**2. Composition of essential oils**

Essential oils are very complex natural mixtures which can contain about 20–60 components at quite different concentrations. They are characterized by two or three major components at fairly high concentrations (20–70%) compared to others components present in trace amounts. For example, carvacrol (30%) and thymol (27%) are the major components of the *Origanum compactum* essential oil, linalool (68%) of the *Coriandrum sativum* essential oil, a- and b-thujone (57%)

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

birds, providing photoprotection to the plant, and allelopathy.

The natural mixtures of volatile and aromatic compounds (Essential oils) are secondary aromatic plant metabolites. Essential oils are complex, multi-component systems composed mainly of terpenes in addition to some other non-terpene components. Several techniques can be used to extract essential oils from different parts of the aromatic plant, including hydrodistillation (HD), solvent extraction and supercritical fluid extraction (SFE) [2]. Essential oils are derived from various parts of the plant, including leaves, flowers, fruits, seeds, rhizomes, roots, and bark. In the plant, these constituents serve several physiological purposes for the plant protection from pests and microorganisms, attraction of pollinating insects or

Essential oils are usually obtained by steam or hydro-distillation first developed in the Middle Ages by Arabs. Known for their antiseptic, i.e., bactericidal, virucidal and fungicidal, and medicinal properties and their fragrance, they are used in embalmment, preservation of foods and as antimicrobial, analgesic, sedative, anti-inflammatory, spasmolytic and locally anesthesic remedies. Up to the present day, these characteristics have not changed much except that more is now known about some of their mechanisms of action, particularly at the antimicrobial level. In nature, essential oils play an important role in the protection of the plants as antibacterials, antivirals, antifungals, insecticides, and also against herbivores by reducing their appetite for such plants. They also may attract some insects to favor the dispersion of pollens and seeds, or repel undesirable others. Essential oils are extracted from various aromatic plants generally localized in temperate to warm countries like Mediterranean and tropical countries where they represent an important part of the traditional pharmacopeia. They are liquid, volatile, limpid and rarely colored, lipid soluble and soluble in organic solvents. Essential oils can be synthesized by all plant organs, i.e., buds, flowers, leaves, stems, twigs, seeds, fruits, roots, wood or bark, and are stored in storage cells like cavities, canals, epidermic cells or glandular trichomes [3, 4]. Most of the commercialized essential oils are chemotyped by gas chromatography and mass spectrometry analysis. Analytical monographs have been published (European Pharmacopoeia, ISO, WHO, Council of Europe; [5]) to ensure

Essential oils have been largely employed for their properties already observed in nature, i.e., for their antibacterial, antifungal and insecticidal activities. At present, approximately 3000 essential oils are known, 300 of which are commercially important especially for the pharmaceutical, agronomic, food, sanitary, cosmetic and perfume industries. Essential oils or some of their components are used in perfumes and make-up products, in sanitary products, in dentistry, in agriculture, as food preservers and additives, and as natural remedies. For example, d-limonene, geranyl acetate or d-carvone are employed in perfumes, creams, soaps, as fragrant components and in food, as natural flavoring agents fragrances for household cleaning products and as industrial solvents. Moreover, essential oils are used in massages as mixtures with vegetal oil or in baths but most frequently in aromatherapy. Some essential oils appear to exhibit particular medicinal properties that have been claimed to cure one or another

Essential oils have traditionally been used to impart flavoring or preservative effects to foods, or to instill fragrances in cosmetics and aromatherapy. Since ancient times, numerous civilizations have also valued essential oils for their therapeutic qualities in disease prevention and treatment. Later, the Greeks and Romans absorbed Egyptian practices of using essential oils in aromatherapy and expanded it to their baths for promotion of well-being. For instance, baths infused

**154**

good quality of essential oils.

organ dysfunction or systemic disorder [6–8].

and camphor (24%) of the *Artemisia herba-alba* essential oil, 1,8-cineole (50%) of the *Cinnamomum camphora* essential oil, α-phellandrene (36%) and limonene (31%) of leaf (of what)? and carvone (58%) and limonene (37%) of seed *Anethum graveolens* essential oil, menthol (59%) and menthone (19%) of *Mentha piperita* essential oil. Generally, these major components determine the biological properties of the essential oils. The components include two groups of distinct biosynthetical origin [36–39]. The main group is composed of terpenes and terpenoids and the other of aromatic and aliphatic constituents, all characterized by low molecular (see **Figure 1**).

