Preparation and Characterization of Casein–Soy Protein Hybrid Gels Cross-Linked by Transglutaminase

To enhance the gelling functionality of plant proteins, this study developed hybrid gels by blending casein with soy protein isolate (SPI) at various ratios using microbial transglutaminase (MTG) as a cross-linking catalyst. The gels were systematically characterized in terms of microstructure, water distribution, rheological and textural properties, secondary structure, and intermolecular interactions. Incorporation of casein significantly improved gel strength, water-holding capacity, and network uniformity. Notably, the 1:1 casein-to-SPI ratio yielded the highest performance, featuring extensive inter-protein cross-linking, an increased proportion of ordered secondary structures, and a finely porous matrix that effectively immobilized water. Mechanistically, MTG-catalyzed covalent bonding established the primary network scaffold, while hydrophobic interactions and disulfide bonds further stabilized the gel matrix. These findings demonstrate that MTG-induced Casein–SPI hybrid gels can enhance the functional properties of plant proteins and offer a viable strategy for designing sustainable protein-based food structures with tailored performance.