**Abstract**

Essential oils (EO) are volatile compounds produced by the secondary metabolism of aromatic plants. They are complex mixtures whose main components are synthesized by the mevalonic acid and the methyl erythritol phosphate pathways, which lead to the biosynthesis of terpenes, and the shikimic acid pathway, responsible for the biosynthesis of phenylpropanoid compounds. In nature, EOs are stored in the aerial parts of the plant, being of vital importance for their survival due to their antimicrobial properties. In addition, EOs provide protection against herbivores to the aromatic plants and allow them to repel or attract insects because of their strong fragrance, as well as compete with other plants of the same environment. Humans have exploited the properties of their EOs since ancient times, being used as medicinal remedies, among other uses. Currently, aromatic plants are used in pharmaceutical and food industries. One of the most commonly used aromatic plants is thyme. Thyme is a perennial aromatic plant, taxonomically belonging to the genera *Thymus* and *Thymbra*, belonging to the family Lamiaceae. These plants are very abundant in the Mediterranean Region. In this review, we focus on the study of the properties and use of EOs of *Thymbra capitata* (L) Cav. and *Thymus hyemalis* Lange., whose EOs are rich in phenolic monoterpenes. These compounds are responsible for their antioxidant, anti-inflammatory, anticarcinogenic, antibacterial, antifungal, and antiparasitic properties.

**Keywords:** essential oil, *Thymus hyemalis*, *Thymbra capitata*, aromatic plant, antioxidant, antimicrobial, carvacrol, thymol

#### **1. Introduction**

Essential oils (EOs) are volatile odorous compounds, are liquids at room temperature, and are produced by aromatic plants, as a result of their secondary metabolism [1, 2].

In nature, the EOs are stored in the secretory cells, cavities, channels, epidermal cells and trichomes of all the aerial organs of the plants, since they are of vital importance for plant survival, due to their antifungal, antibacterial, and antiviral activities. Also, they provide plants protection against herbivores and allow plants to compete with other plants, acting as allelopathic compounds. In addition, they are involved in pollination, attracting insects which favor the dispersal of seeds and pollen [3].

On the other hand, aromatic plants which produce these EOs have been used since ancient times to treat diseases, due to their healing properties. In fact, there are studies that claim that already in Ancient Egypt (2000 BC), these compounds were used as medicinal remedies, beauty products, and in religious rituals. Likewise, Hippocrates (460–377 BC), the father of medicine, studied and documented the properties of 300 aromatic plants, confirming the use of EOs in Ancient Greece. The Romans also showed great interest in the fragrance and properties of the EOs. Dioscorides, a Greek physician and botanist, described in Ancient Rome more than 500 aromatic plants and their EOs, in his book *De Materia Medica* [4]. In the tenth century, the Arabs also began to extract the EOs and use them in medicine. The use of EOs began to expand due to their pleasant fragrances, and at the end of the twelfth century, they began to be used in Europe. Their popularity was such that when the bubonic plague reached England in the mid-fourteenth century, it was ordered to burn aromatic plants in the streets, to fight the infection. At the beginning of the eighteenth century, EOs were already used to treat many diseases.

In the nineteenth century, EO composition was investigated [5]. It is now known that the major components of EOs are synthesized from three biosynthetic pathways: the mevalonic acid pathway, active in the cytosol, and the methyl erythritol phosphate pathway, active in the chloroplast, both of which lead to the biosynthesis of terpenes. A third route is the shikimic acid pathway, responsible for the biosynthesis of phenylpropanoid compounds [6]. EOs of terpene nature are synthesized from isopentenyl pyrophosphate and its isomer dimethylallyl pyrophosphate, which give rise to geranyl diphosphate, precursor of monoterpenes, and farnesyl diphosphate, precursor of sesquiterpenes, as shown in **Figure 1**. Among the numerous compounds present in the EOs derived from this biosynthetic pathway, thymol and carvacrol (isomers) stand out due to the numerous properties that are granted to them. As seen in **Figure 2**, both are phenolic monoterpenes synthesized from p-cymene, whose precursor is γ-terpinene [7].

Thymol and carvacrol are usually found in thyme EOs. On the other hand, compounds such as alcohols, aldehydes, ketones, esters, and, less frequently, carboxylic acids, as well as aromatic compounds such as phenolic ethers and aromatic esters are also present, although in a significantly lower proportion than the previous ones [6]. The properties of the EOs are mainly attributed to the major compounds, thymol and carvacrol; however it has been observed that these compounds can interact with the minority compounds, causing synergistic or antagonistic effects, thus influencing the properties of the EO [8]. In addition, the chemical composition varies according to environmental and genetic factors, influencing the phenological stage in which the harvest is made on the quality and quantity of the EO [9, 10]. The high variability occurs even within the same species, there being different major compounds among the specimens of the populations, which gives rise to the existence of different chemotypes [11].

