The present study demonstrates the effects of pH-shifting treatments and magnetic field-assisted pH-shifting treatments on the properties of myofibrillar protein (MP) in frozen meat. The solubility results indicate that the pH-shifting treatments increased the solubility of MP from 16.8% to a maximum of 21.0% (pH 9). The values of surface hydrophobicity and protein particle size distribution indicate that the pH-shifting treatment effectively inhibited protein aggregation through electrostatic interactions. However, under higher pH conditions (pH 10, 11), the treatments assisted by the magnetic field increased the degree of aggregation. The total thiol content and SDS-PAGE results further suggest that the magnetic field-assisted pH-shifting treatment accelerated the formation of covalent bonds among MPs under the alkaline environment. The results of the Differential Scanning Calorimetry (DSC) and protein secondary structure analysis indicate that the magnetic field promoted the unfolding of protein structures in an alkaline environment, markedly reducing the effective pH levels of pH-shifting. Electron paramagnetic resonance (EPR) data indicate that the phenomenon might be associated with the increased concentration of free radicals caused by the magnetic field treatment. In summary, the application of magnetic field-assisted pH-shifting treatments could emerge as a potent and promising strategy to improve the protein properties in frozen meat.

This study compared the effects of air freezing (AF), immersion freezing (IF), and ultrasound-assisted immersion freezing (UIF) at different power levels on the physicochemical properties, oxidative characteristics, and the structural and functional properties of myofibrillar protein (MP) of fresh beef. The results showed that UIF treatments generally outperformed AF and IF, with UIF-400 W exhibiting the best performance in maintaining beef quality. Specifically, UIF-400 W mitigated muscle fiber damage by inhibiting ice crystal growth, significantly improved water-holding capacity and color stability, reduced shear force, and effectively delayed protein and lipid oxidation. In addition, UIF-400 W brought the rheological properties, emulsifying properties, water distribution, and gel microstructure of MP closer to those of fresh beef. These findings highlight that UIF-400 W can effectively preserve beef quality as well as the structural and functional characteristics of MP, reduce oxidative damage, and thus become a promising technology for quality improvement of beef during freezing.