Inulin's paradoxical effect on Ca(ii)-alginate encapsulation of phenolic extracts

Abstract

Encapsulating polyphenols in Ca(II)-alginate beads requires suitable co-carriers, such as inulin, to reduce bead porosity and enhance protective efficiency. However, the real impact of inulin remains unknown, with some studies showing improved encapsulation efficiency (EE) and others indicating a decline. This study investigated the actual effect of inulin on the EE, physical properties, and release behaviour of Ca(II)-alginate beads loaded with spent sour cherry pomace extracts. EE based on total phenolic content (TPC) determined from bead dissolution in sodium citrate increased significantly with inulin addition, exceeding 90%. In contrast, TPC-based EE calculated from unloaded compounds in the residual calcium chloride solution decreased with inulin, dropping below 40%. This inconsistency was attributed to interference from inulin in the Folin–Ciocalteu assay, as its incorporation into the bead matrix may lead to overestimated TPC values. Supporting this, pure inulin exhibited a measurable TPC of 74.79 ± 2.28 µg GAE per g and notable reducing activity in the ferric-reducing antioxidant assay. Moreover, beads containing inulin exhibited reduced hardness, due to increased viscosity of the feed solution, which likely hindered alginate-calcium cross-linking. However, inulin significantly enhanced anthocyanin-based EE from 47.18 ± 2.54% to 53.26 ± 0.91% (p ≤ 0.05). It also increased the beads' diameter, filling the pores within the Ca(II)-alginate matrix, as confirmed by SEM imaging. Release kinetics of TPC and anthocyanins followed the Korsmeyer–Peppas model (R² > 0.9), with n values indicating Fickian diffusion. Although inulin reduced apparent TPC-based EE and bead rigidity, its functional advantages warrant further optimization of formulation parameters.