Microwave power and temperature are interrelated because high microwave power can bring up the temperature of the system and result in the increase of the extraction yield until it becomes insignificant or declines [ 4, 42, 45 ] . It is known that the temperature is controlled by incident microwave power that controls the amount of energy provided to the matrix, which is converted to heat energy in the dielectric material.
At high temperatures the solvent power increases because of a drop in viscosity and surface tension, facilitating the solvent to solubilize solutes, and improving matrix wetting and penetration [ 13, 43, 46 ] . In addition, when MAE is performed in closed vessels, the temperature may reach far above the boiling point of the solvent, leading to better extraction ef fi ciency by the desorption of solutes from actives sites in the matrix [ 8 ] . However, Routray and Orsat [ 7 ] state that the ef fi ciency increases with the increase in temperature until an optimum temperature is reached and then starts decreasing with the further increase in temperature: this happens because the selection of ideal extraction temperature is directly linked with the stability and, therefore, with the yield of the target compound.
Microwave power is directly related to the quantity of sample and the extraction time required. However, the power provides localized heating in the sample, which acts as a driving force for MAE to destroy the plant matrix so that the solute can diffuse out and dissolve in the solvent. Therefore, increasing the power will generally improve the extraction yield and result in shorter extraction time [ 4, 28 ] . On the other hand, high microwave power can cause poor extraction yield because of the degradation of thermally sensitive compounds. Also, rapid rupture of the cell wall takes place at a higher temperature when using higher power, and as a result impurities can also be leached out into the solvent together with the desired solute [ 13 ] . Therefore, it is important to properly select the MAE power to minimize the time needed to reach the set temperature and avoid a “bumping” phenomenon in temperature during the extraction [ 8 ] . Moreover, the overexposure to microwave radiation, even at low temperature or low operating power, was found to decrease the extraction yield because of the loss of chemical structure of the active compounds.