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The word ‘aroma’ was originally intended to describe the pleasant odours emitted by substances and organisms, but over a period of time, the word has come to include both agreeable and disagreeable smells.
The aroma of the aromatic substances and organisms is due to the presence of chemical compounds that vapourise into the atmosphere at the ambient temperature. Large quantities of the aromatic compounds in plants are thus ‘wasted’ through this volatile nature of these compounds.
Determining if a species is aromatic is not always an easy matter. Plants that cannot be considered as
aromatic have their own smells, as for example, a freshly mowed lawn or crushed grass. Any odour, other than the ‘normal smell’ emitted by a bruised green plant, is regarded as aroma. A more scientific basis is the occurrence of an aromatic compound which is associated with the characteristic smell of a species.
AROMATIC COMPOUNDS
The chemistry of a number of aromatic compounds, particularly those with applications in the industry, has been well studied. Aromatic compounds are generally a closed chain class of organic compounds derived from the benzene ring, as opposed to the fatty or aliphatic compounds. While it is generally true that aromatic compounds owe their odour to an aromatic side chain, compounds with such a structure are not always perceptibly aromatic and also some compounds that do not have an aromatic component, may be aromatic.
Aromatic compounds in plants belong to a variety of classes of chemical compounds such as aromatic ketones, terpenoids, phenolic acids, carboxylic acids, esters, alcohols, aldehydes, etc., but the bulk of them belong are generally called essential or ethreal or volatile oils (Gibbs, 1974). However, a chemist does not recognise essential oils as a class of chemical compounds.
The esters almost invariably impart a pleasant smell. Strong, sometimes pungent or otherwise disagreeable, smells arise from the sulphur compounds while the really foetid odours are due to methyl mercaptans.
ESSENTIAL OILS
Essential oils are produced by over 2,000 species of flowering plants. The diversity of their taxonomic distribution is reflected in Appendix 23. If all organisms are considered together, the number of species with essential oils may exceed 400,000 species. Essential oils evaporate on contact with air and emit the odour. Fatty oils do not behave this way, although some of them do evaporate. The precussors of essential oils are usually formed in the leaves during several biosynthetic processes and the final products are assembled in special glandular structures where they are stored and from which they are released into the atmosphere. These structures are concentrated in different parts of the plant body in different species.
Chemically,essential oils are different from the true fats and oils, the latter being comprised of fatty acids. However, as the physical properties of the essential oils resemble those of fats and as they are soluble in fat solvents, essential oils are often treated along with fats and oils.
Over 500 different chemical compounds have been identified among essential oils, that include both terpenoid and non-terpenoid derivatives. A number of species of aromatic plants contain more than one, sometimes several, aromatic compounds, as for example, the all-spice tree (Pimenta dioica, Myrtaceae).
ROLE OF AROMATIC COMPOUNDS IN PLANT BIOLOGY
Except as attractants of pollen vectors, and as pest and pathogen deterrents, aromatic compounds have no known role in the life of plants. They are by products, often considered as waste products, of plant metabolism. Synthesis of aromatic compounds by plants is subject to genetic and environmental factors, which regulate their quality and quantity.
DISTRIBUTION OF AROMATIC COMPOUNDS IN THE PLANT BODY
In general, any part of a plant, or the whole plant, can contain aromatic compounds, but they are usually concentrated in one or few related organs in different species as in the following instances:
Vegetativeparts:
Coriandrum sativumRoots: Vetiveria zizanioides
Rhizome:Zingiber officinale, Curcuma domestica
Stem bark: Cinnamumum zeylanicum
Wood: Cinnamomum camphora, Santalum album
Leaves: species of Eucalyptus and Cymbopogon
Flowering tops: Mentha spicata
Flower buds: Syzygium aromaticum
Petals: most aromatic species
Styles: Crocus sativus
Green fruits: Capsicum frutescens
Dried fruits: Cuminum cyminum, Trachyspermum ammi
Seeds: Myristica fragrans, Eletteria cardamomum
Aril: Myristica fragrans