CONTROL, SEGUIMIENTO, EVALUACIÓN Y ACTUALIZACIÓN

Microwaves have been recognized as an outstanding source of energy to promote extraction. Microwave-assisted solvent extraction, developed in the late 1980s, is an alternative to conventional extraction techniques for natural products. In this method, extraction is carried out with a suitable solvent and microwave radiation. Yield is comparable and even better

Table 2.2. Applications of accelerated solvent extraction for the extraction of phytochemicals.

Phytochemicals Plant/plant parts Extraction conditions Reference Taxanes (paclitaxel,

baccatin III and 10-deacetylbaccatin III) Taxus cuspidata (Japanese yew) bark Methanol–water (90:10); temperature: 150°C; pressure: 10.13 MPa Kawamura et al., 1999 Total anthocyanins

and total phenolics Dried red grape skin Acidified water, methanol–acetone– water–hydrochloric acid (40:40:20:0.1); temperature: 50°C; pressure: 10.1 MPa Ju and Howard, 2003 Antioxidants and

polyphenols Apple pomace Ethanol (60%); temperature: 160–193°C/75–125°C; pressure: 10.3 MPa

Wijngaard and Brunton, 2009 Isoflavones Radix puerariae Ethanol (60%);

temperature: 60–100°C; pressure: 1400 psi

Lee and Lin, 2007 Furanocoumarins Pastinaca sativa Petrol, methanol;

temperature: 60–100°C; pressure: 60 bar Waksmundzka- Hajnos et al., 2004 Flavonoid glycosides and amino acids

Scutellaria lanteriflora Water; temperature: 85–190°C; pressure: 10 MPa Bergeron et al., 2005 Cocaine and

benzoylecgonine Erythroxylum coca Methanol; temperature: 80°C; pressure: 20 MPa Brachet et al., 2001 Silymarin Silybum

marianum Water; temperature: 110–180°C Bunnel et al., 2010 Kavain Piper

than conventional methods, but in less time. Ganzler and co-workers were the first to report the use of microwave radiation for the extraction of nat- ural products (Ganzler et al., 1986a,b; Ganzler and Salgo, 1987). During MASE, the solvent is heated as a result of the absorption of microwave ra- diations by dipoles existing in the solvent or sample material. Microwave treatment leads to vaporization of water in the sample, destroying the cel- lular system and thereby enabling an exhaustive extraction. In contrast to conventional heating, no temperature gradient exists in the case of MASE; therefore, an even heating of the solution is ensured, thus facilitating the heating of the whole sample simultaneously. Disruption of the hydrogen bond due to the dipole rotation of the molecule is an added advantage in the case of extraction using microwaves. For MASE, the sample and/or solvent must have sufficient dielectric constant as microwave absorption ability depends on the dielectric constant. The larger the dielectric con- stant of the solvent, the more optimal the heating. Also, in some cases the matrix itself interacts with the microwaves as the surrounding solvent has a low dielectric constant (Jassie et al., 1997). Solvents used for MASE cover a wide range of polarity; however, to improve extraction selectivity and the ability of the medium to interact with the microwave, a combination of more than one solvent can be used (Renoe, 1994). Water has the highest microwave absorption, whereas hexane is literally transparent to micro- waves and thus does not heat up (Tatke and Jaiswal, 2011). Consequently, the moisture content in the sample matrix promotes better extraction as water superheats locally and promotes the release of analytes into the sur- rounding medium. Further control of the moisture of the matrix ensures reproducible extractions (Onuska and Terry, 1993; Budzinski et al., 1996; Jassie et al., 1997).

Microwave radiation is usually applied for short intervals, followed by an interval for cooling so that overheating is avoided. The temperature is also monitored externally by infrared sensor. Two types of instruments are available for MASE: (i) closed-vessel microwave extractor (CV-MAE); and (ii) focused open-vessel microwave extractor (FOV-MAE) (Fig. 2.2). Instrumentation is the main difference between CV-MAE and FOV-MAE. In closed-vessel extractors, the vessel containing the sample and solvent is closed, whereas in FOV-MAE it is open. In the case of CV-MAE, extrac- tion is carried out under controlled pressure and temperature increase is achieved rapidly. Pressure depends on the volume and boiling point of the solvent as the solvents may be heated to about 100°C above their boiling point under atmospheric conditions in CV-MAE. The temperature can be fixed by adjusting the microwave power (100–1000 W), and power should be suitably chosen to avoid overheating, leading to degradation of analyte and overpressure. Both extraction efficiency and speed are re- ported to be enhanced in this procedure (Pare, 1990, 1991; Barnabas et al., 1995; Young, 1995; Jassie et al., 1997).

MASE in FOV-MAE is carried out at atmospheric pressure, and the max- imum temperature depends on the boiling point of the solvent. The solvent is heated and refluxed through the sample. Here, the microwave is focused

on the sample placed in the vessel; this generates homogeneous and ef- fective heating. It has been reported that, as compared to the closed-vessel system, open-vessel MASE offers better safety and allows a larger sample to be extracted (Renoe, 1994; Letellier et al., 1999; Kaufmann and Christen, 2002).

