Essential oil (Latin oleum aetherium, oleum aethereum) – a liquid, volatile fragrance, most often found in special cells of plant secretory tissue. Such cells are characteristic of oil-bearing plants, for example, species of the pine family, lamiaceae, myrtle, rutaceous and umbellate family. In terms of composition, the oil is a mixture of various chemical compounds, such as ketones, aldehydes, alcohols, esters, lactones, terpenes, and other organic compounds, including nitrogen and sulfur containing compounds with an unpleasant odor (amines, thiols).

Until the mid-twentieth century, oils were considered by botanists as a waste product of metabolism, not affecting the development of plants. The situation changed with the publication of Fraenkel’s (1959) paper, which stated that secondary metabolites determine the nutritional behavior of insects [footnote needed]. The observation initiated the intensive development of research in the field of ecology, concerning the biochemical co-evolution of animals and plants, and the role of semiochemical substances in shaping the structure of modern ecosystems.

Volatile oils are obtained on an industrial scale from fresh or dried plants. They are extracted from the appropriate plant material most often by steam distillation or extraction. They are used, along with other fragrances, in perfumery (perfumes, cologne). Oils or whole oil-bearing plants are also used as spices, therapeutic agents (herbal medicine) and in aromatherapy.
Occurrence of essential oils in plants

Currently, about 2,000 species of oil-bearing plants are known to occur in all climatic zones, although most often in the tropical zone. They represent various vascular plant families, most of which belong to: pine, cypress, laurel, ginger, rosaceous, lamiaceae, myrtle, olive, rutaceous, umbellate and asteraceous.

Essential oils are produced and accumulated in various plant organs, although usually within specific families in strictly defined, for example umbellate, they are mainly fruits and roots, in light-leafy – leaves and flowers, in calamus and rhizome-like rhizomes. Sometimes various oils are found in different parts of the plant, for example, bitter orange produces three different oils in flowers, leaves and young shoots. Oils are produced in secretory tissues in the outer part of plants (e.g. in glandular hairs or flowers), or inside plants (in canals and oil tanks). The content of oils in plant organs can be very different and changeable depending on the environmental conditions and even the time of day. Usually the largest amounts of oils occur in plants just before flowering, or in the case of plants that accumulate oils in fruit – during their ripening. Maximum oils can constitute up to 20% of the composition of especially aromatic fruits, flowers or leaves. Usually, in oil-bearing plants, oils range from a few milligrams to several dozen grams in one kilogram of raw material.

The importance of fragrance oils in nature
The composition of essential oils, determining their biological activity (used by man), has been evolving since the beginning of the evolution of plants, as evidenced by their existence in gymnosperms, and even within algae. For a long time they were considered as by-products of the metabolism of plants, deprived of or at most accidentally performing some secondary functions in their lives. Over time, the wide range and significance of the functions performed by these substances was discovered and proved. The presence of semiochemicals turned out to be the result of evolution favoring plants more effectively attracting insects pollinating flowers, discouraging herbivores from eating their organs and weakening the development of competing plants.

Attracting pollinators
The mechanisms of chemical communication between different species have so far been scarcely studied. For many years, the attention of biochemists has focused on pheromones, allowing intraspecific interactions. The role of chemical, olfactory and taste stimuli in various interspecies non-antagonistic and antagonistic interactions has so far been most thoroughly investigated in relation to interactions between flowers and pollinators. In many cases it has been confirmed that insects are attracted to the smell of essential oils, which include pheromone compounds of these insects.
A characteristic example are the interactions between some orchid species and pollinating pollinates, studied in the 1970s by Kullenberg, Berbström and Tenga. The research concerned, among others behaviors (behavior) of wild bees (Andrena) that pollinate nectar-producing two-leaf orchids (Ophrys). Only male bees participate in pollination, performing so-called pseudocopulation on the flowers. Earlier it was thought that two-leaflets attracted insects thanks to their evolutionary adaptation, consisting in making the appearance of flowers (color and shape) similar to the appearance of females. The development of chemical analysis methods has shown that olfactory attraction is also important. Essential oils of dicotyledons contain stereochemically constituents corresponding to the pheromones extracted from the female gonads of female bees.

It has been shown that the presented mechanism of attracting pollinators is quite popular among two-leafers. Occurs in at least 15 species or subspecies. Several types of bees and wasps participate in pollination. Different species of flowers are usually visited by different species of insects, however, there are more complex situations. An unusual case is the yellow dicotyledon (Ophrys lutea), which secretes oil containing compounds attractive to more than one species of bees. It also happens that substances that are attractive to one insect species are secreted by different flower species. During studies performed in Panama in the 1980s, it was found that orchids belonging to eleven species are pollinated by males of eight species of bees. A particularly intriguing example of evolutionary biochemical adaptations was the fact that, in addition to the mentioned orchids, bee pollinate a taxonomically distant species: Dalechampia spathulata from the Euphorbia family, whose flowers give off a similar smell (Whitten et al., 1986).

Allelopathic interactions
Essential oils play an important role in some ecosystems as allelopathic substances – harmful to the development of certain groups of plants. They are often secreted by plants growing in hardwood communities. In California’s chaparral, it has been observed that in years when the oils are secreted in smaller quantities due to higher humidity – the grass grows in communities. In dry years, when fragrance oils are secreted intensely – paniculata disappear. Similarly, the influence of oils explains the variable share of grass and hardwood plants as a result of fires. Before hardwood species regenerate from burning vegetation, chaparral is infested with grasses. Over time, as the share of oil-bearing plants increases, the share of grasses clearly decreases and it is not associated with other ecological factors (e.g. limitation of access to light), as there are no significant changes in their scope. The allelopathic effect of oils can be so pronounced that around clusters of oil-bearing plants it is often possible to observe in chaparral a strip of soil 1-2 m wide devoid of almost other plants.

