Preparation of pH-responsive hydrogel microspheres based on fibrillated soy protein isolate and sodium alginate and investigation of polyphenol release behavior from Ascophyllum nodosum

Polyphenols from Ascophyllum nodosum (PEAn) exhibit bioactivity but are prone to structural instability due to their polyhydroxy groups. To address this limitation, we developed pH-responsive hydrogel microspheres through enzymatic fibrillation modification of soybean protein isolate (SPI). Trypsin-induced fibrillation of SPI at neutral pH (7.0) produced β-sheet-rich nanofibrils (SNF), which were crosslinked with sodium alginate (SA) to form composite microspheres. The optimized microspheres achieved an encapsulation efficiency of 73.89 ± 1.07 %, along with enhanced water retention capacity, freeze-thaw stability, and significantly improved DPPH radical scavenging activity. The microspheres exhibited pH-dependent release behavior: less than 10 % of PEAn was released in pH 1.2 within 48 h, whereas a cumulative release of 88.4 % was observed in pH 7.4 via a swelling-driven mechanism. Release kinetics followed the Higuchi model in SGF and the Korsmeyer–Peppas model in SIF, indicating diffusion-controlled and erosion-controlled mechanisms, respectively. This strategy demonstrates a green, solvent-free approach to fabricate biopolymer-based delivery systems that protect unstable bioactives and enable controlled intestinal release, offering potential applications in functional foods and nutraceuticals.