Effect of edible oils on in-vitro starch digestibility and functional properties of cooked rice starch

This study investigated the combined effects of sunflower oil (SO) and coconut oil (CO) at 3% and 6% (w/w) on the in-vitro starch digestibility and functional properties of five rice varieties (XKL, XKS, NG46, NP, and YPT) that had a wide variety of amylose (AM) content and molecular fine structure. Structural, physicochemical, and textural characteristics were analyzed using size exclusion chromatography (SEC), fluorophore-assisted carbohydrate electrophoresis (FACE), X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), a rapid visco analyzer (RVA), differential scanning calorimetry (DSC), texture profile analysis (TPA) and in vitro digestion. Pronounced varietal differences were observed in starch structure and digestion behavior. The low-AM variety NG46 showed the fastest digestion rate coefficient (k = 1.7 min⁻¹). The XRD results confirmed the formation of starch–lipid complexes, indicated by the appearance of V-type diffraction pattern, with the highest relative crystallinity (R_c) in YPT (17.5%) with 6% CO. Incorporation of CO markedly increased the undigested starch fraction (C_res), reaching 23.4% in YPT cooked with 6% CO. In contrast, SO moderately increased resistant starch (RS) but improved textural quality by balancing hardness and stickiness. Overall, 6% CO combined with the high AM, long-chain indica variety YPT was most effective for forming starch–lipid complexes and enhancing RS content, whereas 3% SO improved consumer-preferred texture without greatly reducing digestibility. These findings provide insight into optimizing oil type, concentration, and rice variety to achieve desirable nutritional and functional properties in cooked rice.