Supercritical fluid extraction (SFE) with carbon dioxide (CO2) is already established as a process for the decaffeination of coffee beans and tea. Recent studies have focused on other applications, especially with botanical materials and thermally liable substances (Walker et al., 1999; Rizvi et al., 1994; Rozzi and Singh, 2002; Brunner, 2005; Moura et al., 2005; McHugh and Krukonis, 1994; Mukhopadhyay, 2000). Many excellent review articles of supercritical fluid technology exist. Subjects covered include: a general overview (Chester et al., 1998), the food industry (Brunner, 2004; Rozzi and Singh, 2002), Chinese herbal medicine (Chen and Ling, 2000), pharmaceutical research (Vasukumar and Bansal, 2003), biotechnology (Williams and Clifford, 2000) and natural product studies (Lang and Wai, 2001; Sovova, 2005).
The development of new separation techniques for pharmaceutical and food industries has received a lot of attention due to rising concerns about the health, environmental and safety hazards associated with traditional solvent techniques. Organic solvent residues can remain in the final product as well as promote oxidative degradation of these sensitive nutraceutical compounds during processing; these disadvantages are of special concern in the purification of pharmaceuticals, nutraceuticals and food products. Additional set-backs in the use of these potentially toxic organic solvents include the high initial investment and regeneration. SFE with CO2 is a focus of interest due to the advantages of limiting auto-oxidation and thermal decomposition of the products and offers complete removal of the non-toxic, inexpensive and inert solvent. SFE with CO2 is used in the extraction of desired components from natural materials for eventual use in food, perfumery, pharmaceutical and nutraceutical industries (Goto et al., 1998; Mukhopadhyay, 2000; Rozzi and Singh, 2002).
The SFE process in solids is based on the contact between a solid raw material and pressurized solvent, which removes the compounds of interest from the solid phase. After this removal, the extract separates from the solvent through a pressure reduction. The kinetics of the solute extraction consists of releasing the solutes from porous matrices into the supercritical fluid via mass transfer mechanisms. The extraction of the solute involves the dissolution of the solid component, diffusion of the solute to the solid surface and external mass transfer around the solid particle. Mathematical models have been proposed to correlate overall extraction curves (OEC) during the SFE process (Goto et al., 1993). However, no single model has been universally accepted.
CO2 is an ideal, alternative, extraction solvent because it prevents harmful oxidation reactions and can enter a supercritical state under low temperature and pressure conditions (31.1 °C and 7.38 MPa) (Rizvi, 1994; Krukonis, 1988). Supercritical CO2 has the additional advantages of low cost, non-toxicity, high diffusivity and low viscosity. Solvent separation from the extract is easily accomplished by reducing the pressure and returning the CO2 to a gaseous state. SFE using CO2 is a promising technique with the advantages of limiting the auto-oxidation, decomposition, and polymerization of polyunsaturated fatty acids (PUFAs) found in animal, fish, and fungal oils (Amano et al., 1992; Cohen and Heimer, 1992; Leman, 1997). Application of SFE with CO2 has increased since CO2 is a non-toxic, non-flammable, inexpensive, 'green' and generally regarded as safe (GRAS) solvent (Rizvi et al., 1994; Rozzi and Singh, 2002; Letisse et al., 2006). The application of SFE to fish oil fatty acids has been studied by many authors (Stinson et al., 1991; O'Brien and Senske, 1994; O'Brien et al., 1993; Bajpai et al., 1991; Yazawa, 1996; Letisse et al., 2006) and recent detailed studies have focused on other applications, especially botanical materials and other thermally labile substances (Yongmanitchai and Ward, 1989; Certik and Sajbidor, 1996; Singh and Ward, 1997; Kendrick and Ratledge, 1998; Vazhappilly and Chen, 1998; Molina Grima et al., 2003).
SFE utilizes the ability of normally gaseous chemicals to become excellent solvents for certain solutes under a combination of tunable properties in terms of temperature and pressure. The solvent becomes supercritical when it is raised above its critical point for both temperature and pressure (Tc and Pc, respectively). For CO2, the Tc is 31.1 °C and Pc is 7.38 MPa. Only one phase exists in the critical region (hash marks in Fig. 10.1) that possesses both gas- and liquid-like properties. A supercritical fluid has liquidlike densities and a viscosity close to that of normal gases. The diffusivity for a supercritical fluid is about two orders of magnitude higher than that
for typical liquids: (0.2-0.7) x 10 3 cm2/s compared with (0.2-2.0) x 10 5 cm2/s (Brunner, 1994, 2005; Mukhopadhyay, 2000; Rozzi and Singh, 2002). The low viscosity and other 'gas-like' properties allow for the solvent to diffuse more readily through the solid matrix. These characteristics facilitate rapid mass transfer and faster completion of extractions compared with traditional liquid extraction techniques.
McHugh and Krukonis (1994) have given a detailed historical perspective of the developments related to supercritical fluids. SFE technology, after initial ups and downs in its developments, started to become a possible alternative extraction technology in many fields in the late 1980s and early 1990s.
The most commonly used supercritical fluid, as an extraction solvent, is CO2. The phase diagram given in Fig. 10.1 shows the supercritical region for CO2. CO2 is a non-polar fluid, has a solvating power comparable with hexane and is widely used for the extraction of non-polar compounds. Modifiers in the form of appropriate solvents such as ethanol may be used to extract polar compounds using CO2. The major problems associated with the conventional solvent extraction industry (flammability, possibilities of toxic residues, waste disposal regulations and environmental concerns) have resulted in increased attention for SFE. SFE, apart from overcoming problems associated with conventional solvent extraction, also offers additional advantages such as selective extraction and frac-tionation of high-value components in the extract under optimized extraction conditions.
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