**2. Classification of essential oils**

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

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


**Table 2.**

*Classification of EOs based on consistencies.*


**53**

quality.

[46, 47].

**3.2 Steam distillation**

**3.1 Hydro-distillation**

*Essential Oils: Partnering with Antibiotics DOI: http://dx.doi.org/10.5772/intechopen.86575*

**3. Essential oil extraction**

enriching the essence with extra components (can be one or more). The synthetic EOs, however, are obtained by combining many chemical substances together [16].

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 ultra-

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

Hydro-distillation is the most commonly used method of extraction of EOs in which the plant is boiled in water [41, 42]. This method takes 1 h of distillation for fresh samples and 1 h and 15 min for dried samples. In the hydro-distillation method, a round-bottomed flask is used to place the plant material in with distilled water; if the plant material is dry, 1000 ml of distilled water should be used for 75 g of plant material, and if it is fresh material, 400 ml of distilled water should be used with 200 g of plant material; if the sample of plant is smaller, however, they can adjust the amount of water using this ratio: 13.3 ml of distilled water for each gram of dry plant. For water distillation, the modified Clevenger trap should be used to extract EO, and at the end the volume of the oil should be determined, and the EO should be analyzed immediately [43–45]. An advanced distillation method which is the microwave-assisted hydro-distillation can be used to shorten extraction time

Steam distillation is the traditional method of extraction of EOs from plants [37]. The fundamental principle of steam distillation is that the mixture is allowed to be distilled at a temperature that is lower than the boiling point of the component; EO substances have a high boiling point that can reach 200°C; however, these substances will be volatile when steam or boiling water is present which is in 100°C; then the hot gas mixture will be condensed to form oil if it passes through a cooling system [48]. In steam distillation, the steam is first passed into a flask that contains the plant material; after that the condensate at the bottom of the flask should be collected which will be the water and oil; then the extract is condensated three times with ethyl ether to ensure that the essential oil is fully extracted; then the moisture

See **Table 3** for comparison between natural and synthetic EOs.

sonic extraction methods are also routinely used [17].

#### **Table 3.**

*Classification of EOs based on their origin.*

enriching the essence with extra components (can be one or more). The synthetic EOs, however, are obtained by combining many chemical substances together [16]. See **Table 3** for comparison between natural and synthetic EOs.
