*2.1.1 Biosynthesis of terpenoids*

*Essential Oils - Oils of Nature*

inherent in the plant itself.

yield [5, 11, 12].

system [13–17].

sensitive) and to oxygen (oxidable).

non-transmissible diseases.

**2. Chemical composition**

**2.1 Biosynthesis**

Biotic factors group the living organisms that can affect the plant's metabolite productions. These factors include, in one part, soil organism and microorganism. In fact, secondary metabolites present in essential oils are produced to help the plant to fight against plant invaders, interact with the symbiotic organism, and attract insects for pollination, among others. In another part, biotic factors are

Postharvest treatment include all the procedure that occurs between the plant collection and essential oil extraction. There are numerous reports that highlight the fact that drying plant material before extraction increases drastically the yield of extraction [5, 11, 12]. The counterpart of this method is the fact that after plant collection, there is a biochemical reaction that occurs between secondary metabolites. The output of this biochemical interactions is the changes in the chemistry of the obtained essential oil in comparison with that originally present in plant. At that point, the balance is between biological activity and the

The extraction method can also lead to significant modification of the chemical profile of the essential oil. There are a huge number of approaches that can help to obtain an extract from the aromatic plant, including distillation methods, expression, solvent extraction, enfleurage, and microwave-assisted extraction. Apart from hydrodistillation, microwave assisted, and expression (specifically the citrus pericarp expression), all the other methods lead to a product that is not recognized as essential oil sensu stricto. In fact, with all the other methods, the extract obtained usually contains nonvolatile compounds that are extractable by the process and must be further removed. Also, the hydrodistillation method can lead to the chemical transformation of the compounds such as ester and other compounds that are highly hydrolyzable due to the long stay of the essential oil in water in the extraction

Regarding the conservation method that group all the process between the extraction and chemical analysis, the nature of the oil can lead to some modifications. In fact, most of the compounds that are present in essential oils are unsaturated (contain double bound) and most of them are sensitive to light (photo-

Essential oils are used for their wonderful biological properties. The biological properties include the effect on humans, animals, plants, insects, and microorganisms. In humans, every single part of the human life can be affected by essential oils. In fact, essential oil is employed in nutrition as a food preservative or flavoring, in cosmetics as an odorant part, and in pharmacology as an active ingredient. The essential oil's pharmacological properties comprise their effect on transmissible and

This chapter aims at going through the mechanism of essential secondary metabolite biosynthesis, essential oil extraction, essential oil chemical profile, and its pharmacological potential against the top list of human killer diseases as presented by WHO.

Terpenoid and phenylpropanoid derivatives are the main components found in essential oils. In most plants, their essential oils contain terpenoids at around 80%. But the presence of phenylpropanoid derivatives affords the essential oils significant flavor, odor, and piquant. These two groups of compounds are derived from

two different pathways from different primary metabolites [18].

**14**

The name terpene was first attributed to the compounds with molecular formula C10H16 by Kekule, a German chemist, due to its abundance in turpentine oil. The derivatives C10H16O and C10H18O were named generically camphor and related to terpenes. Wallach, Kekule's assistant, characterized pinenes, limonene, dipentene, phellandrene, fenchone, terpinolene, and sylvestrene, which at that period were recognized as artifacts for turpentine oil [19]. But nowadays, they are considered as compounds of some essential oils.

Terpenoids are a heterogeneous group of terpenes (compounds with double bonds) and their oxygenated derivative. Sensu stricto, an essential oil terpene, is a group of compounds derived from isoprene. Isoprene in its part is an organic scaffold with 5 carbon units with 1 double bond. Terpenoids are also called isoprenoids.

To start at the beginning, there are two pathways that lead to the synthesis of isoprene. These two pathways occur in the different parts of the upstate specialized cell. In the cytosol, the so-called mevalonic acid pathway is used, while in the plastid, the Rohmer or 1-deoxy-d-xylulose-5-phosphate is used. As this part is the most important part to understand the differences between essential oils' chemistry and thereby their pharmacological properties, let us walk you through some chemistry (**Figure 1**).

According to plant species and foremost to the plant enzymatic ability, there is interconversion between certain compounds. The most observed case is that of conversion of thymol into carvacrol or vice versa. This depends on what plants need and environmental conditions [22, 23]. This leads in some case to a huge difference between chemotype of the same plants collected in different areas or different period of time during the same day or during different maturity stages. This has been reported for *Allium sativum* [24–26] and for *Lavandula angustifolia* [14, 27–30] and *Melissa officinalis* [4, 12, 13, 16, 31–36].
