Controlled-release behavior of Fe(II) dependent on the structure of sodium alginate-soy protein hydrogels: Effects of in-situ ionic crosslinking degree

To address the technical challenge of balancing the stability and bioavailability of conventional iron supplements, this study developed a plant-based composite gel system in which Fe(II) served as both a nutritional factor and an ionic crosslinker. The results showed that the three-dimensional gel network was primarily maintained by hydrophobic interactions. Two-dimensional correlation analysis further indicated that Fe(II) may coordinate with the carboxyl groups of sodium alginate (SA) to form ionic bridges, triggering the dissociation and reconstruction of the hydrogen-bonding network, subsequently inducing soy protein isolate (SPI) conformational unfolding, thereby regulating gel network formation. At an Fe(II) concentration of 0.25%, the in-situ ionic crosslinking degree reached the optimum level, yielding a gel with a hardness of 55.72 ± 0.27 g, the highest storage modulus, and the most uniform pore distribution. The β-sheet content in the SPI secondary structure reached its maximum (38.40%), effectively retarding Fe(II) diffusion and gastrointestinal loss. Overall, this study elucidated the mechanism of Fe(II)-mediated in-situ ionic crosslinking and established the structure–function relationship among crosslinking degree, gel structure and Fe(II) controlled-release behavior. These findings provide a novel food-grade iron supplement for alleviating iron-deficiency anemia and expanding the application of plant protein-based composite gels in active mineral delivery.