compared with the control group. The cross-linking and aggregation of MPs were promoted to form a more stable and compact protein structure. In conclusion, these findings demonstrated that the PFMH treatment significantly affected the physicochemical properties of MPs in Nemipterus virgatus. The protein denaturation and aggregation also induced the structural changes to enhance the thermal stability and the formation of dense gel networks. The heating processing of surimi products can also offer potential reference data for the future application of PFMH technology.