The essential oil of juniper berry (*Juniperus drupacea* L.) was analyzed by chromatographic analysis and it was found that α-pinene (44.2%), thymol methyl ether (22.2%) and camphor (10.2%) present in higher concentration [40]. The major volatile compounds found in caper (*Capparis ovata* desf. Var*. caescens*) bud oil were benzyl alcohol (20.4%), furfural (7.4%), ethanol methyl pentyl acetal (5.9%), thymol (5.1%) as well as the major volatile compound found in capers leaves were methyl isocyanate (20.0%), thymol (15.5%) [41].

The following paragraphs will descript some features that characterize each chemical group constituent of essential oils.

## **2.1 Terpenes**

Terpenes form structurally and functionally different classes. They are made from combinations of several 5-carbon-base (C5) units called isoprene. The biosynthesis of the terpenes consists of synthesis of the isopentenyl diphosphate (IPP) precursor, repetitive addition of IPPs to form the prenyldiphosphate precursor of the various classes of terpenes, modification of the allylic prenyldiphosphate by terpene specific synthetases to form the terpene skeleton and finally, secondary enzymatic modification (redox reaction) of the skeleton to attribute functional properties to the different terpenes. The main terpenes are the monoterpenes (C10) and sesquiterpenes (C15), but hemiterpenes (C5), diterpenes (C20), triterpenes

#### **Figure 1.**

*The structure of the chemicals discussed in this chapter with respect to their biological activity in alphabetical order.*

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*Essential Oil as Antimicrobial Agents: Efficacy, Stability, and Safety Issues for Food Application*

(C30) and tetraterpenes (C40) also exist. A terpene containing oxygen is called a

and allow a great variety of structures. They consist of several functions:

• Monocyclic: e.g., terpinenes, p-cimene and phellandrenes, etc.

• Acyclic: e.g., geraniol, linalol, citronellol, lavandulol, and nerol, etc.

• Bicyclic: e.g., borneol, fenchol, chrysanthenol, thuyan-3-ol, etc.

• Monocyclic: e.g., menthones, carvone, pulegone and piperitone, etc.

• Acyclic: e.g., linalyl acetate or propionate, citronellyl acetate, etc.

• Monocyclic: e.g., menthyl or α-terpinyl acetate, etc.

• Phenols (aromatic ethers): e.g., thymol, carvacrol, etc.

When the molecule is optically active, the two enantiomers are very often present in different plants. For example, (+)α-pinene from *Pinus palustris*, (−)

• Bicyclic: e.g., isobornyl acetate, etc.

• 1,8-cineole, menthofurane, etc.

• Peroxydes: e.g., ascaridole, etc.

• Bicyclic: e.g., camphor, fenchone, thujone, ombellulone, pinocamphone and

• Bicyclic: e.g., pinenes, −3-carene, camphene, sabinene, etc.

• Monocyclic: e.g., menthol, α-terpineol and carveol.

• Acyclic: e.g., geranial, neral, citronellal, etc.

• Acyclic: e.g., tegetone, etc.

pinocarvone, etc.

The monoterpenes are formed from the coupling of two isoprene units (C10). They are the most representative molecules constituting 90% of the essential oils

*DOI: http://dx.doi.org/10.5772/intechopen.92305*

• Acyclic: e.g., myrcene and ocimene, etc.

terpenoid.

*2.1.1 Carbures*

*2.1.2 Alcohols*

*2.1.3 Aldehydes*

*2.1.4 Ketone*

*2.1.5 Esters*

*2.1.6 Ethers*

*Essential Oil as Antimicrobial Agents: Efficacy, Stability, and Safety Issues for Food Application DOI: http://dx.doi.org/10.5772/intechopen.92305*

(C30) and tetraterpenes (C40) also exist. A terpene containing oxygen is called a terpenoid.

The monoterpenes are formed from the coupling of two isoprene units (C10). They are the most representative molecules constituting 90% of the essential oils and allow a great variety of structures. They consist of several functions:
