Optimization of Enzyme-Assisted Ultrasonic Extraction of a Quercetin-Containing Crude Extract from Sophora japonica Flower Buds and Evaluation of Its In Vitro Antioxidant and Antibacterial Activities

Flower buds of Sophora japonica are flavonoid-rich edible botanical materials. This study developed an enzyme-assisted ultrasonic protocol for producing a quercetin-containing crude extract. A Box–Behnken design evaluated β-glucosidase concentration, digestion time, and the cellulase-to-pectinase mass ratio. The quadratic model was significant (p < 0.0001; R2 = 0.9848; adjusted R2 = 0.9652). At 14 U/mL β-glucosidase, 72 min of digestion, and a mass ratio of 1.25:1 (w/w), the UV-estimated quercetin-equivalent yield reached 7.34%, compared with 4.17% for ultrasound alone (p < 0.001). HPLC based on retention-time agreement and external calibration estimated quercetin at 71.93 ± 1.35 mg/g initial dry plant material. The dried extract contained 82.93 ± 3.75 mg gallic acid equivalents/g and 154.68 ± 5.56 mg quercetin equivalents/g. It showed concentration-dependent DPPH•, •OH, and ABTS•+ scavenging, with IC50 values of 1.339 ± 0.061, 3.931 ± 0.108, and 1.004 ± 0.032 mg/mL, respectively, and preliminary antibacterial activity against Salmonella enterica, Escherichia coli, and Staphylococcus aureus. These findings establish a defined extraction route while distinguishing the UV optimization endpoint from HPLC-based quercetin estimation.