Microwave-assisted extraction of Eucalyptus robusta leaf for the optimal yield of total phenolic compounds

<i>Eucalyptus robusta (E. robusta)</i> has a significant value in traditional medicine and recently has been shown to possess many pharmacological properties <i>in vitro</i>. This study was designed to utilise microwave-assisted extraction (MAE) to yield optimal total phenolic content (TPC), total flavonoid content (TFC), proanthocyanidin levels and antioxidant capacity from <i>E. robusta</i> using water as the solvent, facilitated by the use of response surface methodology (RSM). A three-level-three-factor Box–Behnken design was implemented to elucidate the effect of irradiation time, power and sample-to-solvent ratio on the yields of these phytochemicals. The results highlighted the accuracy and reliability of RSM as a tool for predicting the yields of TPC, TFC, proanthocyanidins and total antioxidants using MAE. Sample-to-solvent ratio had the greatest impact on the TPC yield followed by power and irradiation time. The optimal MAE conditions for TPC and TFC were 3 min, 600 W power and 2 g/100 mL sample-to-solvent ratio. The experimental yield of TPC was 58.40 ± 1.03 mg GAE/g, and 19.15 ± 1.06 mg RE/g of TFC was obtained under these optimal conditions. These conditions, optimised for maximum TPC yield also liberated 62%, 64.6%, 66.3% and 67% of the maximum proanthocyanidins, ABTS, DPPH and CUPRAC values, respectively. This study revealed that MAE is a reliable and efficient method for extracting high yields of phytochemicals from <i>E. robusta</i>, with significant potential to be up-scaled for industrial, nutraceutical or pharmaceutical applications