Effects of pH and Ionic Strength on the Structure and Functional Characteristics of Tilapia Myofibrillar Protein

This research focused on the myofibrillar protein (MP) and heated myofibrillar protein (HMP) of tilapia, and the effects of diverse pH values and NaCl concentrations on their secondary structure, protein composition, sulfhydryl content, surface hydrophobicity, solubility, and emulsifying characteristics were intensively investigated. The results showed that in terms of solubility, the lowest value was exhibited by MP at pH=5.0, while the maximum solubility (86.78%) was reached at pH=11.0. When the NaCl concentration was 1.0 mol/L, solubilities of 93.31% and 74.60% were observed for MP and HMP, respectively. Through SDS-PAGE gel electrophoresis analysis, it was found that with the increase in pH and NaCl concentration, relatively high protein abundances at 100~200 kDa and 35~48 kDa were displayed by HMP. Regarding the emulsifying properties, as the NaCl concentration increased, the emulsifying activity index (EAI) and emulsifying stability index (ESI) of HMP were first increased and then decreased. When the NaCl concentration was higher than 0.6 mol/L, a decrease in the EAI of HMP from 119.66 m²/g to 93.59 m²/g was observed. The sulfhydryl contents of both MP and HMP were increased with the increase in pH and NaCl concentration. The change rules of their surface hydrophobicities were similar, that was, the surface hydrophobicity was decreased when the pH was increased. For example, at pH=11.0, MP was relatively stable, and its surface hydrophobicity was decreased to the minimum value of 76.16 μg, while the surface hydrophobicity of HMP was decreased from 303 μg to 97.75 μg. Moreover, an increase in surface hydrophobicity was caused by the increase in NaCl concentration. In conclusion, significant impacts on the structural and functional characteristics of MP and HMP were exerted by pH and NaCl concentration. Through systematic study of relevant physical and chemical indexes and analysis of the internal relationship between protein structure changes and functional characteristics, theoretical guidance and data support can be provided for the optimization of tilapia thermal processing parameters, which is helpful for improving the quality of tilapia products and enhancing the technical level of its processing industry.