5. Extraction and Isolation Methods

Aromatic components and their precursors are generally present in aqueous solution or as droplets in the cell sap, although some essential oils may exist in oil sacs, glandular hairs, etc. It is necessary to extract or to isolate the odour / flavour complex as completely as possible from the mass of inert cellular matter with the minimum amount of chemical change.

1. Expression - the physical extraction of aqueous juices from plant tissues, of particular interest in studies on fruit flavours.
2. Solvent Extraction - the solvent used may be either water from which the aromatic components may be recovered by high vacuum vaporization, or low-boiling point nonpolar solvents (e.g., ether, cyclohexane, methylene dichloride, hexane), or liquified gaseous solvents (e.g., Freons or liquified carbon dioxide). The solvent depends on the physical nature of the start material and its susceptibility to oxidative.
3. Steam Distillation at Atmospheric Pressure - is the most used method of isolation and recovery of aromatic compounds from plant materials, although precautions must be taken to limit thermal degradation of components.
4. Vacuum Distillation - is used for the distillation of high molecular weight substances, which need hidg temperatures for their distillation at atmospheric pressures, resulting in chemical decomposition.
5. High Vacuum Degassing - applicable to the recovery of volatiles from fixed oils and foods having a high lipid content.
6. Headspace Vapour Collection - this technique is important for the examination of the low volatile components. If the material under examination is allowed to stand in a suitable vessel, the low-boiling volatile components will achieve the equilibrium in the head-space. These vapors can then be used for direct identification using gas chromatography. The results obtained are, of coarse not representative of the full odour / flavour of the start material.