Steytler bicarbonate anion( Lovell, 1988). The pressure of Co 2 could, however, be used to control the pH of water in a unique fashion since no chemical residues(of acids or bases)remain The potential applications of this technique have not been widely explored A less familiar reaction of CO, with water is the formation of a solid hydrate below about CO2+6H,O=CO2 6H,O (2.4) This restricts the use of NCF CO in the extraction of aqueous systems to temperat up to 10C higher than the freezing point of water. (Note: This depends on the type concentration of the solute. At modest levels CO? is non-toxic and so represents a completely safe NCF solvent fo food applications with no legislative restrictions governing its use. The only possible, but unlikely, physiological hazard involves asphyxiation by displacement of air following a considerable leak in a confined area The combined effects of high hydrostatic pressure and low acidity in water-containing systems can be beneficially employed to prevent food spoilage by destroying bacteria Kamihira et aL., 1987: Taniguchi, 1987a). Rapid decompression of dissolved gas is sometimes used to expand and disrupt the cell structure of natural materials and could also be used as a means of sterilisation. Although SCF CO2 can be an effective apol medium for enzyme reactions(van Eijs et aL., 1988; Steytler et aL., 1991), it has also been used to selectively inactivate enzymes(Taniguchi, 1987b; Weber, 1980). In practice these chniques could be applied either in situ, during an extraction process, or as a separate unit operation 2. 3 PROPERTIES OF NCF SOLUTIONS 2.3.1 Solubilities in NCFs There has been much confusion in some of the literature concerning the solvent properties of NCF CO2. An impression is often given that NCFs are universal solvents which can be to extract virtually any component of a mixture by selecting a suitable set of conditions of temperature and pressure. Statements to the effect that NCFs are 'good solvents, implying that solute loadings are high, are also prevalent and highly misleading. Before examining the solvent properties of NCFs in detail, it is worth stating a few basic principles (1) To be'supercritical'intermolecular attractive interactions must be relatively weak compared with thermal energy. This necessitates an absence of all polar inter actions, such as hydrogen bonding, and defines a medium of low dielectric onstant. All NCFs are therefore essentially apolar solvents (2) The absence of strong attractive interactions between molecules means that solvation energies are generally low and solubilities in NCFs are thus often much lower than in liquid solvents24 D. Steytler bicarbonate anion (Lovell, 1988). The pressure of C02 could, however, be used to control the pH of water in a unique fashion since no chemical residues (of acids or bases) remain. The potential applications of this technique have not been widely explored. A less familiar reaction of C02 with water is the formation of a solid hydrate below about 10°C: C02 + 6H2O = C02.6H20 (2.4) (g) (1) (SI This restricts the use of NCF CO, in the extraction of aqueous systems to temperatures up to 10°C higher than the freezing point of water. (Note: This depends on the type and concentration of the solute.) Biochemical properties At modest levels C02 is non-toxic and so represents a completely safe NCF solvent for food applications with no legislative restrictions governing its use. The only possible, but unlikely, physiological hazard involves asphyxiation by displacement of air following a considerable leak in a confined area. The combined effects of high hydrostatic pressure and low acidity in water-containing systems can be beneficially employed to prevent food spoilage by destroying bacteria (Kamihira et al., 1987; Taniguchi, 1987a). Rapid decompression of dissolved gas is sometimes used to expand and disrupt the cell structure of natural materials and could also be used as a means of sterilisation. Although SCF C02 can be an effective apolar medium for enzyme reactions (van Eijs et al., 1988; Steytler et al., 1991), it has also been used to selectively inactivate enzymes (Taniguchi, 1987b; Weber, 1980). In practice these techniques could be applied either in situ, during an extraction process, or as a separate unit operation. 2.3 PROPERTIES OF NCF SOLUTIONS 2.3.1 Solubilities in NCFs There has been much confusion in some of the literature concerning the solvent properties of NCF C02. An impression is often given that NCFs are universal solvents which can be ‘tuned’ to extract virtually any component of a mixture by selecting a suitable set of conditions of temperature and pressure. Statements to the effect that NCFs are ‘good’ solvents, implying that solute loadings are high, are also prevalent and highly misleading. Before examining the solvent properties of NCFs in detail, it is worth stating a few basic principles: (1) To be ‘supercritical’ intermolecular attractive interactions must be relatively weak compared with thermal energy. This necessitates an absence of all polar inter￾actions, such as hydrogen bonding, and defines a medium of low dielectric constant. All NCFs are therefore essentially apolar solvents. The absence of strong attractive interactions between molecules means that solvation energies are generally low and solubilities in NCFs are thus often much lower than in liquid solvents. (2)