## 4 Discussion  
Speculating the reason for the success behind the SMW-WE methodology, previous authors have described that microwave-assisted extraction was a more effective technique compared to the maceration method, reflux method, Soxhet method and ultrasonic methods for the extraction of bioactive compounds from _Urtica dioica_ leaves and stems. Conventional extraction methods have been associated with high solvent requirements of toxic organic solvents, usually required long extraction time which increased risk of degradation of thermo-labile constituents thus resulting in lower extraction yields [44; 45; 46]. In the present study we have limited our extraction solvent to water, in order to rid the use of toxic organic solvents. When previous authors [46] used microwave extraction, they have reported that the extraction time was reduced, less solvent was used and the amounts of extracted compounds were increased. During microwave extraction, the solvent penetrates into the solid matrix by diffusion (effective), and the solute is dissolved until a concentration limited by the characteristics of the solid is attained. Then the solution containing the solute diffuses to the surface  
Figure 3: UV-Visible spectra of the extracts showing peaks pertaining to the flavonoids and phenolics present in Graviola. by effective diffusion. Finally, by natural or forced convection, the solution is transferred from the surface to the bulk solution, effecting successful extraction. It is reported that in microwave extraction, the process acceleration and high extraction yield are a result of the synergistic combination of two transport phenomena namely, heat and mass gradients working in the same direction [47; 48; 49]. However, in conventional extractions the mass transfer occurs from inside to the outside, although the heat transfer occurs from the outside to inside, this largely slows down the efficiency of the conventional extractions. By increasing the extraction efficiency, we can abstain from the use of efficient yet toxic organic solvents and stick to the use of water as the solvent system. With this idea in mind we have combined two effective extraction systems to facilitate more effective extraction. Prior to microwave extraction we have rendered a 5 min pulse of sonication, this is to facilitate the release of the cages bioactive compounds prior to microwave digestion. Sonication works on acoustic cavitation [49]: the formation, growth, and implosive collapse of bubbles in a liquid leading to rupturing of cell walls and glands for releasing the bioactive compounds easily during the subsequent microwave treatment. As observed from our results standalone sonication (S-WE) or microwave (MW-WE) did not yield as much extraction efficiency as when combined (SMW-WE). Combining both these advanced and accomplished extraction systems is that which lead to the enhanced extraction of Graviola bioactive components as demonstrated in this study. As Figure 4 demonstrates, the mode of action following the sono-microwave method (SMW-WE) for the effective extraction of the bioactive compounds in Graviola in water is as follows: (i) the 5 min bath sonication process, leads to the rupturing of cell walls through localized acoustic cavitation, creating the pathway for the release of the bioactive compounds locked within and (ii) the 2 min microwave treatment, enables the interaction of the solvent with the leaf material, the sonication process paved way for the enhanced penetration of the solvent, enabling easy and effective solubilization of the caged compounds, resulting in effective extraction. Figure 4: Schematic showing the mode of action for enhanced release of bioactive compounds by the demonstrated sono-microwave methodology for effective extraction of bioactive compounds from Graviola.