Nowadays, many of the active ingredients used in the development of both traditional and modern drugs are extracted from plant species [12]. For the extraction of the EO, the technique most often used is the hydrodistillation, which consists of submerging fragments of the aromatic plant in boiling water, and so, the volatile compounds are dragged with the vapor, arriving at a condenser which separates them, and thus, the EO is obtained. Other conventional techniques such as steam distillation or extraction with volatile solvents as well as hydrodistillation by microwaves or extraction by supercritical fluids can be used [4]. After extracting, EOs can be analyzed by gas chromatography and mass spectrophotometry (GC–MS). GC allows the separation of the components of a complex mixture from the EO, and the MS serves for the identification of the individual components, already separated [13].

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**Figure 2.**

*Thymol and carvacrol chemical structure and their precursors.*

**Figure 1.**

*Essential Oils of* Thymbra capitata *and* Thymus hyemalis *and Their Uses Based on Their…*

*Biosynthetic pathways responsible for the biosynthesis of the compounds present in the EOs.*

*DOI: http://dx.doi.org/10.5772/intechopen.89309*

*Essential Oils of* Thymbra capitata *and* Thymus hyemalis *and Their Uses Based on Their… DOI: http://dx.doi.org/10.5772/intechopen.89309*

**Figure 1.**

*Thymus*

On the other hand, aromatic plants which produce these EOs have been used since ancient times to treat diseases, due to their healing properties. In fact, there are studies that claim that already in Ancient Egypt (2000 BC), these compounds were used as medicinal remedies, beauty products, and in religious rituals. Likewise, Hippocrates (460–377 BC), the father of medicine, studied and documented the properties of 300 aromatic plants, confirming the use of EOs in Ancient Greece. The Romans also showed great interest in the fragrance and properties of the EOs. Dioscorides, a Greek physician and botanist, described in Ancient Rome more than 500 aromatic plants and their EOs, in his book *De Materia Medica* [4]. In the tenth century, the Arabs also began to extract the EOs and use them in medicine. The use of EOs began to expand due to their pleasant fragrances, and at the end of the twelfth century, they began to be used in Europe. Their popularity was such that when the bubonic plague reached England in the mid-fourteenth century, it was ordered to burn aromatic plants in the streets, to fight the infection. At the beginning of the eighteenth century, EOs were already used to treat many diseases.

In the nineteenth century, EO composition was investigated [5]. It is now known that the major components of EOs are synthesized from three biosynthetic pathways: the mevalonic acid pathway, active in the cytosol, and the methyl erythritol phosphate pathway, active in the chloroplast, both of which lead to the biosynthesis of terpenes. A third route is the shikimic acid pathway, responsible for the biosynthesis of phenylpropanoid compounds [6]. EOs of terpene nature are synthesized from isopentenyl pyrophosphate and its isomer dimethylallyl pyrophosphate, which give rise to geranyl diphosphate, precursor of monoterpenes, and farnesyl diphosphate, precursor of sesquiterpenes, as shown in **Figure 1**. Among the numerous compounds present in the EOs derived from this biosynthetic pathway, thymol and carvacrol (isomers) stand out due to the numerous properties that are granted to them. As seen in **Figure 2**, both are phenolic monoterpenes synthesized from

Thymol and carvacrol are usually found in thyme EOs. On the other hand, compounds such as alcohols, aldehydes, ketones, esters, and, less frequently, carboxylic acids, as well as aromatic compounds such as phenolic ethers and aromatic esters are also present, although in a significantly lower proportion than the previous ones [6]. The properties of the EOs are mainly attributed to the major compounds, thymol and carvacrol; however it has been observed that these compounds can interact with the minority compounds, causing synergistic or antagonistic effects, thus influencing the properties of the EO [8]. In addition, the chemical composition varies according to environmental and genetic factors, influencing the phenological stage in which the harvest is made on the quality and quantity of the EO [9, 10]. The high variability occurs even within the same species, there being different major compounds among the specimens of the populations, which gives rise to the

Nowadays, many of the active ingredients used in the development of both traditional and modern drugs are extracted from plant species [12]. For the extraction of the EO, the technique most often used is the hydrodistillation, which consists of submerging fragments of the aromatic plant in boiling water, and so, the volatile compounds are dragged with the vapor, arriving at a condenser which separates them, and thus, the EO is obtained. Other conventional techniques such as steam distillation or extraction with volatile solvents as well as hydrodistillation by microwaves or extraction by supercritical fluids can be used [4]. After extracting, EOs can be analyzed by gas chromatography and mass spectrophotometry (GC–MS). GC allows the separation of the components of a complex mixture from the EO, and the MS serves for the identification of the individual components,

p-cymene, whose precursor is γ-terpinene [7].

existence of different chemotypes [11].

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already separated [13].

*Biosynthetic pathways responsible for the biosynthesis of the compounds present in the EOs.*

**Figure 2.**

Interest in EOs has skyrocketed in recent times. The demand for "natural" products increases year after year, and aromatic plants and EOs are becoming part of daily life. Likewise, more and more people are investigating the use of compounds obtained from plant extracts in medicine, such is the case of the EOs of many aromatic plants such as lavender (*Lavandula angustifolia*), eucalyptus (*Eucalyptus globulus*), or mint (*Mentha piperita*), which are being investigated for their neuroprotective effects [14]. Others such as EOs from oregano (*Origanum spp*.) are studied for their antioxidant and antibacterial activities [15].

