Comparative Study on Conventional and Microwave-Assisted Ethanol Extraction of Phenolics and Flavonoids from Waste Onion Leaves

Abstract

This study explored the utilization of waste onion leaves for the extraction of important bioactive compounds known for their health benefits such as phenolics and flavonoids. Conventional solvent (CSE) and microwave extraction (MAE) of flavonoids and phenolics from waste onion leaves using aqueous ethanol as solvent were performed. The effects of different factors such as ethanol concentration (50 %v/v & 80 %v/v), temperature (60 °C & 80 °C for CSE) or power level (10 % & 50 % for MAE), and extraction time (1 hr & 4 hrs for CSE, 10 sec & 60 sec for MAE) on the total flavonoid yield (TFY) yield and total phenolic yield (TPY) were evaluated via a two-level factorial experiment. Results showed that for CSE, ethanol concentration, extraction time and the interactions of the factors significantly affected the TFY (p ≤ 0.05), while all the three factors, ethanol concentration-temperature interaction and ethanol concentration-extraction time interaction significantly affected the TPY (p ≤ 0.05). On the other hand, for MAE, ethanol concentration and power level have significant effects on the TFY (p ≤ 0.05) while the ethanol concentration significantly affected the TPY (p ≤ 0.05). The maximum extractable flavonoid and phenolic yields from the onion leaves were found to be 8.61 mg quercetin equivalent (QE)/g dry weight (DW) and 8.95 mg gallic acid equivalent (GAE) /g DW, respectively. For CSE, high levels of both ethanol concentration (80 %v/v) and low level of extraction time (1 hr) resulted in higher values of TFY (2.63 mg QE/g DW) and TPY (6.15 mg GAE/g DW). For MAE, higher values of TFY (2.98 mg QE/g DW) and TPY (4.24 mg GAE/g DW) were obtained at the high level of ethanol concentration (80 %v/v ethanol), high power level (50 %), and high level of extraction time (50 sec). MAE was found to be a more advantageous extraction method than the conventional one, because of its comparable flavonoids and phenolics recovery at a shorter extraction time.