A food-grade ionic network microcapsule for marine collagen-derived peptide‑zinc chelate stabilization and digestive delivery

Marine by-products are a valuable source of bioactive peptides with significant nutritional and functional potential. However, metal-chelate complexes derived from such peptides exhibit poor stability during processing and gastrointestinal digestion. In this study, a food-grade, ionically self-assembled microcapsule was developed using chitosan oligosaccharide (COS) and sodium tripolyphosphate (TPP) to encapsulate a peptide‑zinc chelate obtained from Takifugu rubripes bone. The COS-TPP system self-assembled in aqueous solution via electrostatic interactions between -NH 3 + and -O-P=O − groups, forming a stable, solvent-free 3D network. Structural analyses (FTIR, XRD, SEM, DLS, TGA) demonstrated that COS chain length critically governed cross-linking density, hydrogen-bond network regularity, and matrix compactness. COS3 (molecular weight < 3 kDa) exhibited superior zinc-loading capacity (9.94%) and thermal stability due to a balanced hydrogen-bond network, whereas COS5 formed a compact amorphous structure, providing enhanced barrier and controlled-release properties. Both COS3 and COS5 microcapsules exhibited significantly improved zinc retention and delayed release compared with the free chelate during in vitro digestion. Moreover, both systems maintained high zinc stability under varying temperature and centrifugation conditions, confirming their strong environmental resistance prior to digestion. Overall, this work provides mechanistic insights into the ionically driven self-assembly and structure-function relationship of food-grade biopolymer carriers and proposes a sustainable strategy for delivering metal-peptide nutrients in functional food applications.