In all experiments, the extraction yield was significantly timedependant and increased with extended ultrasonic times, especially
from 2 to 20 min, but slowly from 20 to 40 min. Thus, the efficient extraction period for achieving maximum yield of pomegranate
seed oil was about 20 min. This can be attributed to the fact that extraction presents two stages; the first stage, which is
characterized by a rapid rate, involves the penetration of the solvent into the cellular structure followed by the dissolution of soluble
constituents in the solvent, whereas the second one involves the external diffusion of soluble constituents through the porous
structure of the residual solids and its transfer from the solution in contact with the particles to the bulk of the solution. Zhang
et al. (2008), who studied the ultrasound-assisted extraction of oil from flaxseeds, also reported that the effect of ultrasound is
more effective in the first 30 min. They attributed this observation to the fact that ultrasonic wave could disrupt the cell walls, so larger contact area between solvent and material was created and more oil was appeared on the surface. However, this effect would be increasingly weak on the inner cell walls as the distance is increased. Thus, the ultrasonic waves affect the mass transfer rate mainly in the solvent penetration stage. A similar trend was reported by Pan et al. (2012), who extracted antioxidants from the
dry peels of pomegranate marc applying ultrasonic irradiation in continuous and pulsed modes.