*2.1.2 Biosynthesis of phenylpropanoids*

The synthesis of this class of compounds in aromatic plant leads to a wide variety of compounds, but in this chapter, we will focus on the pathway that leads to volatile compounds.

**Figure 1.** *Biosynthesis of terpenoids summarized from [20, 21].*

In comparison to isoprenoids, volatile phenylpropanoid compound synthesis occurs less often. But this is not the case for almost all plants; in fact for clove oil, eugenol is the major compounds [37].

The enzymatic arsenal involved in the synthesis of volatile compounds in the plant is not well known. Therefore, there are many approaches to explain the synthesis of those groups of compounds. But from a metabolite point of view, the starting point of these volatile phenols is the phenyl-alanine that is transformed into cinnamic acid by the phenylalanine amino lyase. The cinnamic acid in turn is transformed into para-COUMARIC acid. This latter compound depending on the enzymatic ability of the plants can undergo two different ways of synthesis (**Figure 2**).

#### **2.2 Essential oil extraction**

Essential oil extraction is one of the critical points that can affect the chemical profile of the essential oil. Sensu stricto, essential oils are a volatile odorant complex mixture obtained by distillation. Many techniques have been developed to obtain essential oil such as microwave-assisted extraction, expression, enfleurage, and solvent extraction. Most of these extraction methods lead most of the time to artefactual products as well as transformed products. To better understand, the next paragraphs will present the most used methods and their principal limit in the way of modification of the original chemical profile of the essential oil.

#### *2.2.1 Distillation methods*

Distillation methods are a group of methods using steam as compound vector or transporter. In fact, in distillation method, the plant material may be immersed or not in water, and after heating to water boiling point, the impression created in the reactor by steam as well as the high temperature will create the vaporization of these volatile compounds from their stockade cell to the environment of the reactor. The gas is pouched throughout a cooler. The condensation of the water and volatilized compounds from their vapor to water phase form a mixture that can be separated according to their density. There are two varieties of distillation methods: the hydrodistillation and the so-called steam distillation methods [38–40].

**17**

*2.2.3 Expression*

*2.2.4 Enfleurage*

*Essential Oil's Chemical Composition and Pharmacological Properties*

Hydrodistillation is an essential oil extraction method in which the material is immersed in water, but in the steam distillation, the plant material is not in direct contact with water but will go through, for example, steam flow before entering the cooler. The principal limits of this method regarding the impact on the chemical profile

• The thermosensitive compounds will undergo a transformation or simply degradation. In fact, the high temperature can catalyze some chemical reactions that the normal cell will not, and this will lead to the chemical entities

• Lasting contact with water increases hydrolysis. Esterified compounds are highly sensitive to water as they will be broken down into compounds that do

• When the glass Clevenger is used, the essential oil is exposed to light and can, therefore, undergo photo-oxidation and the chemical profile will reveal a high

Many reports have highlighted these observations and one of them is that of Kamii et al. [14]. In fact when analyzing the chemical profile of *Lavandula angustifolia* extract using two different technics including hydrodistillation, they obtained 25.3% of linalyl acetate, 16.4% of terpene-4-on, 13% of linalool, and 13.6% of ocimene for hydrodistillation and 30.6% of linalyl acetate, 14.1% of terpene-4-on, 8.4% of lavandulyl acetate, 7.3% of β-caryophyllene, and 7.2% of β-farnesene for supercritical fluid extraction as well as lavandulol and phytol that were solely present in that later method. Many other

Microwave-assisted essential oil extraction is a variant of the distillation method where the heating source has been changed from the normal electric heating cap by the microwave. The plus here is the hypothetic increasing in extraction yield: hypothetic because the increase in yield is not as spectacular as tough [41–43]. It is true that it is better to crush the plant material, but in comparison to the classic distillation method, the essential oil yield is systematically the same. The principle of this method is based on the change of the polarity of water by the waves and of course the heating that will play the same role as in classic distillation method. This method has in addition the limit of the normal distillation method, the fact that the microwave can lead to chemical stereo switching from one isomer to another.

