**3. Essential oils and their antioxidant properties**

The innumerable uses or importance of EOs calls for the best and most efficient methods in exploring aromatic plants. The therapeutics and other biological properties of EOs depend on chemical composition, molecular structure, position or location, and stereochemistry of the functional groups inherent in the molecule. It is crucial to get these various chemical constituents of EOs out of the aromatic plants well preserved. The method of steam distillation, solvent extraction, maceration, cold press extraction CO2, and water distillation are employed. Modern techniques such as supercritical fluid extraction, microwave-assisted extraction, and ultrasound are more efficient, with greater yield for the composition of EOs [23, 24]. The vast chemical constituents of EOs have been expressed to have antioxidant activity. EOs are mainly classified into two structural families of hydrocarbon skeleton, namely, Phenylpropanoid and terpenoids, both of which contain an antioxidant phenolic, a principal compound inherent in several EOs [15]. A few structures of the different chemical components of Eos are given as an example in **Figure 1**.

Today, evaluating chemical components or constituents of natural products is an essential aspect of drug discovery and development. Determining the antioxidant properties of EOs is very important, attributed to the composition of different constituents. Various methods with their unique properties or mechanism are used

*Essential Oils and Their Antioxidant Importance: The* In Vitro *and* In Vivo *Treatment... DOI: http://dx.doi.org/10.5772/intechopen.113031*

to determine the antioxidant of EOs. Several scientific works have been done to evaluate antioxidant properties in vitro and in vivo assays, and many methods, especially in vitro assay, give reliable results in a short time without using animals [25]. Different in vitro assays that are commonly used for the determination of the antioxidant activity of natural products, including EOs are 2,2-DPPH (2,2-diphenyl-1-picryl-hydrazyl-hydrate) antioxidant compound's ability to behave as a hydrogen donor or free radical scavenger, ABTS (2,2′-azinobis-(3-ethylbenzthiazolin-6-sulfonic acid), antioxidant compound stabilize the ABTS radical cation (ABTS·+) acting as electron transfer, FRAP(Ferric reducing antioxidant power) acting as reductant of ferric tripyridyltriazine complex [26] among others.

The identification of the components of EOs is done using chromatographic techniques, and gas chromatography-mass spectrometry (GC-MS) is usually or majorly employed technique to separate and identify EOs from different substances. This identification is possible using a unique fragmentation pattern of each separated component For example, GC-MS was used to identify the chemical composition of the EOs obtained from six Lamiaceae plants. The results revealed 167 components were identified from the six EOs using GC-MS [4]. Basically, in the review of the preclinical and clinical studies, the therapeutic potential of EOs from plants that delineates their biological actions on the CNS is highlighted in **Table 1**.



#### **Table 1.**

*Summary of biological actions of some EOs antioxidant constituents on the CNS.*
