This process entails the extraction of solid MAPs by liquid sol- vents. This is a typical solid-liquid extraction process. Two factors that affect the extent and rate of extraction are the thermodynamics and kinetics (rate of mass transfer) of the process.
8.3.1 Thermodynamics of Solvent Extraction and Choice of Solvent
Relative sorption of solutes in the solvent depends on the in- teractions between the solutes and the solvent. Solubility or miscibility of a component with the solvent depends on their relative solubility parameters.
For mutual solubility of two components, their free energy of mixing, ΔGm should be negative. ΔGm is defi ned as
ΔGm = ΔHm – T ΔSm (3.1) Enthalpy of mixing, ΔHm can be correlated to cohesive energy density, i.e. solubility parameter (δ) as:
ΔHm = n1 n2 V1 (δ1 – δ2)2 (3.2) In equation (3.2), the solubility parameter is that due to only dispersive forces between structural units of the concerned solute and sol- vent, since the original regular solution theory of Scatchard and Hildebrand was restricted to non-polar, non-hydrogen bonding solute-solvent systems.
However, for many liquids and solutes, contributions from polar and hydro- gen bonding forces need to be considered. Accordingly, equation (3.2) be- comes:
ΔHm = n1 n2 V1 [(Δδd)2 + (Δδp)2 + (Δδh)2] (3.3) From equations (3.2) and (3.3), it is clear that to make ΔGm
negative, the difference between δi (solvent) and δi (solute), i.e. (Δδ) for all the three forces of interactions, should be as small as possible. It im- plies that the solvent and the desired solute to be extracted should have comparable polarity and hydrogen bonding capabilities to achieve similar solubility parameter values. Grulke has given an exhaustive tabulation of solubility parameters for the most common chemical compounds. The in-
dividual δh, δd and δp values for compounds not listed in the tabulation can be obtained using the group contributions due to various different groups given by Grulke. When the individual δi (solvent) and δi (solute) values are very close, a high solubility of the solute in the solvent is obtained. For instance, it is well known that non-polar solvents dissolve terpene frac- tions more than oxygenated compounds because both are non-polar. On the other hand, mixed solvents of polar and non-polar compounds can yield better results for oxygenated compounds. Bio-ethanol is a good solvent for such oxygenated compounds on two accounts: (i) it is natural, and (ii) it is
“green” (renewable). However, most MAPs contain water and the complete miscibility of ethanol with water implies dilution of the solvent after each use. This is further complicated by the fact that ethanol forms an azeotrope at high concentration (~95 wt%). As a result, ingress of small quantities of water is suffi cient to reach the azeotropic composition. Implementation of the Montreal Protocol, the Clean Air Act, and the Pollution Prevention Act of 1990 has resulted in increased awareness of organic solvent use in chemi- cal processing.
8.3.2 Solid-liquid Mass Transfer
The MAPs to be processed are in solid form. Solid-liquid extrac- tion is a typical heterogeneous mass transfer process. In such processes, the rate of extraction depends upon: (i) the interface area, and (ii) the mass transfer coeffi cient. Both should be high. High effective interface area can be obtained by comminuting the solid material to be processed. During com- minution, the ensuing friction can increase the temperature of the solid and thereby possibly lead to degradation of thermally labile components. To avoid this, special water-cooled roll crushers are used.
The mass transfer coeffi cient depends on the diffusivity of the solute in the solid matrix (main resistance) and the level of turbulence in the extractor. Traditional extraction has relied upon percolation or extraction in stirred vessels. In the case of percolation, the solid is packed in a vessel which is fi lled with solvent. The latter is allowed to percolate in the solid ma- trix under stagnant conditions. In the case of extraction in stirred vessels, different types of agitators are used to suspend the solid in the solvent and accelerate the mass transfer process. In both percolation and extraction in stirred vessels, the solvent is fi rst sorbed by the matrix of the solid. This sorption, which causes swelling of the matrix, is a relatively slow process.
However, once the matrix is swollen, the diffusion coeffi cient increases sev- eral fold or even by an order of magnitude as compared to the dry matrix.
