Since the dawn of civilisation, the essential oils, or volatile oils as they are commonly called, have been used extensively in perfumes, flavourings and medicines. So inextricably are they linked with our customs that it is hard to imagine life without them. The senses of smell and taste have not only added much to man's enjoyment and pleasure, but olfaction has also played a much greater role in animal evolution, especially in the recognition of young ones, to detect enemies and other harmful objects, to find mating partners (through the release of pheromones) and for hunting sources of food. Although, for humans these functions are no longer crucially important, they have retained the ability to detect odours and tastes.The man has selected, cultivated and utilised various fragrant plants that appeal to his olfactory senses. Over 3000 essential oils have been identified from a large number of plants belonging to some 87 angiospermic families, notably Asteraceae, Geraniaceae, Poaceae, Myrtaceae, Lamiaceae, Rutaceae and Apiaceae.

Essential oils (also sometimes called ethereal oils) are a class of vegetable oils, which are usually chemically complex mixtures of organic substances. Mostly they are terpene derivatives, phenylpropanoids, various hydrocarbons and straight chain compounds (seldom longer than 20 carbon atoms) (Figure 17.1). They are distinguished from the fixed oils in their physical and chemical properties.

Essential oils have a characteristic aroma or taste that they impart to the plants in which they occur. Unlike fixed oils, they do not become rancid but instead tend to polymerise or resinify, depositing a little solid matter (resins and gums) on standing. Since they are volatile, they do not leave a greasy stain on the paper. Usually, the essential oils are liquid at ordinary temperature and volatilise without undergoing decomposition. They are generally insoluble in water (rose and padang or keora are, however, soluble) but freely soluble in organic solvents and fatty and mineral oils. Because of their lipophilicity (high lipid solubility), these oils are easily absorbed through skin and mucosa of mouth, nose, pharynx and gastro-intestines. Most of them are optically active, having a high refractive index and a high boiling point and do not form soaps with alkalis.

The essential oils seem to be the by-products of plant metabolism rather than reserve foods as in oil-bearing seeds.