It is estimated that more than 250,000 hectares are currently used to produce about 250 different plant extracts and, so on, different EOs, so they have a high socioeconomic importance in the places where they are produced, being generally rural areas in developing countries. These EOs are often used in the food industry as well as in other products of daily use such as bath gels, soaps, detergents, oral care products and body lotions. They are also widely used in aromatherapy (International Trade Center 2014). This justifies that, on a global level, 45,000 tons of EOs are produced annually, which implies an investment of more than 600 million euros, according to a study carried out by the Ministry of Agriculture of France. The main exporters are China, the USA, Brazil, EU countries, India, and Indonesia, and the largest imports are Switzerland, the USA, EU countries, Japan, and Canada [16].

In the industry of the EOs, one of the aromatic plants with greater use is thyme. Thyme is a small shrub and perennial aromatic plant, belonging taxonomically to the genera *Thymus* and *Thymbra*, of the family *Lamiaceae*, which includes 220 genera with plants such as mint, peppermint, basil, oregano, or pennyroyal, known throughout the world [17]. Thyme is very abundant in the Mediterranean Region. In the Iberian Peninsula, there is a high number of endemism, and it is common to find them in groups of thickets commonly known as "tomillares" [18]. Spain is one of the main suppliers of thyme worldwide [19], being the provinces with higher production Almeria, Murcia, and Granada, although it is also important in other areas of Andalusia, Castilla-La Mancha, and other provinces of interior, as Teruel [20].

Within the Region of Murcia, we found several species of thyme, two of them being of special relevance, both for their properties and for their environmental situation [18]: (1) *Thymbra capitata* (L.) Cav., commonly known as the Andalusian thyme, has a compact and stiff, fairly branched shrubby appearance, with pink flowers arranged in pineapple-shaped heads and leaves that are linear, glandular, and fleshy-looking, with a flat margin (**Figure 3**) [21]. (2) *Thymus hyemalis* Lange, commonly known as purple thyme or winter thyme, since the flowering stage

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*Essential Oils of* Thymbra capitata *and* Thymus hyemalis *and Their Uses Based on Their…*

occurs in this season of the year, is a much branched woody shrub whose flowers have a pink corolla and a calyx with ciliated teeth, with leaves that are of small size,

T. hyemalis *inflorescence (left), characteristical disposal, and leaf morphology (right).*

This work focuses on these two species, due to the fact that *T. capitata*, a species of Mediterranean distribution, is found in the Region of Murcia in a retrograde situation and *T. hyemalis* is an endemic species from southeastern Spain, being mainly found in Murcia and Almería [18]. In addition, the properties and the possible uses

According to several studies, the EO of *T. capitata* is characterized by its high chemical homogeneity. Russo et al. [23], in an experiment carried out with wild populations of *T. capitata* in Calabria (Italy), observed that all the collected specimens, despite having grown under different environmental conditions, had a very similar chemical composition and all the specimens were of chemotype carvacrol (81.5–78.4%). A relatively high percentage of *p*-cymene, γ-terpinene, and β-caryophyllene were also found in these specimens. In addition, it has been observed that the percentages of *p*-cymene and γ-terpinene decreased when the percentage of carvacrol increased, which indicated that both compounds were its precursors [24]. Saija et al. [25], studying the chemical composition of this EO found that, all the wild specimens of *T. capitata* analyzed were of chemotype carvacrol. These results agree with studies conducted by Miguel et al. [9, 26, 27], where the major component was carvacrol, regardless of both the part of the plant used and the state of development. Likewise, Tuttolomondo et al. [28], in Sicily (Italy), found 38 compounds, being the most representative α-pinene, myrcene, α-terpinene, *p*-cymene, γ-terpinene, borneol, β-caryophyllene and carvacrol (67.4–79.5%), being the 13 biotypes studied of carvacrol chemotype. These results suggest that there is no polymorphism in the EO of *T. capitata*. However, other studies are contradictory to the results mentioned above, showing the existence

linear, and with revolute margin [22] (**Figure 4**).

**2. Bioactive compounds of** *T. capitata* **EO**

of their EOs are the aims of this review.

**Figure 4.**

*DOI: http://dx.doi.org/10.5772/intechopen.89309*

**Figure 3.** T. capitata *inflorescence, characteristical disposal, and leaf morphology.*

*Essential Oils of* Thymbra capitata *and* Thymus hyemalis *and Their Uses Based on Their… DOI: http://dx.doi.org/10.5772/intechopen.89309*

**Figure 4.** T. hyemalis *inflorescence (left), characteristical disposal, and leaf morphology (right).*

occurs in this season of the year, is a much branched woody shrub whose flowers have a pink corolla and a calyx with ciliated teeth, with leaves that are of small size, linear, and with revolute margin [22] (**Figure 4**).

This work focuses on these two species, due to the fact that *T. capitata*, a species of Mediterranean distribution, is found in the Region of Murcia in a retrograde situation and *T. hyemalis* is an endemic species from southeastern Spain, being mainly found in Murcia and Almería [18]. In addition, the properties and the possible uses of their EOs are the aims of this review.
