GLOSARIO DE TÉRMINOS

As with microwaves, ultrasound (frequency range 18–40  kHz) has been recognized as an excellent energy source for promoting extraction. UASE is much faster than conventional extraction methods due to the high con- tact surface area between solid and liquid phase. High-frequency sound Table 2.3. Application of MASE for the extraction of phytochemicals.

Phytochemical Plant Solvent Reference Azadirachtin-related limonoids Azadirachta indica Methanol–water Dai et al., 2001 Cocaine and benzoylecgonine Coca leaves Methanol Brachet et al., 2002 Alkamides Echinacea purpurea

(L.) Monarch

Methanol–water Hudaib et al., 2003 Ginsenosides Panax ginseng Methanol–water,

ethanol–water

Shu et al., 2005 Camptothecin,

9-methoxycamptothecin

Nothapodytes foetida Methanol (90%),

ethanol (90%)

Fulzele and Satdive, 2005

Taxanes Taxus spp. Mattina et al., 1997

Saponins Cicer arietinum n-Butanol and

water mixture Karem et al., 2005 Essential oil Rosmarinus

officinalis No solvent Bousbia et al., 2009

Glycyrrhizic acid Licorice root Ethanol

energy releases phytochemicals from plant materials through cavitation. Cavitations, or the formation and collapse of microscopic bubbles, release tremendous energy as heat, pressure and mechanical shear, thereby enhan- cing mass transfer and facilitating solvent access to the cell (Chemat et al., 2004; Novak et al., 2008). Ultrasound energy has been shown to improve extraction from vegetal tissues through the action of accelerating the rehy- dration or swelling of the plant cell that is accompanied by the fragmen- tation of the tissue matrix through mass transfer and penetration of the solvent into the cell, thereby promoting the absorption of the cell contents in the solvent (Vinatoru et al., 1997; Toma et al., 2001). The amount of ultrasonic cavitations in the solvent mixture is affected by surface tension, viscosity and vapour pressure (Chen et al., 2007). Low vapour pressure solvents produce few cavitations, whereas high vapour pressure solvents produce more cavitations, but they collapse with less intensity and are therefore not very effective in extraction. Cavitations occur more easily in low-viscosity solvents, and also the ultrasonic intensity applied could ex- ceed the molecular forces of the solvent more easily. As low-viscosity liquids have lower density and high diffusivity, they can diffuse easily in the pores of plant materials. Surface tension also influences cavitational effects. Low surface tension solvents require low energy to produce cavitations (Mason et al., 1996; Gutiérrez et al., 2008). Ultrasound-assisted extraction (UAE) is fast compared with traditional extraction methods like Soxhlet and cold percolation methods, and has the potential to increase analytical throughput while keeping solvent volumes and sample masses low.

In this method, finely powdered plant material mixed with solvent is sonicated in an ultrasonicator for about 10–60 min at a particular tem- perature (Fig. 2.3). If the compound is thermally unstable, extraction is carried out at low temperature to avoid thermal degradation or damage. Non-polar, polar and a combination of non-polar and polar solvents can be used in UASE. A combination of non-polar and polar solvents with ultrasonic energy has been reported to act like an emulsification–extraction, resulting in rapid and efficient extraction of total lipids from solid mat- rices (Perez-Serradila et al., 2007). Ultrasonic power, volume and polarity of solvent, solvent and sample ratio, extraction time and pH are the im- portant parameters influencing UASE.

Table 2.4. Comparison of Soxhlet extraction, MASE, ASE and SFE.

Factor Soxhlet MASE ASE SFE

1. Investment 2. Process time 3. Solvent consumption 4. Method development 5. Sample treatment Small Long (up to 48 h) High (200–500 ml) Simple Required Medium Short (<30 min) Low (40 ml) Simple Required Large Short (<30 min) Medium (<100 ml) Simple Required Large Short (<60 min) Minimal (<5 ml) Labour-intensive Not required

The effects of ultrasonics have been studied for more than 100 plant species, and UASE has been reported for the extraction of phenols, gin- senosides, anthraquinones and polycylic hydrocarbons (Wu et al., 2001; Chriestensen et al., 2005; Hemwimol et al., 2006; Richter et al., 2006; Ahh

et al., 2007). Quan et al. (2009) reported that UASE was highly efficient

in the extraction of ferulic acid from Angelica sinensis. Chen et al. (2007) reported that UAE was more efficient and rapid for the extraction of anthocyanins from red raspberry in comparison to conventional solvent extraction, possibly due to strong disruption of the fruit tissue structure under ultrasonic acoustic cavitation. The findings were further corroborated by scanning electron microscopic (SEM) studies. However, no changes in the composition of anthocyanins, as confirmed by high-performance liquid chromatography (HPLC), were observed. Conventional extraction and UASE were compared for the extraction of steroids and triterpenoids from three Chresta species, namely C. exsucca, C. scapigera and C. sphaer-

ocephala (Schinor et al., 2004). Total extraction time was reduced sig-

nificantly in the case of UASE, and also this method was reported to be more effective for the extraction of steroids and most of the triterpenes. Li et al. (2005) reported that UASE was highly efficient for the extraction of chlorogenic acid from Eucommia ulmoides and also from other Chinese medicines as compared to classical methods. The influence of four vari- ables was investigated with regard to the extraction efficiency of chloro- genic acid from fresh leaves and fresh and dried barks of E. ulmoides. Aqueous methanol (70%) in a solvent–sample ratio equal to 20:1 (v/w) with an extraction time of 90 min (3 × 30 min) were the optimum extraction conditions. The extraction efficiency of ethanol was improved for the isola- tion of carnosic acid from Rosmarinus officinalis. Ethanol, which is a poor

Ultrasonic probe

Solvent Solid sample

solvent for the extraction of carnosic acid under conventional conditions, had a similar level of extraction to that of ethyl acetate and butanone in MASE. The yield of carnosic acid was also improved and the extraction time was shortened at the same temperature (Albu et al., 2004). Stavarache

et al. (2005) reported that the base-catalysed transesterification of vegetable

oil with short chain alcohols with ultrasound (28  kHz and 40  kHz) was much shorter than with mechanical stirring. The quantity of catalyst re- quired for transesterification was also reduced by two to three times. Forty kilohertz ultrasound was found much more effective in shortening the reac- tion time, but the yield was better at 28 kHz. However, higher frequencies were not useful for the transesterification of fatty acids. Extraction tempera- ture is a factor that must be taken into account. This can be illustrated with the extraction of saponins from ginseng roots. UASE was found to be more efficient and three times faster than conventional Soxhlet extraction (Wu

et al., 2001). Together with total saponin content, the yield of ginsenosides

Rb1, Rb2, Rc, Rd and Rf were determined individually after using both an

ultrasonic bath (38.5  kHz, 810  W) and an ultrasonic probe (3  mm diam- eter tip, 20 kHz, 600 W) during extraction. Formation of ginsenoside Rg3

and Rh2 was reported because of thermal extraction process from the more

abundant ginsenosides, Rb1 and Rc (Popovich and Kitts, 2004).

The application of ultrasound irradiation facilitates low temperature rupturing of plant cell membranes, thereby liberating molecules from cellular structures, but extract cannot be separated completely from the solvent at the end. In general, UASE is carried out at a lower temperature than other forms of extraction, and this helps to avoid the degradation of thermally unstable ingredients in plant materials. Some applications of UASE for the extraction of phytochemicals are summarized in Table 2.5.