Multiscale characterization of citrate-acidified hawthorn pectin emulsion gels: Structure, storage behavior, and phenanthroline-detectable ferrous iron in digestion supernatants

Fortifying lipid-containing foods with ferrous iron remains challenging because Fe²⁺ is readily oxidized and can promote matrix destabilization and lipid oxidation. In this study, hawthorn pectin (HP) was used to construct Fe²⁺-loaded emulsion gels, and five systems were compared: HP-Fe, HP-O, HP-Fe-O, HP-Fe-O-CA, and commercial low-methoxyl pectin (LMP)-Fe-O-CA. A multiscale characterization strategy, including apparent particle/aggregate size, ζ-potential, rheology, texture, low-field nuclear magnetic resonance (LF-NMR), Fourier transform infrared spectroscopy (FTIR), microscopy, storage behavior, apparent oxidation-related indices, and INFOGEST static digestion, was used to relate gel structure to phenanthroline-detectable Fe²⁺ retention and a nominal-volume-normalized supernatant Fe²⁺ index during static in vitro digestion. The citrate-containing acidified HP emulsion gel showed an intermediate apparent droplet/aggregate size (D[4,3] = 4.86 ± 0.21 μm), a lower absolute ζ-potential (− 8.63 ± 0.28 mV), and the highest hardness (153.6 ± 6.5 g), indicating a more deformation-resistant gel-like matrix. During storage, this formulation exhibited the lowest separated-phase release, the highest assay-detectable Fe²⁺ retention, and lower reagent-blank-corrected apparent peroxide value (POV) and thiobarbituric acid-reactive substances (TBARS) signals than HP-Fe-O under the defined assay conditions. During in vitro digestion, HP-Fe-O-CA showed a lower early-stage nominal-volume-normalized supernatant Fe²⁺ index and the highest final intestinal index under the defined digestion and assay conditions. These results support the potential of HP-Fe-O-CA as a hawthorn-pectin-based matrix for Fe²⁺-fortified lipid-containing systems under the defined storage and digestion-assay conditions.