Polar compounds supercritical fluid extraction

Supercritical fluid extraction — During the past two decades, important progress was registered in the extraction of bioactive phytochemicals from plant or food matrices. Most of the work in this area focused on non-polar compounds (terpenoid flavors, hydrocarbons, carotenes) where a supercritical (SFE) method with CO2 offered high extraction efficiencies. Co-solvent systems combining CO2 with one or more modifiers extended the utility of the SFE-CO2 system to polar and even ionic compounds, e.g., supercritical water to extract polar compounds. This last technique claims the additional advantage of combining extraction and destruction of contaminants via the supercritical water oxidation process."... 

Supercritical fluid extraction (SEE) is another modern separation technology usually employed to extract lipophilic compounds such as cranberry seed oil, lycopene, coumarins, and other seed oils. Anthocyanins generally and glycosylated anthocyanins in particular were considered unsuitable for SEE due to their hydrophilic properties, since SEE is applicable for non-polar analytes. However, a small amount of methanol was applied as co-solvent to increase CO2 polarity in anthocyanin extraction from grape pomace. New applications of SEE for anthocyanin purification have been reported for cosmetic applications from red fruits. ... 

As its name suggests, supercritical fluid extraction (SEE) relies on the solubilizing properties of supercritical fluids. The lower viscosities and higher diffusion rates of supercritical fluids, when compared with those of liquids, make them ideal for the extraction of diffusion-controlled matrices, such as plant tissues. Advantages of the method are lower solvent consumption, controllable selectivity, and less thermal or chemical degradation than methods such as Soxhlet extraction. Numerous applications in the extraction of natural products have been reported, with supercritical carbon dioxide being the most widely used extraction solvent. However, to allow for the extraction of polar compounds such as flavonoids, polar solvents (like methanol) have to be added as modifiers. There is consequently a substantial reduction in selectivity. This explains why there are relatively few applications to polyphenols in the literature. Even with pressures of up to 689 bar and 20% modifier (usually methanol) in the extraction fluid, yields of polyphenolic compounds remain low, as shown for marigold Calendula officinalis, Asteraceae) and chamomile Matricaria recutita, Asteraceae). " ... 

Extraction with supercritical CO2 is a technical process of increasing importance. It provides a mild and rapid technique for the extraction of low- or medium-polarity substances. Supercritical CO2 is used for supercritical fluid extraction (SFE) in important technical processes such as the decaffeination of coffee and the extraction of hops, as well as the extraction of naturally occurring compounds from biomaterials. As many applications are performed in the pharmaceutical, polymer, environmental and nutritional fields, direct on-line SFE-NMR would be an ideal tool to monitor the various extraction processes. 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

In more recent years, supercritical fluid extraction has been found to be useful for the extraction of low to moderately polar compounds. The requirements in any particular case depend on the characteristics of the matrix, the drug, and the drug metabolites. The concentration range of the drug is also obviously important, as it will determine the methods selected for the separation and analytical procedures. 

Moore WN, Taylor LT. Analytical inverse supercritical-fluid extraction of polar pharmaceutical compounds from cream and ointment matrices. J Pharm Biomed Anal 1994 12 1227-1232. 

One of the problems with using a liquid as the extraction solvent is its removal when the extraction is finished. The most recent way to eliminate this problem is to use a supercritical gas, COj being the gas of choice at the moment. A gas in the supercritical state has solvent properties comparable to a liquid but it is less viscous, so it can penetrate the sample faster. When the extraction is complete, the pressure is released, and the gas evaporates away from the extracted components. CO2 is nonpolar so more polar compounds such as methanol are sometimes added in small amounts. This exceWeni supercritical fluid extraction (SEE) technique is described in Chapter 13. 

Hills et al. (1991) applied simultaneous supercritical fluid extraction to roasted coffee beans. This technique can be used with or without a derivatizing reagent In a dissociative mechanism, the adsorbed analyte must first desorb from a matrix active site and be dissolved in the supercritical fluid and then react to form the less polar derivative, which favors solvation in the supercritical carbon dioxide. In the associative mechanism, derivatization occurs while the analyte is adsorbed on the active site of the matrix. Reaction with the adsorbed analyte results in the desorption of the non-polar derivative into the supercritical fluid . Thus 2-hexenedioic acid (E.49) was identified for the first time as a native compound (without the use of a derivatizing agent). Benzenic and furanic compounds and caffeine were also identified. 

Luque de Castro, M. D. and Tena, M. T., Strategies for supercritical fluid extraction of polar and ionic compounds, Trends Anal. Chem., 15, 32-31, 1996. 

Another widely-used extraction technique is supercritical fluid extraction (SFE) with supercritical CO2. The extraction capability of supercritical CO2 is dependent upon the applied pressure. For pressures in the range of 40 bar, CO2 has an extraction polarity similar to that of hexane. For pressures near 400 bar, the extraction behavior is similar to that of dichloromethane. Thus, by using a pressure gradient, effective extraction of lipophilic compounds can be performed. Figure 7-10 shows a feasible arrangement for online SFE-NMR coupling [13], [14]. 

Apart from these techniques, supercritical fluids extraction (SEE) is the preferred technique in many areas of active compounds extraction. However, in what concerns to flavonoids, its utility is highly influenced by the matrix s composition. For example, in the case of isolation of non-polar flavonoid aglycones by SEE, when compared with classical techniques, SEE provided identical or sometimes better results. Nevertheless, when water-soluble glycosides are considered, the use of SEE results in considerably different yields.Still, the use of aqueous methanol constitutes a useful and balanced extraction solvent that allows the extraction of both aglycones and flavonoid glycosides, depending on the conditions used, such as time and temperature, among others. Other reported solvents used for extraction are acetone, ethanol, and dimethyl sulfoxide (DMSO), with... 

Such findings are in agreement with other works that compare different extraction methods [38-39]. Overall, SFE affords richer extracts, with higher number of separated compounds the use of EtOH as co-solvent increases the antioxidant potential of the extract, if compared with pure CO2 extraction, due to the increase in solvent polarity the supercritical fluid is able to extract important compounds not detected in conventional extracts, such as oleic acid and phytol, used in cosmetic products. 

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