Microwave-assisted extraction of antioxidant compounds from Russian olive leaves and flowers: Optimization, HPLC characterization and comparison with other methods

Russian olive is a medicinal plant containing phytochemicals with antioxidant activities, which are of great interest due to their beneficial health effects. Our preliminary studies produced extracts from Russian olive leaves and flowers and determined their potential antioxidant activity and their most abundant phenolic compounds were detected. However, the maximum retention of these antioxidant compounds during microwave-assisted extraction (MAE) requires optimized process conditions. Therefore, two-level fractional factorial designs in preliminary tests were used for assessing the magnitude of microwave extraction factors' effects on the extraction of bioactive compounds from Russian olive samples and the experimental domains were determined. Subsequently, Response surface methodology (RSM) was conducted to determine the optimized MAE process variables including citric acid’s molarity (X1: 1–5 M), extraction temperature (X2: 60–110 °C), ethanol concentration (X3: 0–100%) and solid to solvent ratio (X4: 5–15) for maximum recovery of total phenolic content, total flavonoid content and total antioxidant compounds in the extracts. Based on the results, the optimal extraction conditions were as follows: solid to solvent ratio of 7.5 (w/v), citric acid molarity of 2 M, ethanol concentration of 59.8% and 66.4%, and temperature of 97.4 °C and 97.5 °C for Russian olive leaves and flowers, respectively. In addition, the influence of extraction parameters on the responses was modeled using a second-order regression equation and a good agreement between the predicted values and the experimentally obtained values was indicated. Finally, among phenolic compounds which were previously identified through Quadrupole Time of Flight Mass Spectrometer (QTOF-MS) analysis, three abundant flavonoids of the extracts with therapeutic properties including isorhamnetin, luteolin and rutin contained in the optimized extract from MAE were compared with that of ultrasonic extraction and conventional extraction using high-performance liquid chromatography (HPLC) analysis. Results confirmed MAE as an efficient method for extraction of bioactive phytochemicals from Russian olive.