Effect of Fe(III) on soybean protein-gallic acid hydrogel: Structure, functional properties, and digestive properties

A soy protein isolate-gallic acid (SPI-GA) complex was synthesized, mediated using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide. Different amounts of Fe3+ were then added to the SPI-GA complex to produce hydrogels (SPA@Fe) containing metal-phenolic network without heating the protein. Spectral analysis revealed that these hydrogels contained multiple modes of cross-linking, such as Schiff bases, hydrogen bonds, and coordination bonds. Coordination bonds formed between GA and Fe3+ mainly had a coordination number of three, and they changed the structure of the protein. Furthermore, these coordination bonds also endowed SPA@Fe with a more compact crosslinked network, thus improving the hydrogel's textural characteristics, thermal stability, storage modulus, and glucose adsorption capacity. The SPA@Fe hydrogel exhibited the best performance when the molar ratio of GA to Fe3+ was 1, with the thermal stability, hardness, springiness, and glucose adsorption capacity being 3.9, 1.84, 1.22, and 1.93 times higher than those of the SPI-GA hydrogel, respectively. Moreover, in-vitro digestion tests showed that SPA@Fe could control the release of loaded curcumin, had good capacity for adsorbing bile salt, and showed delayed degradation. This hydrogel system may offer a safe and feasible way to deliver bioactive substances such as curcumin.