The origin of natural aromatic materials

 Some of these chemicals make up essential oils which comprise of materials from a number of chemical groups. Only as recently as fifty years ago the isolation of natural materials had to be undertaken physically, by chemical reactions to identify compounds. This limited the ability to identify more than just a few compounds in essential oils. The advent of spectroscopic methods created a revolution in natural product chemistry which enabled the identification of trace constituents and more detailed understanding of the chemistry of plants. The spectrum of odorous substances is very narrow where only materials with a molecular weight below 300-400 and an appreciable vapour pressure at room temperature have noticeable odours to humans. Relatively few organic materials have pleasant odours, with the majority of materials diffusing acetic, propionic, butyric and lactic odours. Essential oils are not the only chemical substances found in plants. Metabolites, like fats, fatty acids, waxes, oils, coumarins, anthraquinones and alkaloids are also soluble in ethanol and other solvents and can be extracted by distillation. Thus, materials extracted from plants contain both volatile aromatic and odourless substances. Generally, essential oils can be physically distinguished from other compounds because a drop of a volatile oil on paper will completely evaporate, unlike fatty oils. Essential oils are generally a pale to clear or slightly yellowish liquid, mostly insoluble in water, with specific gravities between 0.8 to 1.2. The odour of an essential oil will resemble the source flora, made up of a large number of constituents, sometimes into the hundreds. Some essential oil odours are dominated by a single constituent, like citral in lemongrass oil, but most oils rely on a complex mixture of constituents to provide the overall odour profile. Constituents in essential oils can be put into three classes:

•  Those greater than 1%, which are main constituents.
• Those present in parts per thousand, which are minor constituents.
•  And those less than one part per thousand, which are trace constituents.

Given the vast number of different odours and chemical structures in essential oils, most compounds are biosynthesised by a small number of metabolic pathways. Although these pathways are common to all plants, small genetic differences introduce important differences in these pathways, thereby producing variances in biosynthesis.1 These numerous reactions and transformations create exotic fragrance blends, which we call essential oils.