Tailoring the structure and function of soy protein isolate hydrogels via cinnamaldehyde cross-linking: from...

This study prepared soy protein isolate (SPI)-cinnamaldehyde (CA) composites and hydrogels, to systematically elucidate the interaction mechanism between SPI and CA while investigating the structural and functional properties of the composite hydrogels. Fluorescence and UV-Vis spectroscopy revealed that covalent crosslinking with cinnamaldehyde enhanced the UV-Vis absorption peaks and induced fluorescence quenching. SDS-PAGE analysis confirmed the formation of high molecular weight polymers. In vitro experiments demonstrated that CA covalent modification increased SPI particle size by 40.4 %. Infrared spectroscopy revealed blue shifts in the composite gel's amide I stretching vibration peak to 1623.769 cm^-1 and amide II stretching vibration peak to 1578.45 cm^-1. A characteristic CN peak appeared at 1447.797 cm^-1; The free amino content decreased from 457.73 nmol/mg to 308.43 nmol/mg, and the free sulfhydryl content decreased from 4.64 nmol/mg to 3.24 nmol/mg, confirming covalent crosslinking between CA and SPI via Schiff base and Michael addition reactions. At the optimal CA concentration (3 % w/v), the composite gel exhibited a significant 126 % increase in hardness and a 22 % improvement in water-holding capacity, alongside markedly enhanced thermal stability and rheological properties. This study elucidates the dual covalent cross-linking mechanism between CA and SPI based on Schiff base/Michael addition reactions, confirming that the composite gel formed at 3 % CA concentration exhibits an optimal gel network structure, textural properties, thermal stability, and rheological performance. These findings provide theoretical support for developing CA as a natural modifier to enhance soy protein gel properties and offer new insights for modifying other plant proteins.