Irradiation co-succinylation modification of caprine casein: Impact on physicochemical properties, molecular structure, and functional properties

This study aimed to systematically characterize the effects of electron beam irradiation (EBI) and succinic anhydride chemical conjugation on the physicochemical properties, molecular structure and functional properties of caprine casein employing multi-scale analytical approaches. The degree of succinylation achieved with EBI indicated effective promotion of the succinylation reaction, making the dual modification better for the protein than EBI or succinylation alone. The increase in succinylation degree corresponded to significant changes in the physicochemical and functional properties of casein. Notably, the lysine, β-sheet, and free sulfhydryl contents of casein, along with endogenous fluorescence intensity and surface hydrophobicity decreased significantly. In contrast, β-turn content markedly increased, while molecular flexibility, thermal stability, particle size, and absolute ζ-potential exhibited a biphasic response to succinylation treatment; an initial increase followed by a subsequent decline. Additionally, protein structures underwent a conformational transition from ordered to disordered, with the surface presenting a rough, loose, and porous state, and the solubility, emulsification properties, and water–oil binding ability showing improvement. The results of static rheological properties revealed succinylation-mediated alterations in the protein–protein and protein–water interactions, along with reduced apparent viscosity and shear stress. Concurrently, the newly observed hydrogen peak in the nuclear magnetic resonance (NMR) hydrogen spectrum indicated successful succinylation of casein. Altogether, the results of this study show that synergistic effects of EBI and succinylation can significantly improve the functional properties of casein, providing new insights into broadening its application in food processing.