Supercritical fluid extraction 23 the value of the diffusion coefficient under these conditions(D=4x10-8m2s-)with hexane at NTP(D=4x10-9m2s-). These values are fairly representative and it is generally observed that self-diffusion coefficients for NCFs under typical extraction con ditions are about an order of magnitude greater than in liquid solvents. Diffusion coeffi cients of solutes in NCFs are generally enhanced to a similar extent( Section 2.3.3) Volatility(vapour pressure) In conventional extraction processes liquid solvents are recovered by distillation at elevated temperature(and/ or reduced pressure)in which valuable volatile components of he extract can be lost. Near-critical fluids are highly volatile and can be completely removed and recycled at low temperatures during an extraction process. This has important implications for improving the quality of extracts, since (1) Highly volatile components in the extract are retained. This is of particular gnificance in the extraction of flavours and fragrances ( 2) The extract is not subjected to thermal or chemical degradation(e.g. oxidation)at the elevated temperatures employed in distillation 3) The high volatility ensures'complete'removal of solvent residues. Any legislative restrictions regarding residual solvent levels are thereby avoided Chemical properties Of all NCFs, CO2 is the safest medium for use in solvent extraction as it provides a non flammable, non-oxidative environment. CO2 does, however, undergo chemical reactions with water which often need to be considered when extracting food materials. One familiar set of reactions is the dissolution of CO2 in water to produce carbonic acid The carbonic acid then dissociates and lowers the ph of the aqueous phase in contact ith CO2 H,CO2+Ho H3O*+ HCO3 HCO +HO H20+C The pH of water is therefore primarily determined by the partial pressure of Coz with which it is in contact. Water in contact with atmospheric CO2 has a pH of approximately 5.7 at 20C and 3. 8 when contacted with pure CO2 at the same pressure. With increasing pressure the pH falls further, so that when in contact with liquid CO at 100 bar the pH is about 3. This represents a fairly typical acidity for water in an NCF CO2 extraction ince K2>K3 the hydrogen ion concentration is primarily determined by the dissociation of carbonic acid. If the increased acidity is problematic it is possible to suppress the dissociation and buffer the coexisting aqueous phase by addition ofSupercritical fluid extraction 23 the value of the diffusion coefficient under these conditions (D = 4 x lo-* m2 s-l) with hexane at NTP (D = 4 x lo-' m2 s-'). These values are fairly representative and it is generally observed that self-diffusion coefficients for NCFs under typical extraction con￾ditions are about an order of magnitude greater than in liquid solvents. Diffusion coeffi￾cients of solutes in NCFs are generally enhanced to a similar extent (Section 2.3.3). Volatility (vapour pressure) In conventional extraction processes liquid solvents are recovered by distillation at elevated temperature (and/or reduced pressure) in which valuable volatile components of the extract can be lost. Near-critical fluids are highly volatile and can be completely removed and recycled at low temperatures during an extraction process. This has important implications for improving the quality of extracts, since: (1) (2) (3) Highly volatile components in the extract are retained. This is of particular significance in the extraction of flavours and fragrances. The extract is not subjected to thermal or chemical degradation (e.g. oxidation) at the elevated temperatures employed in distillation. The high volatility ensures 'complete' removal of solvent residues. Any legislative restrictions regarding residual solvent levels are thereby avoided. Chemical properties Of all NCFs, C02 is the safest medium for use in solvent extraction as it provides a non￾flammable, non-oxidative environment. C02 does, however, undergo chemical reactions with water which often need to be considered when extracting food materials. One familiar set of reactions is the dissolution of C02 in water to produce carbonic acid: (2.1) The carbonic acid then dissociates and lowers the pH of the aqueous phase in contact with C02: K, C02+H20 4 H2C03 K* H2C03 + H20 e H30+ + HCOT (2.2) HCOC + H20 e H30+ + C0:- K3 (2.3) The pH of water is therefore primarily determined by the partial pressure of C02 with which it is in contact. Water in contact with atmospheric C02 has a pH of approximately 5.7 at 20°C and 3.8 when contacted with pure C02 at the same pressure. With increasing pressure the pH falls further, so that when in contact with liquid C02 at 100 bar the pH is about 3. This represents a fairly typical acidity for water in an NCF C02 extraction process. Since K2 S K3 the hydrogen ion concentration is primarily determined by the initial dissociation of carbonic acid. If the increased acidity is problematic it is possible to suppress the dissociation and buffer the coexisting aqueous phase by addition of