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PR and SRK equations gave similar solubility values. This is expected since, in general, these mixtures are formed from the same family of substances.

Different substances have different critical temperatures and pressures, important in selecting the appropriate fluid and operating conditions for an application (Table 5). If the critical pressure is too high, processing will be more difficult and costly because equipment must be designed for high pressures. If the critical temperature is too high, heat-sensitive materials such as many food constituents will be altered or destroyed.

Carbon dioxide is the solvent of choice for food applications for many reasons (25). It is (1) inert and noncor-rosive in dry environments, (2) nonflammable and nonexplosive, (3) abundant and inexpensive, (4) nontoxic and accepted as a harmless ingredient of foods and beverages, (5) easy to separate the solute from the solvent, and (6) has desirable physical properties such as low Tc (31°C) and low Pc (7.39 MPa), low viscosity, low surface tension, and high diffusivity. When pressure is decreased, C02 will separate out from the solute as gas. C02 can also protect foods from oxygen and limit oxidation reactions. The solubilities of some substances in liquid C02 are given in Table 6. This can be extrapolated to the SC C02 extraction. Since the C02 molecule is nonpolar, at high densities it is a good solvent of lipophilic organic substances and esters, ethers, and lactones. In general, hydroxyl and carboxyl groups reduce the solubility of a substance in SC C02. Polar or charged materials such as sugars and amino acids are not soluble. Large molecules such as cellulose and proteins are also insoluble. The solubilities of substances such as waxes, resins, and pigments increase with an increase in the density of SC C02. Solubility is also affected by the intermolecular forces of the solute molecules, which enable the C02 molecules to surround and hold them in solution (5). In general, as the number of carbon-carbon double

Table 5. Critical Property Data for Some Supercritical Solvents
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