Ultrasound-assisted extraction of flavonoids and phenolic compounds from Ocimum tenuiflorum leaves

Ultrasound-assisted extraction (UAE) was optimized for extraction of flavonoids and phenolics from dried leaves of Ocimum tenuiflorum using response surface methodology with a central composite rotatable design and independent variables of ethanol concentration (30-70%), sonication time (5-15 min), and ultrasonic energy density (UED) (0.15-0.35 W/cm3). A significant (p<0.0001) quadratic model described experimental data with a non-significant lack of fit (p>0.05). Linear and quadratic effects of independent variables significantly (p<0.001) influenced flavonoid content (FC) and total phenolic content (TPC) yields. Optimized UAE conditions of ethanol concentration, sonication time, and UED were 55.34%, 11.71 min, and 0.26 W/cm3, respectively, resulting in predicted FC and TPC yields of 6.69 mg of quercetin equivalents QE/g and 9.41 mg of gallic acid equivalents GAE/g, respectively. HPLC analysis revealed different phenolic and flavonoid compounds. UAE extracts exhibited better in vitro antioxidant activities and thermo-oxidative stability values than conventional solvent extracts.