Effects of ultrasound and pH-shift modification on the freezing properties of potato protein gels

Potato protein has been recognized as the high-quality protein source from plants in the food industry, due to its high digestibility, balanced amino acid composition, and particularly high lysine content. Compared with other plant proteins, potato protein also exhibits favorable functional properties, including excellent solubility, emulsification, and gelation capacity. Potato protein has been widely applied in various processed foods, especially frozen and refrigerated products. Protein gels can often serve as structural matrices to determine texture, water retention, and product stability. However, the formation and growth of ice crystals within the gel matrix can disrupt the protein network during freezing, resulting in structural damage, moisture migration, and deterioration of textural quality. Therefore, it is often required to improve the freezing resistance of potato protein gels in frozen food systems. Among them, pH-shift and ultrasonic treatment have been widely reported to alter protein conformation, molecular unfolding, and intermolecular interactions, thereby leading to gel formation behavior after modification. In the present study, potato protein was modified using two physical–chemical strategies: pH-shift and ultrasonic treatment, as well as their combined application. A systematic analysis was conducted to explore the textural properties, color difference, gel-participating protein content, and microstructure of potato protein gels before and after freezing. There was a relationship between the modified potato proteins microstructures and the gel properties during freezing. An investigation was also made to clarify the effects of pH-shift and ultrasonic modification on freezing behavior and stability of potato protein gels. The results showed that untreated potato protein gel exhibited a gelation temperature of 89.06 °C, a gel hardness of 0.327 N, and minimal cohesiveness. The gel presented the roughest surface morphology, with visible granular aggregates and uniform distribution of pore size. The gelation temperature remained unchanged at 81.94 °C, while gel hardness increased to 0.356 N, thereby producing a slightly smoother gel structure. Gels exhibited a finer microstructure and lower phase-change enthalpy after pH-shift treatment, with 84.14% of the proteins participating in gel formation. The gel exhibited the most refined structure after combined pH-shift and ultrasonic treatment, indicating smooth surface, uniformly small pores, as well as the lowest phase-change enthalpy and temperature. In addition, the soluble protein content decreased to 16.39%. The water-holding capacity of untreated potato protein gel decreased to 76.8% after freezing. Water-holding capacity was reduced to 79.16% after ultrasonic treatment. In contrast, there were only slight and uniform variations in pore size, with water-holding capacity maintained at 82.43%. There was the smallest variation in pore size and the minimal decrease in water-holding capacity after integrated pH-shift and ultrasonic treatment. The soluble protein content decreased by only 16.39%, the freezing enthalpy of water in the gel was −212.86 J/g, and the average pore size was 3 μm. Overall, a fine and intact gel network structure formed in the potato protein after integrated pH-shift and ultrasonic treatment. Water migration and ice crystal formation were restricted during freezing, leading to the high freezing resistance. This combined modification strategy improved the freezing stability of potato protein gels. Protein molecular structure and gelation patterns were obtained to regulate the freeze resistance. The finding can provide theoretical basis and insights to develop potato-based frozen products.