**2. Definition and localization of essential oils**

Volatile oils are aromatic compounds which occur only in 10% of the plant kingdom and are stored in plants in specific secretory cells such as glands, hairs, ducts, cavities, or resin ducts [10–13]. Essential oils are hydrophobic in nature; they can be dissolved by polar solvent like alcohols and nonpolar solvents, waxes, and oils. Most of them are pale yellow or with no color with the exception of the blue volatile oil of *Matricaria chamomilla* L., and most are liquid and of low density than water except the essential oil of *Cinnamomum verum* Blume. and *Syzygium aromaticum* L. [14, 15].

Volatile oils are easily oxidizable by light, heat, and air due to the presence of olefenic double bonds and functional groups such as hydroxyl, aldehyde, and ester [16, 17].
