Volatile oils are the odorous principles found in various plant parts. Essential oils comprise the volatile, distillable or expressable fraction responsible for the characteristic odour found in many plants, and are mainly terpenoid in nature, though other classes of chemical substances may be present along with terpenes.
Depending on the plant family, they may occur in specialised secretory structures, such as glandular trychomes (Labiatae), modified parenchymal cells (Piperaceae), oil tubes called vittae (Umbelliferae) or in lysigeneous or schizogeneous cavities and passages (Pinaceae, Rutaceae) in the plant. Even though not enough is known about these oil secretory structures to carefully categorize them, from a practical stand point they can be divided into superficial and subcutaneous oils, and based on currently known information, it seems safe to say, that oils of the Labiatae, Verbenaceae and Geraniaceae are the only superficial oils known, while the others should be considered subcutaneous. For example, if you rub a thyme bush or a sage leaf slightly, you can smell the scent immediately and strong. That is, because the essential oil is contained in glandular hairs, trychomes, which you disrupt by touching.
A Bay laurel leaf, however, only reveals its scent when you crush the leaves, as the volatile oil is contained in oil cells within the leaf.
Volatile oils are generally mixtures of non polar mono- and sesquiterpene hydrocarbons and their polar oxygenated derivatives, like alcohols, acids, esters, aldehydes and ketones. Diterpenes are found in a few essential oils.
In some oils, the hydrocarbons predominate, while others are mainly composed of oxygenated compounds. The scent and taste of volatile oils is mainly determined by these oxygenated constituents, and even though the non- polar hydrocarbons might be present in the largest proportions, the former, along with Nitrogen containing compounds are mainly responsible for the characteristic aroma associated with a specific essential oil.
Sesquiterpenes are generally higher boiling and less volatile, however, they are an essential part of most volatile oils, subtly influencing the scent. Most oils have one to several major components, which impart the characteristic feature of the odor, but the many minor constituents play a very important part in the final product.
Essential oils can be composed of hundreds of different compounds.
Essential Oil production has in 1984 been estimated to be ca 45000 tonnes yearly. 15 products, however, account for approximately 90% of the total production, these being of the citrus family, the mints, Cymbopogon (Lemongrass, citronella) species, Eucalyptus species, lavender, clove, Litsea cubeba, sassafras, patchouli and Osmanthus. 65% of the world production stems from perennial woody species, the remaining originate from mostly cultivated herbaceous plants. Less than 1000 tonnes are obtained from plants collected in the Wild.
Generally, more than 95% of all essential oils produced worldwide are produced for the flavor, fragrance and toiletry industry.
Most natural essential oils are obtained by hydrodistillation of the plant material, however, some fragrance materials from especially flowers (jasmine, violet) are obtained by solvent extraction, and the citrus peel oils are mainly obtained by simple expression.
There are several factors influencing the yield and composition of any given oil. Apart from the plant's infra-specific variation, genotype and ontogeny, there are also several extrinsic, geographical and environmental factors influencing yield and composition, such as climate, rainfall, soil and the like However, oils found in secretory ducts, formed on roots, seeds and fruits, are least affected by these extrinsic factors, as oils produced by plants bearing these type of oil glands vary less from one season to another, and that it can be stated, that all seed planted, non vegetatively reproduced plants possess a higher degree of variability in essential oil content and composition.
On the other hand, also the harvesting, storing and processing of the material can have substantial influence. It is important to find the optimal conditions for each essential oil producing plant with respect to the desired product. Most of the techniques employed in the extraction process have a significant effect on the final composition of the product obtained. The degree of comminution, the amount of herb in the distillation vessel, extraction time, hydrolysis and decomposition of herb components and compositional changes during distillation are all aspects of the distillation process which deserve particular attention.
The use of essential oils in religious rituals, as perfumes and therapeutic agents, dates back to ancient times, and is well documented from then, through the middle ages until the 19th century. Single chemicals, as we have today were not known. Even though the invention of distillation used to be attributed to the Arabs, only 1000 years ago, today we know, that historically, the first record of distillation took place in the Indus Valley, now Pakistan, around 5000 years ago, where a terracotta still was found, revealing traces of a volatile oil. Though the earliest uses of fragrance compounds involved religious rites, this practise introduced the concept of disinfection, cleaning and refreshing the air surrounding a sick person. In many cases, recent findings on essential oils biological activity are compatible with the reported traditional uses. Especially the anti microbial and anti inflammatory activity of a wide range of essential oils is well documented.
Today natural essential oils are used increasingly as natural food flavours, meeting the growing consumer demand for natural additives in food products, and in quickly expanding phytotherapeutic systems, called aromatherapy, while their use in perfumery today is limited, due to the increasing competition with synthetic substitutes.
Regarding the therapeutic uses of essential oils, the antimicrobal activity has already been mentioned. Essential oils are often used to treat coughs and unspecific irritations of the respiratory tract, and in clinical tests the secretolytic and secretomotoric effect of several essential oils were clearly demonstrated. Another use of volatile oils is in the stimulation or tranquilization of the nervous system. This quasi thymo-leptic activity can be used to influence the psychic state of people. Anxiolytic (against phobias), spasmolytic, sedative and tranquilizing properties can be attributed to these oils and their components, and another modern use of fragrance materials is in the control, regulation or suppression of the appetite of overweight people.
Oils with hyperaemic properties cause a better bloodcirculation not only in the skin, but also influencing the inner organs due to a vasodilatory activity, and many oils have appetite stimulating, choleretic, cholekinetic and carminative properties- also as spices in the food. Some essential oils increase the secretion in the stomach and intestines, others stimulate the liver or gall bladder. Perhaps surprisingly, hormone like activity is wide spread in the plant kingdom. trans-Anethole for example, is related to estrone methyl ether, a chemical with estrogenic properties found in several plants. Polymerised anethole seems to stimulate lactation, even though the hormone like action of this compound, as well as anise oil and fennel oil, demonstrated in rodents is relatively weak.
Certain oils exhibit an antifungal activity at lower concentrations than commercial fungicides.
There have been a number of attempts to identify which functional groups or spatial configurations of oil components are responsible for this actions, for example it was found that the cis-figuration around a double bond conferred greater anti microbial activity than the trans-figuration, however, the mechanism of action of the oil components is usually poorly understood.
Essential oils in general exhibit low mammalian toxicity, and undesirable side effects have been observed mainly on mis-use of such oils for abortion, accidental swallowing by children or on application in excessive doses.