Optimized ultrasonic-assisted extraction of flavonoids from Prunella vulgaris L. and evaluation of antioxidant activities in vitro

The Box–Behnken design combined with response surface methodology was used to optimize ultrasonic-assisted extraction of total flavonoids from Prunella vulgaris L. The results indicated that the highest extraction yield of flavonoids by ultrasonic-assisted extraction could obtain to be 3.62% using ethanol concentration of 41% (v/v) as solvent and liquid to solid ratio of 30:1 (mL/g) for 30.5 min at the temperature of 79 °C. The crude extract was then purified by AB-8 macroporous adsorption resin, the flavonoids content in the purified extract increased to 81.58%. The antioxidant activities of the purified flavonoids were evaluated in vitro by scavenging capability of DPPH free radical and hydroxyl free radical, reducing power, total antioxidant capability and β-carotene bleaching test. The results showed that the flavonoids have significant antioxidant activities, which can be used as a source of potential antioxidant.Industrial RelevanceThe effective exploitation and application of the natural plant resources have very important significance for the industrial and economic development. Prunella vulgaris L. is a perennial plant commonly found in China and Europe, which was proved to be rich in flavonoids, triterpenes and phenolic acid. It has long been used as a folk medicine for alleviating sore throat, reducing fever and accelerating wound healing. Recent studies have demonstrated that the methanol or water extract of this herb exhibits many of the activities including antihyperglycemic activity, systemic anaphylaxis inhibition, antioxidative antiviral and antibacterial effects. However, attempts have not been made so far to investigate the ultrasonic-assistant extraction of flavonoids from P. vulgaris L. and evaluate their antioxidant activities in vitro.In this work, a four-variable, three-level experiment Box–Behnken design based on response surface methodology was used to optimize ultrasonic-assistant extraction of flavonoids from P. vulgaris L. The optimized results showed that the highest extraction yield of flavonoids by ultrasonic-assisted extraction could arrive 3.62%, using 41% (v/v) ethanol as solvent and 30:1 (mL/g) liquid to solid ratio for 30.5 min at the temperature of 79 °C. The suitability of the model equation for predicting the optimum response values was tested using the selected optimal conditions. The predicted extraction yield of flavonoids was 3.63%, which was consistent with the practical extraction yield of flavonoids of 3.62%. The crude extract was then purified by AB-8 macroporous adsorption resin, the flavonoids content in the purified extract increased to 81.58%. The antioxidant activities of the purified flavonoids were evaluated in vitro by scavenging capability of DPPH free radical and hydroxyl free radical, reducing power, total antioxidant capability and β-carotene bleaching test. The results indicated that the flavonoids have significant antioxidant activities, which can be used as a source of potential antioxidant or functional food material. We believe that the knowledge gained from this study should be helpful to further exploit and apply this resource.

Research Highlights
► The total flavonoids of Prunella vulgaris L. are extracted by ultrasonic method.
► The response surface methodology is used to optimize the flavonoids extraction.
► The optimum extraction parameters are obtained.
► The highest extaction yiels of the flavonoids is 3.62%.
► The purified flavonoids have significant antioxidant activities.