Natural flavourings from green chemistry for foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 260

Table 12.1 Extraction solvents permitted for use in the preparation of food ingredients
according to EU legislation

Restricted by MRL* Unrestricted Organic

Diethyl ether Propane Water

Hexane Butane Carbon dioxide

Cyclohexane Ethyl acetate Ethanol

Methyl acetate Ethanol

Butan-1-ol Carbon dioxide

Butan-2-ol Acetone

Butan-2-one Nitrous oxide

Dichloromethane Water

Methanol
Propan-1-ol
Propan-2-ol
1,1,1,2-tetrafl uoroethane

Note: *Maximum residue level .

12.2.1 Application of liquid and supercritical CO 2 as an alternative
extraction and fractionation solvent for essential oils and resins
Processes that either reduce or eliminate the use of volatile organic chemicals
(VOCs) can be described as 'green' and supercritical fl uids (SCFs) are generally
considered to be green in terms of their applications and value in both reducing
and replacing VOCs. The most important SCF is carbon dioxide.
Carbon dioxide (CO 2 ) can be used as an extraction solvent both as liquid or
supercritical CO 2 (McHugh and Kroukonis 1994; Reverchon 1997), and
supercritical CO 2 in particular is now well-established as an extraction solvent in
the food industry. A supercritical fl uid is a substance above its critical temperature
( T c) and pressure ( P c); the T c for CO 2 is 304K (31°C) and P c is 72.8 bar
(7.3 MPa), making the supercritical fl uid phase readily accessible (Brogle 1982;
Clifford 1998; Jessop and Leitner 1999; Arai et al. 2002; Kerton 2009). Above the
critical point, CO 2 exists as a supercritical fl uid (Fig. 12.1) and the manipulation
of temperature and pressure produces a highly tuneable solvent that can be used
to selectively extract a wide range of molecules. Most commercial extraction
plants are designed to operate using supercritical CO 2 rather than liquid CO 2 for
this reason.
The properties of SCFs have been described as an intermediate between gas
and liquid and have the advantages of each phase, high diffusivity and low
viscosity. The physical properties of gaseous, liquid and supercritical CO 2 are
shown in Table 12.2.
Supercritical CO 2 is an ideal solvent for extraction as the solvent is easily
removed owing to its 'zero' surface tension and extraction products are recovered
in a solvent-free state. Its rapid diffusion can enhance extraction rates, and after
the extraction, the solvent can be recycled, resulting in nearly zero waste. It is
non-toxic, non-fl ammable, colourless, odourless and tasteless which makes it a

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