in fl uenced by the dielectric properties of the solvent. Solvents such as water, ethanol, and methanol may heat up tremendously on longer exposure, thus risking the future of thermolabile constituents [ 13 ] .
Occasionally, when longer extraction time is required, the samples are extracted in multiple steps using consecutive extraction cycles, which are also an example of the use of a larger amount of solvent and higher microwave application time [ 28, 42 ] . In this case, the fresh solvent is fed to the residue and the process is repeated to guarantee the exhaustion of the matrix. With this procedure, the extraction yield is enhanced, avoiding long heating [ 7, 28 ] . The number of process cycles will depend on the type of matrix and the solute. According to Li et al. [ 43 ] , three cycles of 7 min were appropriate for MAE of triterpene saponins from yellow horn, whereas in optimization of triterpenoid saponins MAE from Ganoderma atrum , cycles of 5 min each were recommended [ 26 ] . Yan et al. [ 44 ] found that three extraction cycles of 5 min each are optimal for extracting astragalosides from Radix astragali. They also found that increasing the irradiation time from 1 to 5 min increases the extraction yield rapidly; extraction reaches its maximum at 5 min, and then the yields decreased with the extension of the irradiation time. In the case of fl avonoids extraction from R. astragali , there was an increase in yield with time up to an exposure of 25 min and then the extraction yield started to decrease [ 42 ] . In the work of Chen et al. [ 26 ] it was observed that triterpenoid saponins yield from Ganoderma atrum reached its maximum at 20 min; after this time, the target compounds easily decomposed because of long exposure to high temperature. The same behavior was found by Song et al. [ 35 ] .