Camu camu (Myrciaria dubia) polyphenols modulate rice starch digestibility after thermal complexation but not during storage

The effect of camu camu (CC, M. dubia) starch ligands on the digestibility of normal rice starch systems (RS, 10 % w/v) after complexation and over time during cold storage was investigated. Specifically, RS and RS-CC composites (2.5 % and 5 % w/v) were gelatinised (90 °C, 20 min), cooled and stored at 4 °C for up to 21 days. Multi-scale characterisation was performed using FTIR, ¹H NMR, ¹³C NMR, TGA, DSC, XRD, SEM and rheology. Additionally, the in vitro digestibility, phenolic release, firmness, and syneresis of these samples were evaluated. FTIR and ¹H NMR spectra confirmed complexation via intermolecular hydrogen bonds between RS and CC-derived hydroxycinnamic acids and/or palmitic acid. DSC (ΔH_{r101–111°C} = 7–15 J g⁻¹), XRD (2θ = 13.0°, 19.8°) and NMR indicated the formation of V₆-type helices, although non-inclusion interactions were predominant. RS-CC composites exhibited soft-gel characteristics (G’∼20 MPa, G’’∼6 MPa) with densely packed microstructures and disrupted short-range molecular order (R_1047/1022∼0.5, R_1022/995∼1.1). CC complexation significantly modulated starch digestibility after gelatinisation and cooling (C_∞∼69 mg∙100 g⁻¹, RDS∼48 %, SDS∼19 %, RSF∼33 %; p < 0.05). However, this effect was not maintained during storage. After 4 days, no significant differences were observed in the digestive properties of the RS gels and composites, due to the inhibitory effect of CC phenolics on amylose retrogradation. Over time, starch structural rearrangements limited the in vitro intestinal release of phenolics, contributing to the loss of the initial modulatory effect. These findings provide new insight into the complexation-retrogradation-digestibility relationship that could inform the reformulation of starch-based foods with sustained and tuned digestibility.