Handbook of Herbs and Spices - Volume 3

Volatiles from herbs and spices 179

occasionally an oxygen functional group (e.g. cinnamaldehyde in cinnamon oil). The

aromatic ring may be substituted with up to four oxygens, which are then further

modified themselves by the addition of a methylenedioxy ring, as in safrole.

11.3 Biosynthesis of the components of volatile oils...........................

Chemicals produced by plants that are characterized by a limited distribution, and an

absence of obvious value in the physiology of the producer plant, are known as

secondary metabolites. The array of secondary metabolites, which of course includes

volatile oils, is enormous. The terpenes constitute a major group, with more than

1000 monoterpene and perhaps 3000 sesquiterpene structures known. By contrast,

the number of phenylpropenes is small, with probably less than 50 being known

(Waterman 1993).

Despite the vast numbers and structural diversity of secondary metabolites, almost

all arise from one of the three biosynthetic pathways, or from a combination of two

or more of these pathways. These are known as the acetate, mevalonate (based on

Camphene a-pinene Carane

Bicyclic monoterpenes

O O

O

(–) Limonene (–) Carvone Pulegone Menthone

Monocyclic monoterpenes

OH

Nerol Citronellol

OH

Geraniol

OH

Acyclic monoterpenes

Fig. 11.2 Structures of selected monoterpenes.