The allelopathic effect of oils occurs as a result of their volatilisation into the atmosphere and their absorption by the covering tissues of neighboring plants, either directly or in a concentrated form with dew. They also adsorb on the surface layers of soil, from where they pass into the soil solution. Allelochemical interaction mechanisms are very diverse. The fine particles of cinnamic acid and α-pinene easily penetrate living tissues and cause mitochondrial damage. Monoterpenes have particular activity in blocking mitosis and limiting the process of cell elongation.

Protection against biting
Essential oils are among the many substances that plants use to protect against herbivores. The very strong odor emitted by them has a deterrent effect on both large herbivores and some invertebrates, e.g. insect larvae.

Characteristics of oils
Oils at room temperature are usually rare, mobile and very volatile liquids, less often they have an oily, oily or exceptionally even solid consistency. They are usually colorless, slightly yellowish or greenish, rarely have a pronounced green, blue or brown coloration. Their specific gravity is almost always lower than water, a few heavier exceptions include, among others cinnamon and clove oil. Mostly they do not dissolve in water, or they only do it to a minimal extent. They dissolve easily in alcohols, ether, chloroform and liquid fats. Oils are characterized by high boiling points, always exceeding 100 ° C, sometimes reaching even 300 ° C. They are all flammable, but they burn very much. A characteristic feature of oils is their strong and usually pleasant aroma, characteristic for oils from various plants or their parts.

In chemical terms, oils are a complex mixture of various chemical compounds.
The use of essential oils

Essential oils have been used for thousands of years as specifics of natural medicine, cosmetics and dermatology. They are widely used in aromatherapy, but they are not treated like typical medicines by doctors and pharmacists. This is due to the fact that they are extremely complex mixtures of up to several hundred diverse chemical compounds with not always completely known and often variable content. This makes explicit confirmation of the actual biological activity of individual natural oils or their mixtures difficult. In many cases it has been confirmed (see e.g. tiger ointment).

Discussing the therapeutic properties of oils A. Rumińska states that oils and oils can be used as medicines:
• irritant – irritate sensory and painful nerve endings, dilate blood vessels, causing hyperemia and inflammation, which speeds up the production of antibodies by the body (ol. Rosemary, ol. Eucalyptus, ol. Mustard, camphor)
• antibacterial – they stop the growth of microorganisms or kill them (thyme oil, mint oil, anise oil, marjoram oil)
• expectorant – stimulate mucus secretion and disinfect the respiratory tract (pine oil, eucalyptus oil, sage oil, mint oil, garlic oil)
• diuretic – stimulate urine secretion and disinfect the urinary tract (juniper olives, parsley olives)
• cholagogue – stimulate the formation and secretion of bile and prevent gallbladder contractions (peppermint oil, thyme oil)
stomach – stimulate the secretion of gastric juice and facilitate digestion (oils from umbelliferous plants)
• sedative and anesthetic – infest the centers of the cerebral cortex and medulla oblongata (valerian oil, lemon balm oil, clove oil, calamus oil)

Due to disinfecting properties, volatile oils, especially those containing alkaloids, inhibit the growth of pathogenic bacteria. Strongly disinfecting properties of thymol, found in oil of thyme and sand thyme, are valued. The oils found in garlic and onions also work disinfectantly. Volatile oils are also used to combat mycoses, scabies and other skin parasites. A very important common feature of essential oils is their high antiseptic effectiveness and the lack of confirmed cases of microbial resistance to their action.

Some components of volatile oils, for example eugenol, camphor and menthol, locally relieve pain; borneol and camphor strengthen the heart muscle and regulate blood circulation.
Volatile oils are used in the production of perfumes, although they are increasingly being replaced by synthetic fragrances. The most valued compositions of perfume ingredients are the secret of the companies. All perfumery compositions are included in one of the seven main classes, separated by the technical committee of the French Perfumers’ Association. There are three to seven groups in different classes, with different fragrance notes.

Oils are also an important ingredient in many plants used as spices in the household and food industry. They improve the taste of dishes and stimulate digestion. They also play an important role in the production of alcoholic and non-alcoholic beverages as components of flavors and aromas. Finally, they are used to flavor sweets and tobacco products.

Volatile oils used in too high doses can cause allergic reactions or local skin irritation, which can cause blisters and inflammation. Spices used in excess may cause irritation of mucous membranes and gastrointestinal disorders. The potential toxicity of oils is characterized by LD50 lethal dose values ​​determined for experimental animals.
Aromatic oils, for example mint and rose, are made from essential oils.

Essential oil technology
A separate department of chemical technology closely related to process engineering is devoted to the methods of producing essential oils on an industrial scale from oil-based plant materials. The technology includes methods for preparing raw materials, extracting useful ingredients and their purification, processing and storage. The quality of products (including their smell) and the efficiency of technological processes depend on the type of industrial equipment (installations) used.
Depending on the type of herbal raw materials, essential oils are obtained by:
• steam distillation
• extrusion
• extraction with volatile solvents
• cold absorption (enfleurage)
• extraction with non-volatile solvents (maceration, enfleurage à chaud).
Processes for processing oils extracted from plants include, among others, their purification or separation to obtain individual substances