New insights into alkaline pH-driven Schiff base hydrogels: A case of soybean protein isolate / dialdehyde sodium alginate mixed systems.

Herein, soybean protein isolate (SPI) and dialdehyde sodium alginate (DSA) were employed as model systems to investigate the formation mechanism of Schiff base hydrogels and their performance regulation under varying alkaline pH conditions (7.0-11.5). Spectral analysis indicated that as pH increased, the SPI conformation gradually unfolded, exposing amino acid residues and enhancing Schiff base cross-linking. The cross-linking degree of the hydrogel at pH 10.5 reached 3.6 times that observed at pH 7.0. However, under strongly alkaline conditions (pH 11.5), SPI and DSA underwent degradation, leading to a localized over-crosslinking of the hydrogel and a reduction in toughness, resulting in brittle fracture occurring near 60% strain. Comprehensive evaluation of macroscopic/microscopic structure, rheology, mechanical properties, swelling and degradation behavior revealed that SPI/DSA hydrogels with optimal overall performance can be obtained at pH 10.5. Finally, molecular dynamics simulations further showed that glycinin (11S) exhibited strong binding stability with DSA.