**3. Essential factors influencing MAE and mechanism of action**

Several studies had been done on optimizing MAE factors to achieve optimal yields from the considered plant samples. The operative parameters influencing MAE include solvent-to-feed ratio, solvent composition, characteristic of the plant sample and its water content, microwave power, irradiation time, stirring effect, microwave energy density, and extraction temperature. These operative parameters determine the efficiency of MAE. Hence, understanding the influences and interactions of these parameters on the extraction process is paramount.

#### **3.1 Solvent-to-feed ratio**

The selection of solvent is the most significant factor that affects microwaveassisted extraction. Adequate solvent selection will produce an efficient extraction process. The solubility of the compound of interest, mass transfer kinetics of the process, and solvent penetration that occurs from the interaction between the dielectric effect and sample matrix are inevitable parameters [24, 25]. Chan et al. reported that the selection of extraction solvent depends on the capacity of that solvent to absorb microwave energy [26]. If the solvent has a high dielectric constant and dielectric loss, the solvent capacity to absorb microwave energy will be high [25]. Tatke and Jaiswal reported that solvents such as methanol, ethanol, and water are excellent microwave-absorbing solvents which possess sufficient polarity to be heated up through microwave power [27]. Studies had shown that the addition of a small quantity of water to polar solvent resulted in higher diffusion of water into the cells of the matrix, leading to effective heating and thus facilitating the transport of compounds into the solvent at higher mass transfer rates [24, 26, 28].

*Microwave-Assisted Extraction of Bioactive Compounds (Review) DOI: http://dx.doi.org/10.5772/intechopen.96092*

Veggi et al. had reported that the extraction solution must not exceed 30–34% (w/v) [29]. In the past studies, the solvent-to-feed ratio between 10:1 (mL/g) and 20:1 (mL/g) had been reported to give optimal yields [29, 30]. The volume of extracting solvent is another important factor, a large volume of solvent requires more energy and time to condense extraction solution in the purification process. MAE may give lower recoveries because of non-uniform distribution and exposure to microwave [29].
