Preparation, characterization, digestibility and antioxidant activity of quercetin grafted Cynanchum auriculatum starch.

In this study, a novel method was developed to conjugate quercetin with Cynanchum auriculatum starch. Quercetin was first succinylized and then grafted onto C. auriculatum starch through a N,N'‑carbonyldiimidazole mediated reaction. The obtained water soluble quercetin grafted starch (quercetin-g-starch) was characterized by several instrumental methods. UV-vis spectrum of quercetin-g-starch aqueous solution exhibited two absorption bands at around 300 and 430nm. Fourier-transform infrared spectrum of quercetin-g-starch showed characteristic bands at 1731 and 1568cm-1, assigning to CO stretching of esterified carboxyl group and CC stretching of aromatic ring, respectively. Proton nuclear magnetic resonance spectrum of quercetin-g-starch showed partial proton signals of starch (3.30-5.47ppm) and quercetin (6.19-7.68ppm). All above results suggested quercetin was successfully grafted onto C. auriculatum starch. Besides, quercetin-g-starch particles were irregular in shape and were in an amorphous state. Notably, the thermal stability, resistant starch content as well as antioxidant activity of C. auriculatum starch was greatly enhanced by grafting with quercetin. This indicated the potential of quercetin-g-starch in the development of a novel resistant starch with antioxidant activity.