and Singh, 2002). However, supercritical CO2 cannot be used for dissolving polar molecules
(e.g., hydroxyl, carboxyl, or nitrogen) because of its non-polar nature (Sihvonen et al., 1999).
For these situations, addition of a polar co-solvent (e.g., ethanol) with supercritical CO2
enhances extraction efficiency (Li and Hartland, 1996). In addition, solubility of CO2 decreases
with increasing molecular weight (Palmer and Ting, 1995). The major disadvantage is that
application of supercritical CO2 requires high pressure extraction vessels leading to high capital
and operating costs. However, costs of supercritical extraction processes are competitive with
other processes, and sometimes supercritical processes are unique in their ability to produce
solvent-free products and handle high viscosity materials (Brunner, 2005).
Because supercritical fluids have the solvating power of a liquid and better mass transfer
characteristics than traditional solvents, interest in extraction with supercritical fluids has been
growing rapidly in recent years (McHugh and Krukonis, 1994; Zhang et al., 1995). Solvents such
as ammonia, ethylene, toluene, and CO2 show promise for supercritical fluid extraction. Of
these, CO2 offers unique advantages: it is abundant, non-reactive, non-toxic, and harmless
(O’Toole et al., 1986). The natural flavor and aroma of the extracts (e.g., oils) are preserved
during supercritical CO2 extraction since it is carried out at low temperatures (as low as 31oC)
and in an inert CO2 atmosphere (Temelli, 1987; Palazoglu and Balaban, 1998).
Conventional extraction methods such as solvent extraction using Soxhlet apparatus, although
effective for extraction of oils, can lead to degradation of heat sensitive compounds as well as
leave traces of toxic solvents in the solute. This is a concern for food and medicinal extracts.
Supercritical CO2 extraction is a viable alternative to solvent extraction methods due to low
critical temperature (31.1oC) (Tonthubthimthong et al., 2001). Moreover, supercritical CO2
extraction is a popular technique for oil extraction due to its high extraction efficiency, short
extracting time, lower refining requirement, and absence of chemical residues or contamination
in the extracted oils (Bhattacharjee et al., 2007).
Currently, commercial plants exist for decaffeinating coffee and tea, extracting beer flavoring
agents from hops, and separating oils and oleoresins from spices using supercritical CO2
(Palmer and Ting, 1995; Brunner, 2005). Supercritical CO2 replaces water or methyl chloride for
the extraction of these materials.
Palmer and Ting (1995) tabulate references in the literature where supercritical CO2 was used
on a wide range of raw materials for: (i) extraction of fats and oils, (ii) extraction of cholesterol,
(iii) fractionation of fats and oils, (iv) refining fats and oils (e.g., deacidification and
deodorization), (v) flavor/aroma extraction, and (vi) extraction of other food constituents such as
pigments and antioxidants. Selected examples from the above categories, and additional
applications of supercritical CO2 extraction from the recent literature are discussed below.
Supercritical CO2 has been used to extract oils from a variety of raw materials such as peanut
(Santerre et al., 1994), almond (Marrone et al., 1998), pistachio nut (Palazoglu and Balaban,
1998), wheat plumule (Zhang et al., 1998), pecan (Li et al., 1999), egg yolk (Wu et al., 2001),
garlic (Xu et al., 2005), hazelnut (Ozkal et al., 2005a), and cotton seed (Bhattacharjee et al.,
2007). In addition, Zhang et al. (1995) lists the references for supercritical CO2 extraction of
soybean oil, peanut butter, corn oil, evening primrose oil, peppermint and spearmint oil, and
fractional separation of marjoram leaves and onion juice. Rozzi and Singh (2002) provide a list
of studies where supercritical CO2 has been used for extraction of lipids from brown seaweed,
butter oil, corn bran, ground beef, fungi, milk fat, oat bran, pecan, pistachio, pork, rapeseed, rice
bran, safflower, soybean, and sunflower.
In several studies, lipids extracted using supercritical CO2 were found to have superior qualities
to those obtained from traditional solvent extraction methods. For example, Friedrich et al.
(1982) found that soybean oil extracted with supercritical CO2 (20.7 to 69.0 MPa and 50oC) was