To enhance and stabilise the edible quality of coated tofu, this study explored the changes in the microstructure and intermolecular forces of coated tofu gel and sol under different heat treatments. It elucidated the phase transformation mechanism of coated tofu gel and sol under heat treatment. The results showed that the protein structure unfolded, the fluorescence intensity decreased, and the protein solubility, surface hydrophobicity, and free sulfhydryl group content increased as the coated tofu gel transformed to sol. Disulfide bonding and hydrophobic interactions were the primary intermolecular forces in the heat-induced gel–sol transition. FTIR showed that the content of β-sheets decreased significantly during gel–sol transformation, while the content of β-turns, α-helices and random coils increased significantly. Most remained relatively stable during the gel–sol transformation process, with only the A and B subunits of the 11S protein decreasing slightly. Their reduction became significant when the temperature reached 200 °C. Additionally, the high-temperature heat treatment promoted the gel–sol transition of the coated tofu, with its cross-section gradually transforming from a porous network structure to a more uniform and smooth texture during heat treatment process. The findings of this study provide a theoretical basis for improving the quality of coated tofu by optimising heat treatment parameters, laying the groundwork for future advancements in the development of pre-heat-treated coated tofu.

Coated tofu is prone to spoilage and degradation during processing, storage, and transportation. As the material basis for gel of coated tofu, proteins determine coated tofu’s unique qualities, such as its colour, flavour, and texture. This study aimed to investigate the changes in the quality of coated tofu and the physicochemical properties of its proteins during cold storage (4 °C and 10 °C, 14 days), as well as the intrinsic correlations between these variables. Quality deterioration and protein structural changes were significantly slower at 4 °C than at 10 °C, with lower temperature effectively delaying quality loss. The results indicated that as storage time increased, the freshness of coated tofu declined, its textural properties significantly deteriorated, and the protein gel network structure became impaired. Meanwhile, the proteins underwent significant oxidative denaturation, characterized by a decrease in the free thiol group content and an increase in surface hydrophobicity. The tertiary structure exhibited unfolding and disruption, while the secondary structure transitioned from an ordered to a disordered state. Specifically, the contents of α-helixes and β-sheets decreased significantly, reaching 34.96% and 8.68%, respectively, after 14 days of storage at 4 °C. In contrast, the contents of β-turns and random coils increased to 30.11% and 26.25%, respectively, under the same storage conditions. The subunit bands of the 11S and 7S proteins gradually weakened, and the protein structure tended to loosen. Correlation analysis revealed that the oxidative denaturation, structural depolymerization, and reaggregation of proteins were highly significantly correlated with the textural breakdown and colour deterioration of coated tofu, which together contributed to the quality degradation of coated tofu during cold storage. The findings of this study provide fundamental data and technical support for the development of cold storage methods for coated tofu.

Under alkaline conditions, most commonly used preservatives exhibit limited efficacy and fail to meet the preservation requirements of coated tofu. This study aims to investigate the effects of metal-phenolic networks (MPNs) on quality deterioration, protein oxidation, conformation, and gel microstructure of coated tofu during cold storage (4 °C and 10 °C). The results showed that, compared with the untreated control group, MPNs treatment effectively inhibited protein oxidation, alleviated quality deterioration, delayed the degradation of color and texture, and reduced protein degradation, as evidenced by soluble protein contents that were 63.55% (4 °C) and 66.65% (10 °C) lower than those of the control group after 20 days of storage. MPNs treatment also improved the orderliness and stability of the protein secondary structure. In addition, electrophoretic analysis showed that MPNs markedly retarded the decline in band optical density of the 11S protein A subunit by 96.19% and 97.28% at 4 °C and 10 °C, respectively, and suppressed the increase in the B subunit by 13.28% and 73.20%, respectively. Moreover, MPNs treatment helped maintain a more compact gel network. Based on physicochemical characterization and Pearson correlation analysis, the preservative effect of MPNs on coated tofu under alkaline conditions was elucidated, revealing the internal correlation between the inhibition of quality deterioration and the regulation of protein oxidation. Specifically, MPNs mitigate protein disulfide bond loss, increase the β-sheet content, preserve the natural protein conformation and the relative proportion of 11S subunits, stabilize the gel microstructure, and thereby achieve quality preservation. These findings provide theoretical support and strategic reference for the development of preservation technologies for alkaline high-protein gel foods.