Effects of quercetin on the structure and functional properties of extrusion rice starch

The properties of starch are determined by its own multiscale structure. Appropriate modification has also been adopted to regulate its properties in the multiscale structure of starch. Extrusion processing can be expected to apply in the food manufacturing industry, due to its continuous production, easy operation, green environmental protection, and safety. The mixing, stirring, crushing, shearing, and thermal effects generated by extrusion can be used to destroy the original structure of starch, and then induce the starch molecules for the new structure and properties. Among them, polyphenol has been added into the starch system as small molecules with physiological activity function in recent years. The hydrophilicity of starch molecular, hydrogen bond and van der Waals force interacted to promote the evolution of starch multi-scale structure for better properties. The polyphenol-starch-based foods have also been applied to increase the nutritional properties of foods. New functional foods can be produced to prevent and treat diseases, such as hyperglycemia. The polyphenols can dominate the structure of rice products and the product properties. This study aims to investigate the functional and structural properties of quercetin (Q) on the formation of complexes of rice (R) under the extrusion field. Q was also added (0-10%) into the rice. Scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared spectroscopy (FTIR), rapid viscosity analyzer (RVA), differential scanning calorimeter (DSC), and Ultraviolet visible spectrophotometer were carried out to characterize the structural properties. Furthermore, a systematic investigation was implemented to explore the effects of Q on water solubility index (WSI), water absorption index (WAI), and particle size of the complexes. The experimental results showed that the relaxation time of T₂₃ was advanced significantly when the Q concentration was 4%. The starch crystal structure was damaged by extrusion, compared with the R, thus leading to the WAI and WSI of ER increased significantly (P<0.05). The DSC results showed that the heat absorption peak outstandingly disappeared in the range of 57.88-76.39 ℃. The crystallinity of starch was destroyed to depolymerise the double-helix orderly arrangement structure. The FTIR results showed that the characteristic absorption bands of benzene ring bond stretching appeared in 1 519.59 and 1 320.15 cm⁻¹. The XRD results showed that a new crystal structure appeared near 27.4° with the addition of Q, and the relative crystallinity increased from 8.31% to 16.81%. Meanwhile, the smaller particle size of E-QR was obtained, compared with the ER. The morphological results showed that the Q was attached to the surface of the complex, indicating more three-dimensional and compact particles. The iodine binding capacity showed that the Q inhibited the recrystallization of starch, and then delayed the regeneration of amylose, compared with the ER. The setback of E-4% QR decreased by 23.16%. In summary, the hydroxyl group of Q was tightly bound to starch via hydrogen bonding. The recrystallization of starch was inhibited to promote molecular rearrangement, thus leading to the modified structural and functional properties of starch. This finding can provide a theoretical basis for the development of new rice products.