Optimization of ultrasound and microwave-assisted extraction of phenolic compounds from olive leaves by response surface methodology

Plant secondary metabolites, phenolic compounds have been extensively studied and are commonly used as antioxidants for a wide range of applications. The response surface methodology (RSM) based on a central composite design was applied to determine the optimum conditions for the ultrasound (UAE) and microwave-assisted extractions (MAE) of polyphenol and flavonoid components from olive leaves (Olea europaea L.). For UAE, two operational variables were evaluated: liquid/solid ratio (l/s ratio) (4, 12 and 20) and the extraction time (10, 25 and 40 min). For MAE, three process variables were evaluated: liquid/solid ratio (4, 12 and 20), the extraction time (1, 3 and 5 min) and the power (100, 200 and 300W). The total polyphenol content (TPC) and total flavonoid content (TFC) were significantly influenced by liquid/solid ratio. In fact, the RSM applied for UAE showed that 37.07 of the liquid/solid ratio and the time of 28.69 min were the optimum conditions. The RSM applied on MAE showed the optimal extraction conditions obtained by using liquid/solid ratio of 16.76, extraction time of 1.81 min and the power of 259.46 W. The MAE represented an interesting alternative protocol for rapid, economic and high eco-extraction using water as green solvent.