Sequential extraction of quercetin-3-O-rhamnoside from Piliostigma thonningii Schum. leaves using microwave technology

Piliostigma thonningii (Schum.) Milne-Redh. is a plant rich in quercetin-3-O-rhamnoside (quercitrin), a flavonoid involved in the antioxidant and antimicrobial processes. Microwave assisted extraction (MAE) is a method which gives better extraction yield, enhance the quality of extracts while decreasing the extraction time. This effect, a sequential optimization by response surface methodology using a central composite design, help to determine the optimal conditions for obtaining more antioxidant compounds of P. thonningii leaves. The response surface curves showed that there was a positive interaction between the extraction time and the solvent concentration on the DPPH scavenging and iron chelating activities of the extracts. The optimal ethanolic extraction parameters for the highest yield of flavonoids were an extraction time of 69 s, an irradiation power of 380 W and a solid–liquid ratio of 1/10 (w/v). On the residue, the optimal extraction parameters for simultaneously obtaining the highest flavonoids yield and the highest antioxidant activity were an extraction time of 49 s, an irradiation power of 520 W and an ethanol concentration of 67% (v/v). HPLC analysis has shown the second optimization helped to further maximize the extraction of active compound quercetin-3-O-rhamnoside. Electron microscopy of the powders before and after extraction has shown that microwave heating causes cellular damage. Compared to the maceration extraction method, the combined extracts of sequential MAE provide higher antioxidant activities.