**2. Essential oils**

### **2.1 Definition**

A plethora of practical definitions of the term essential or volatile oils exist. Essential oils are concentrated aromatic hydrophobic oily volatile liquids characterized by a strong odor and produced by different plant materials such as flowers, peels, rhizomes, buds, seeds, leaves, twigs, bark, herbs or grass, wood, fruits, roots, and whole plant from one single botanic species [7–9]. However, EOs with a specific characteristic (including chemical properties and biological activities) are generally obtained from a single botanical source when the age of the plant, the climate, and the edaphic and harvest period are relatively identical [10]. They are called "essential oils" because they contain the "essence" of the plant material. A few are produced by animals and microorganisms [11]. Mosses, liverworts, seaweeds, and fungi have also been shown to contain EOs. EOs are limpid, rarely colored, and soluble in nonpolar or weakly polar organic solvents and of lower density (lighter) than water, with very few exceptions [12]. They are usually colorless particularly when fresh, but few may also be pale yellow (yellow mandarin), blue (*Matricaria chamomilla* well known as chamomile), orange (sweet orange, *Citrus sinensis*), and green (bergamot, *Citrus bergamia*) [13]. Nevertheless, they may be readily oxidizable with age by light, heat, or air, which resulting to the dark color [14]. Therefore, they need to be stored in a cool and dry place, preferably in amber glass containers. The primary difference between terpenes and EOs is that they contain terpenes and a variety of other compounds as well.

#### **2.2 Distribution, uses, and importance**

The quality and the quantity of EOs in plant material depends on the climate, the soil type, the age and vegetable cycle stage, the preparation method, chemotypes, as well as the plant organ [8]. An estimated 3000 EOs, from about 2000 plants, are of great value and are used in a very large variety of fields [15, 16]. All plants possess principally the ability to produce volatile compounds, quite often, however, only in traces. Those plants that can produce an EO of commercial interest are called essential oils plants [17]. EOs occur specially in higher plants (with about 17,500 known species) but are distributed in good amount in a limited number of families including Myrtaceae, Myristicaceae, Oleaceae, Rosaceae, Acoraceae, Cupressaceae, Lauraceae, Compositae, Rutaceae, Lamiaceae, Asteraceae, Umbelliferae, Apiaceae, Poaceae, Zingiberaceae, etc. [18–21].

In most cases, the biological function of EOs remains obscure. They are nowadays subject of intensive scientific research and also attract attention of diverse industries due to their potentials as active pharmacological compounds or natural preservatives [22]. Their ecological role is however well studied and described. The most known are plant interactions (allelopathic agents, germination inhibitors) and plant–animal interactions for protection against predators (insects, fungi, herbivores) and attraction of pollinating insect to their host [23]. Industries have always

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**Figure 1.**

*Terpenoids as Important Bioactive Constituents of Essential Oils DOI: http://dx.doi.org/10.5772/10.5772/intechopen.91426*

**2.3 Extraction and analysis**

*2.3.1 Extraction of essential oils*

*2.3.1.1 Classical and conventional methods*

plants material being processed (**Figure 1**).

*Diagrammatic illustration of hydrodistillation (HD) method [32].*

tion of EOs.

*2.3.1.1.1 Hydrodistillation*

*2.3.1.1.2 Steam distillation*

had special interest on the microbial safety of cosmetics, as microbial spoilage can lead to product degradation and cause a risk for customers' health. EOs and drugs containing them are of great importance in pharmacy, perfumery (heal, perfume, incense, household cleaning products), food technology (favor for food, drinks, spices, preservative), agriculture (insecticide), and aromatherapy. Their importance is nowadays known and appreciated in plant chemotaxonomy [24, 25].

The world production and consumption of EOs and perfumes are increasing very fast. Production technology of EOs is an essential element to improve their overall yield. They are obtained from raw material by several extraction techniques such as water or steam distillation, solvent extraction, expression under pressure, microwave-assisted extraction, supercritical fluid, or subcritical water extractions [22, 26–28]. The best extraction method to use depends on the ease of evaporating (volatility) and the hydrophilicity or hydrophobicity (polarity) of the desired components. The extraction method chosen greatly affects the chemical composi-

They are the most frequently used method for the extraction of EOs from plants.

It is the oldest and easiest conventional method of extraction of EOs [11, 29–31].

The principle of this technique is that the combined vapor pressure equals the ambient pressure at about 100°C so that the volatile components with the

The principle is based on the isotropic distillation. The plant material soaks up water during the boiling process, and the oil contained in the oil cells diffuses through the cell walls by means of osmosis. The distillation time depends on the had special interest on the microbial safety of cosmetics, as microbial spoilage can lead to product degradation and cause a risk for customers' health. EOs and drugs containing them are of great importance in pharmacy, perfumery (heal, perfume, incense, household cleaning products), food technology (favor for food, drinks, spices, preservative), agriculture (insecticide), and aromatherapy. Their importance is nowadays known and appreciated in plant chemotaxonomy [24, 25].