The MASE method could serve as a good alternative process for the preparation of high-value bioactive extracts, and the method has been utilized to extract a large number of bioactive phytochemicals from plant sources. Dai et al. (2001) reported the influence of various operating param- eters, for example microwave power, solvent and irradiation time, on the recovery of azadirachtin-related limonoids (AZRL) from the seed kernel, seed shell, leaf and leaf stem of the neem tree. The extraction efficiency of MASE was compared with conventional extraction methods. MASE en- hanced the extraction of AZRL from different parts possessing micro- structure. Solvent also influenced the selectivity of microwave- assisted extraction. Cocaine and benzoyleconine were extracted from coca leaves using MASE (Brachet et al., 2002). Quantitative recovery of cocaine from leaves was obtained in 30  s, and MASE-generated extract was similar to extract obtained from conventional extraction methods. Recoveries of lipophilic markers, mainly alkamide, from the roots of Echinacera pur-

pura Monarch was evaluated by applying MASE (Hudaib et al., 2003).

Soxhlet and UASE reference methods were compared with MASE in terms of solvent and extraction time. Using methanol (70%) as the solvent, both MASE and UASE were found to be superior compared to Soxhlet. Both MASE and UASE were further evaluated by applying a different ratio of

Cooling water Condensing coil To drain Energy attenuator Connecting tube Boiling water Microwave cavity

Magnetic stirrer Time controller

Time presetting

Temperature recorder Shielded

thermistor

Fig. 2.2. Schematic diagram of an open-vessel microwave extractor. (From Mandal

methanol–water (60–70%) as the solvent system. Recoveries were higher in MASE than in UASE, over 70–100% methanol range, while values were comparable at 60% methanol. The effects of the degree of grinding, solvent–material ratio and dielectric constant of solvent were optimized for the extraction of artemisnin from Artemisia annua (Hao et al., 2002). Optimal conditions for MASE were: duration of irradiation, 12  min; diameter of raw materials, less than 0.125 mm; and solvent–sample ratio, more than 11.3. Under the optimized conditions of MASE, the yield (92.1%) was higher than with Soxhlet and normal stirring extraction (60%). Notably, supercritical fluid extraction using carbon dioxide had the lowest extraction yield of artemisnin. Four extraction methods in- cluding MASE were compared for recovery of the anticancer drug, camp- tothecin (CPT), from Nothapodytes foetida. The maximum percentage of extraction (2.67%) of CPT was obtained by MASE. The extraction time for MASE (3 min) was less than that for Soxhlet (120 min), UASE (30 min) and stirring extraction (30  min). Also, methanol (90%, v/v) yielded more extract than ethanol (90%, v/v). Shu et al. (2005) also reported that MASE (15 min, 150 W) yielded a better percentage of ginsenosides Rg1

(0.28%, 70% methanol–water) and Rb1 (1.31%, 30% water–ethanol)

in comparison to 10  h solvent extraction (0.22% of Rg1 and 0.87% of

Rb1 obtained in 70% water–ethanol). Similarly, in the case of the tax-

ane class of natural products from the needles of Taxus, MASE reduced both extraction time and solvent consumption considerably while main- taining the qualitative and quantitative recovery of taxane relative to SLE methods. Casazza et al. (2010) compared SLE, UASE and MASE for the extraction and antioxidant power of phenolics from Vitis vinifera wastes. Although the highest content of total polyphenol, o-diphenols and flavon- oids for seeds and skins were obtained using high pressure and tempera- ture extraction, the highest antiradical power was determined in seed extracts obtained using MASE. For saponins extraction from chickpea (Cicer arietinum), MASE proved better compared with Soxhlet extraction. A butanol– water mixture exhibited selectivity towards saponin extrac- tion. MASE contributed stability of heat-sensitive major saponins, that is, DDMP (2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4- one)-conjugated saponins (Karem et al., 2005). Liazid et al. (2007) investigated stability of 22 phenolic compounds of different families (benzoic acid, benzoic alde- hydes, cinnamic acids, catechins, coumarins, stilbenes and flavonoids) under MASE at working temperatures between 50°C and 175°C. It was reported that all the compounds were stable up to 100°C, but at 125°C significant degradation of epicatechin, resveratol and myricetin was ob- served. Chemical structure and relationship studies revealed that com- pounds with a greater number of hydroxyl-type substitutions degraded more easily under extraction conditions.

For essential oil extraction from the leaves of Rosamirunus officianals, Bousbia et al. (2009) reported the microwave hydrodiffusion and gravity (MGH) technique and compared its effectiveness with hydrodistillation.

No solvents or water were used in MGH, and this method yielded es- sential oil containing a high amount of oxygenated compounds, thereby increasing the antimicrobial and antioxidant activities of the essen- tial oil. Tigrine-Kordjani et al. (2006) also reported microwave-assisted, solvent-free distillation of essential oil from different aromatic plants. The direct interaction of microwaves with stem produced from the water pre- sent in fresh plant materials favours the release of essential oils trapped inside the cells of plant tissues. Dai et al. (2010) also reported the enhanced yield of three mint compounds, menthone, menthol and menthofuran, from peppermint leaves.

Ionic liquids, considered as ‘green solvents’, are also being used in microwave-assisted extraction. Du et al. (2009) used ionic liquids (ILs) for the extraction of phenolic compounds from Psidium guajva leaves and

Smilax china tubers, and reported that ILs could replace efficiently the

conventional reflux method with methanol. Some of the applications of MASE for the extraction of phytochemicals are summarized in Table 2.3, and comparisons of MASE with some other extraction methods are sum- marized in Table 2.4.