Extraction Technologies For Medicinal And Aromatic Plants - Unido

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
stirred vessels, on the other hand, provide a high level of turbulence and
hence facilitate transport into the bulk solvent phase. In both percolation
and stirred vessels, the dominant resistance is diffusion through the solid
matrix. It is then clear that even stirred vessels with high power inputs may
not intensify the mass transfer process. Therefore, instead of focusing on
the transport at the solid surface, it is desirable to increase the rate of
transport through the solid matrix by rupturing the cells which contain the
solute or oil and consequently bring the same in direct contact with the
solvent.
8.3.3 Microwave-assisted Extraction
8.3.3.1 Principle of Microwave Heating
Microwave radiation interacts with dipoles of polar and polarizable
materials. The coupled forces of electric and magnetic components
change direction rapidly (2450 MHz). Polar molecules try to orient in the
changing fi eld direction and hence get heated. In non-polar solvents without
polarizable groups, the heating is poor (dielectric absorption only because
of atomic and electronic polarizations). This thermal effect is practically instantaneous
at the molecular level but limited to a small area and depth
near the surface of the material. The rest of the material is heated by conduction.
Thus, large particles or agglomerates of small particles cannot be
heated uniformly, which is a major drawback of microwave heating. It may
be possible to use high power sources to increase the depth of penetration.
However, microwave radiation exhibits an exponential decay once inside a
microwave-absorbing solid.
The various industrial techniques used for heating are listed
in Table 1, which shows that microwaves have the highest effi ciency when
compared with the other competitive techniques.
8.3.3.2 Mechanism of MAE
In microwave-assisted extraction (MAE): 1) the heat of the microwave
irradiation is directly transferred to the solid without absorption by
the microwave-transparent solvent; 2) the intense heating of step 1 causes
instantaneous heating of the residual microwave-absorbing moisture in the
solid; 3) the heated moisture evaporates, creating a high vapor pressure; 4)
the vapor pressure generated by the moisture breaks the cell; and 5) breakage
of cell walls releases the oil trapped within it (Figure 1).
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