**3. Essential oil extraction**

*Essential Oils - Oils of Nature*

*EOs extracted from plant parts.*

EOs can be found in various plants species, in particular those that belong to the Coniferae, Myrtaceae, Rutaceae, Labiatae, Umbelliferae, Alliaceae, and Zingiberaceae families [16, 17]. EOs are derived from different plant parts, such as flowers, leaves, wood, roots, seeds, rhizomes, and fruits [18]. See **Table 1** for

**Part Name of essential oil References** Flowers Lavender, jasmine [18] Leaves Mint, lemongrass [19, 20] Wood Sandal, cedarwood [21, 22] Roots Sassafras, valerian [23, 24] Seeds Fennel, nutmeg [25, 26] Rhizomes Ginger, orris [27, 28] Fruits Orange, juniper [18, 29]

In general, EOs can be classified based on their chemical composition, aroma created by the oil, evaporation speed, taxonomy or the families they belong to, their therapeutic uses, consistency, their origin, and the alphabetical order [16, 30]. Classification based on consistency, for example, can be divided into essences, balms,

Furthermore, there are three classifications of EOs based on their origin which are natural, artificial, and synthetic [16]. The natural EOs are taken from the plant without physical or chemical modifications, while the artificial oils are obtained by

> Copaiba balsam, Peruvian balsam, Canada balsam, Tolu balsam, Cabreuva balsam, Bangui balsam [16, 34]

> > Great smell, helpful for physical and

Cheap, commonly used as fragrance and taste enhancers, long lasting [15, 36]

Patchouli, sandalwood, frankincense [33]

mental health [36]

and resins [16, 31]. See **Table 2** for definition and examples of each.

**Definition Examples**

Balsams Thick very volatile natural extract from tree

Resins Solid or semisolid products that comprise of derivates and abietic acid [16]

Natural Expensive, need a lot of natural sources to create, can cause burns if not diluted [15, 35, 36]

Synthetic No therapeutic properties, damaging the skin and respiratory system [36]

or bush [16]

*Classification of EOs based on consistencies.*

*Classification of EOs based on their origin.*

Essences Volatile liquid at room temperature [16] Lavender, jasmine, geranium, rose [32, 33]

**Disadvantages Advantages**

examples of EOs found in each of the plant parts.

**2. Classification of essential oils**

**52**

**Table 3.**

**Based on consistency**

**Table 1.**

**Table 2.**

**Types of essential oil**

Five thousand years ago, the ancient civilizations have already incorporated the use of machines for EO extraction [11]. However, there has been an expansion of the different extraction methods today. One of the important methods is the hydrodistillation which is divided into water distillation, water-steam distillation, and steam distillation [37, 38]. Hydro-distillation method involves hydro-diffusion, hydrolysis, and decomposition by heat [18]. In addition, steam distillation is performed by using the Clevenger system to extract oil from both fresh and dried plants, and it takes about 3 h [1, 11]. Another method is the expression method which utilizes the machines to compress the EO out of the plant [9, 11]. Additionally, solvent extraction and ultrasonic extraction methods are also routinely used [17].

Throughout the distillation process, water is separated by gravity, and at the end it leaves the volatile liquid behind; this liquid is the EO [16, 39]. EOs that are extracted by the use of chemical solvents cannot be called true EOs according to the National Cancer Institute, because they can cause changes in the clarity, scent, and fragrance of the oil [40]. The four criteria that affect the amount of essential oils produced are (1) time of distillation, (2) temperature, (3) pressure, and (4) plant quality.