Evidently the controlling step is the diffusion of the solute through the solid matrix to the surface of the solid. Once the solute is available at the surface, the solvent can dissolve it depending upon the rate of transport from the solid surface into the bulk of the solvent. In percolation vessels, this latter transport is predominantly by molecular diffusion and hence is slow, although not as slow as the transport through the solid matrix. The
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 HeatingMicrowave radiation interacts with dipoles of polar and polariz- able 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 in- stantaneous 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 con- duction. 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 mi- crowave 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) break- age of cell walls releases the oil trapped within it (Figure 1).
Table 1: Relative effi ciencies of common heating devices
Appliance Temperature,
°C Rating, W Time Energy used,
kWh
Energy cost, US$
Electric oven 177 2000 1 h 2 0.17
Convection
oven 163 1853 45 min 1.39 0.12
Gas oven 177 36 1 h 3.57 0.07
Microwave
oven High 1440 15 min 0.36 0.03
A B
Figure 1: Mint gland: (A) before and (B) after microwave irradiation (Microphotographs courtesy of Radient Technologies Inc.)
It is evident then that the main resistance to solid-liquid mass transfer, the transport of the solute through cell membrane, is eliminated because of the rupture of the cells. Besides cell breakage, the other advan- tages of microwave heating are:
Improved “existing” products 1.
Increased marker recovery 2.
Increased purity of the extract 3.
Reduced heat degradation 4.
Reduced processing costs 5.
Signifi cantly faster extraction 6.
Much lower energy usage 7.
Much lower (order of magnitude) solvent usage 8.
Potential for “new” products 9.
8.3.3.3 Literature on MAE
Some interesting results on MAE have recently been published.
For example, the extraction of vanillin from V. planifolia pods using MAE and ultrasound-assisted extraction has been described. Using absolute ethanol as the solvent at room temperature, the yield of vanillin was 1.25 wt% at each of 3 conventional extractions performed over 24 h. Using ultrasound-assisted extraction, the yield was 0.99 wt%, while it was 1.86 wt% using MAE. These in- vestigations clearly showed that vanillin extraction by MAE is superior to other techniques in terms of yield, purity of vanillin, and the time taken to extract the same percentage of the vanillin from the pods. The extraction of vanillin and p-hydroxy benzaldehyde (PHB) from vanilla beans using MAE has also been studied: MAE was superior to the conventional, offi cial method of extrac- tion in Mexico, which involves maceration of the beans with ethanol for 12 h.
Specifi cally, extraction time decreased 62-fold and vanillin and PHB concentra- tions increased between 40% and 50% with respect to the Mexican extraction method. This study also showed that extraction of commercial samples was superior to extraction of dried and lyophilized beans. This observation illus- trates the role played by moisture in aiding extraction, as discussed in Sec- tion 8.3.3.2. Several other investigations have shown that MAE has gained acceptance as a mild and controllable processing tool. MAE is a simple, rapid and low-solvent-consuming process.
8.3.3.4 Industrial-scale MAE
As mentioned earlier, microwave radiation decays exponentially inside a solid matrix. This aspect must be carefully weighed while designing industrial-scale MAE. The major requirements that must be met are:
Free distribution of particles allows uniform heating of all the 1.
particles in the solid bed. This criterion also enhances the extent and probability of proximity of the substrate to the wall of the sample holder where the microwave exposure is high- est. Most comminuted samples of MAPs which are used for commercial extraction are not of the same shape and size.
Therefore, there is a strong tendency to “segregate”, which must be curbed by regular renewal of the layer.
Thin and uniform spreading of the substrate layers. This per- 2.
mits complete and uniform penetration of microwave radia- tion even at large water contents.
Low depth of the layers. Since microwaves have low penetra- 3.
tion depth (~1.5 cm in H2O at 2.45 GHz), the layers should be
<1.5 cm thick.
Large-scale commercial (3 tonne/hour) MAE is available for in- dustrial use (www.radientinc.com). In view of the advantages of MAE and the development of equipment for large-scale commercial operation, MAE has a bright future. Figure 2 shows a fl owsheet for industrial-scale MAE.
Figure 2: Flowsheet of microwave-assisted extraction (courtesy, Radient Technologies Inc).