Flavonoid Biotransformation in Rutin-Rich Beverages by Magnetic Nanoparticle-Immobilized Endogenous Rutin-Hydrolyzing...

Tartary buckwheat (Fagopyrum tataricum) is a major source of dietary rutin. During processing, endogenous rutin-hydrolyzing enzyme (RHE) can catalyze the biotransformation of rutin, generating valuable flavonoid derivatives that may have enhanced functional properties. However, the use of free RHE in Tartary buckwheat matrices is constrained by its poor stability and low reusability. In this study, RHE was isolated from Tartary buckwheat and covalently immobilized onto bifunctional magnetic iron oxide nanoparticles that were silica-coated, amino-functionalized, and glutaraldehyde-activated. Compared to the free enzyme, the immobilized RHE displayed broader pH tolerance, improved thermal stability, and excellent reusability while retaining its catalytic specificity. When applied to 10 commercial rutin-rich beverages, immobilized RHE effectively promoted rutin conversion. This transformation was accompanied by substantial quercetin accumulation, particularly in Tartary buckwheat tea, where the quercetin concentration increased from approximately 0.20 to 23.67-29.62 µg/mL. In ethanol-containing buckwheat beer, a product assigned as ethyl rutinoside was detected at concentrations of 23.82-33.34 µg/mL. Consequently, enzymatic treatment enhanced the antioxidant capacity of the beverages, with 1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and ferric reducing antioxidant power (FRAP) values increasing by 26.70%-56.97%. Overall, magnetic immobilization enabled the effective application of RHE in complex beverage matrices and offers a practical strategy for flavonoid biotransformation in functional beverage production.