Ultrasound-Assisted Extraction of Antioxidant Polyphenols from Zheng’an White Tea: Process Optimization and Bioactivity

White tea is rich in polyphenols with antioxidant potential, but information on Zheng’an white tea remains limited. This study optimized water-based ultrasound-assisted extraction (UAE) using single-factor experiments and a Box–Behnken design and evaluated the antioxidant activity of the resulting extract. Tea-leaf microstructure, polyphenolic composition, and antioxidant capacity were assessed using scanning electron microscopy (SEM), liquid chromatography–mass spectrometry (LC–MS), in vitro radical-scavenging assays, and a Caenorhabditis elegans model, respectively. The optimized conditions within the investigated experimental domain were a solid-to-liquid ratio of 1:50 (g/mL), an ultrasonic time of 50 min, and an ultrasonic power of 118 W, yielding a total polyphenol content of 37.93 ± 0.17 mg GAE/g DW. SEM revealed marked disruption of tea-leaf cellular structures after UAE. LC–MS enabled the putative annotation of 25 representative polyphenols spanning phenolic acids, flavan-3-ols, procyanidins, flavonols and their glycosides, and related derivatives. At 1.5 mg/mL, the extract exhibited ABTS and DPPH radical-scavenging rates of 81.68 ± 3.23% and 85.43 ± 2.31%, respectively. In C. elegans, 100 μg/mL extract increased catalase activity by 86.03% and decreased malondialdehyde content by 46.40% (p < 0.01). These findings show that the optimized aqueous UAE procedure recovered a polyphenol-rich extract with antioxidant activity in the tested chemical assays and C. elegans model.