Screening and process optimization of ultrasound-assisted extraction of main antioxidants from sweet tea (Lithocarpus litseifolius [Hance] Chun)

Sweet tea (Lithocarpus litseifolius [Hance] Chun) is an herbal tea with multiple health benefits due to its main bioactive compounds. In this work, the DPPH-spiking test based on high-performance liquid chromatography (DPPH-spiking HPLC) was applied to precisely screen antioxidants from sweet tea. Three main antioxidants, including isoquercitrin, phloridzin, and trilobatin, were screened out from sweet tea. Furthermore, their antioxidant capacities were verified by the DPPH radical scavenging assay, and found their antioxidant capacities in the order of isoquercitrin > trilobatin > phloridzin. Ultrasonic-assisted extraction (UAE) was applied to optimize and extract the screened antioxidants, and the optimal extraction conditions were ethanol concentration of 69.0%, sample-to-liquid ratio of 18.0 w/v, extraction time of 18.4 min, extraction temperature of 25 °C, and ultrasonic power of 450 W, yielding 17.1 ± 1.26 mg/g isoquercitrin, 34.8 ± 1.49 mg/g phloridzin, and 163 ± 1.73 mg/g trilobatin. The antioxidant capacities of the extracts based on UAE (11.5 ± 0.04 mg ascorbic acid equivalent/g DW) was much higher than the extracts based on maceration extraction (7.9 mg ± 0.02 ascorbic acid equivalent/g DW). Moreover, it was found that trilobatin, isoquercitrin, and phloridzin accounted for 30.9%, 24.9%, and 5.33% of the total antioxidant capacities of the extracts under the optimal extraction conditions, respectively. Therefore, DPPH-spiking HPLC analysis combined with UAE is an effective method for the screening and extracting of the antioxidants in sweet tea, which should have potential applications in the development of functional foods.