5.4. Extraction with dense or liquefied gases

There are several commercial plants in operation for extracting flavouring materials from herbs using liquid CO₂. Moyler (1993) describes a plant at Bletchtley in England, in which extraction is coming out at about 8ºC and 60 bar (point 3 on the figure below). Product recovery involves boiling off this liquid carbon dioxide at 13 ºC, 48 bar.

Range of solvent conditions over which extraction operations with compressed and liquefied gases have been reported. (except where stated, carbon dioxide is the solvent and the process is used commercially).

Liquid CO₂ has several advantages over most conventional organic solvents, namely:

non flammability;
because it is gaseous under ambient conditions, it is easily removable;
colourless and without odour or taste;
cheap and easy to obtain;
Extracts obtained by this method cover a broader spectrum of volatilities than do those obtained by hydrodistillation, which confers to the oil the absolute character;
Extracts are free of residues from the extraction solvent.

In the subcritical state, liquid CO₂, behaves as a non-polar solvent, with temperatures between -55ºC and 31ºC, and pressure between 5 and 74 bar.

Under normal working conditions (temperature 0º-10ºC, pressure 50-80 bar) it is a selective solvent, dissolving mainly non polar and slightly polar components with molecular weights up to about 400 (Moyler, 1988).

Components such as chlorophyll, carotenoids and alkaloids are virtually insoluble in liquid CO₂. Water solubility is 0.1% (w/w) at 20ºC (Sims, 1982).

The relative solubilities in liquid CO₂ of some classes of compounds typically present in herbs and other plant materials are summarised in the table below (Moyler, 1988). It is seen from this table that virtually all the useful flavour components are extracted.

Relative solubility in liquid CO₂ of compounds found in plants (Moyler, 1988)
Miscible/very miscible	Low solubility	Insoluble
Non-polar and slightly polar organic compounds of low molecular weight (< 250). Examples include: Aliphatic hydrocarbons Alcohols Aldehydes Acetones Esters Monoterpenes Sesquiterpenes	Higher molecular weight compounds: Aliphatic hydrocarbons Alcohols Aldehydes Acetones Esters Substituted terpenes and sesquiterpenes Carboxylic acids Relatively polar compounds with amino groups	Sugars, proteins Polyphenols Tannins, some waxes, inorganic salts Chlorophyll, carotenoids, citric, malic acids Higher molecular weight compounds