This method is specific for citrus pericarp. In fact, to avoid the thermal destruction, cold pressing of the citrus fruit rinds as the essential oil is store at that part of the plant has led to a good quality oil. The limit of this technique is the fact that it is

Enfleurage is an old technique of essential aroma extraction. It is based on the solubilization of the essential oil's component on a greasy wax. When using cold extraction, it can take more than a month, but this can be reduced to a few

number of oxygenated derivatives that do not really exist in the plant.

reports are available in the literature for many other plants [1, 15–17].

*DOI: http://dx.doi.org/10.5772/intechopen.86573*

that were not present in the plants.

not exist in the essential oil storage cells.

*2.2.2 Microwave-assisted essential oil extraction*

not applicable to other parts of the plant [38–40].

of the essential oil are as follow:

**Figure 2.** *Biosynthesis of phenylpropanoids summarized from [20].*

#### *Essential Oil's Chemical Composition and Pharmacological Properties DOI: http://dx.doi.org/10.5772/intechopen.86573*

*Essential Oils - Oils of Nature*

of synthesis (**Figure 2**).

*2.2.1 Distillation methods*

distillation methods [38–40].

*Biosynthesis of phenylpropanoids summarized from [20].*

**2.2 Essential oil extraction**

eugenol is the major compounds [37].

In comparison to isoprenoids, volatile phenylpropanoid compound synthesis occurs less often. But this is not the case for almost all plants; in fact for clove oil,

Essential oil extraction is one of the critical points that can affect the chemical profile of the essential oil. Sensu stricto, essential oils are a volatile odorant complex mixture obtained by distillation. Many techniques have been developed to obtain essential oil such as microwave-assisted extraction, expression, enfleurage, and solvent extraction. Most of these extraction methods lead most of the time to artefactual products as well as transformed products. To better understand, the next paragraphs will present the most used methods and their principal limit in the way

Distillation methods are a group of methods using steam as compound vector or transporter. In fact, in distillation method, the plant material may be immersed or not in water, and after heating to water boiling point, the impression created in the reactor by steam as well as the high temperature will create the vaporization of these volatile compounds from their stockade cell to the environment of the reactor. The gas is pouched throughout a cooler. The condensation of the water and volatilized compounds from their vapor to water phase form a mixture that can be separated according to their density. There are two varieties of distillation methods: the hydrodistillation and the so-called steam

of modification of the original chemical profile of the essential oil.

The enzymatic arsenal involved in the synthesis of volatile compounds in the plant is not well known. Therefore, there are many approaches to explain the synthesis of those groups of compounds. But from a metabolite point of view, the starting point of these volatile phenols is the phenyl-alanine that is transformed into cinnamic acid by the phenylalanine amino lyase. The cinnamic acid in turn is transformed into para-COUMARIC acid. This latter compound depending on the enzymatic ability of the plants can undergo two different ways

**16**

**Figure 2.**

Hydrodistillation is an essential oil extraction method in which the material is immersed in water, but in the steam distillation, the plant material is not in direct contact with water but will go through, for example, steam flow before entering the cooler.

The principal limits of this method regarding the impact on the chemical profile of the essential oil are as follow:


Many reports have highlighted these observations and one of them is that of Kamii et al. [14]. In fact when analyzing the chemical profile of *Lavandula angustifolia* extract using two different technics including hydrodistillation, they obtained 25.3% of linalyl acetate, 16.4% of terpene-4-on, 13% of linalool, and 13.6% of ocimene for hydrodistillation and 30.6% of linalyl acetate, 14.1% of terpene-4-on, 8.4% of lavandulyl acetate, 7.3% of β-caryophyllene, and 7.2% of β-farnesene for supercritical fluid extraction as well as lavandulol and phytol that were solely present in that later method. Many other reports are available in the literature for many other plants [1, 15–17].
