Introduction
The word essential oil comes from the Greek root ”essential”. These compounds have a very strong odor and fragrant. most of them are colorless as long as they are fresh and pure. Essential oils evaporate easily at room temperature when exposed to air. [1, 2, 3, 4].
Essential oils are also known as odoriferous oils and can be extracted from various plant sources, for example flower (Jasmine, Rose, Violet and Lavender), leaves (Thymes, Eucalyptus, Salvia), bark (Cinnamon), buds (Clove), herbs, fruits (Orange, Lemon), twigs, seeds (Cardamom), wood (Sandal), rhizome and roots (Ginger) and etc. [4, 5, 6].
Herbal essential oils are widely used worldwide in industries such as cosmetics, pharmaceutical, food, agriculture, industry and etc. as flavors or fragrances [3, 7]. Most of them have properties such as antibacterial, antioxidant, fungicide, anti-virus, anti-parasite, insect repellent, fragrance, flavors and etc. [7]. So, they have received a lot of attention in various industries. Compounds produced by plants in nature can be classified into two main groups. The main or primary metabolites are those constituents that are present in the whole family of plants and animals and constitute the main elements of life. Including carbohydrates, proteins, lipids and nucleic acids. In plants, there are other metabolic ways that the production of these ways is not completely known to the organism. These compounds are referred to as secondary metabolites and their production way is called secondary metabolism. Essential oils are secondary metabolites of plants [7, 8, 9].
The composition of the essential oils of plants is different due to the influence of factors such as genetics and environment. Also, these compounds vary according to the methods of extraction from plants and how the plant grow [10].
Plant essential oils compounds are composed of relatively low molecular weight organic molecules including carbon, oxygen, hydrogen. Other elements such as nitrogen, sulfur, chlorine and bromine may be rarely found in this compounds [7, 11]. Plant essential oils make up a small part (less than 5%) of plant composition mainly composed of terpenes (monoterpenes (C10) represent up more than 80% of the composition of plant essential oils and sesquerpenes (C15)), aromatic compounds (aldehyde, alcohol, phenol, methoxy derivative, and etc.), and terpenoids (isoprenoids) [4, 6, 7, 12].
Compounds of essential oils can be divided into 2 major groups: terpene hydrocarbons and oxygenated compounds. Hydrocarbons consist of carbon and hydrogen atoms arranged in chains. These hydrocarbons can have non-circular, annular and aromatic structure of carbon and hydrogen atoms arranged in chains. These hydrocarbons can have acyclic, alicyclic (monocyclic, bicyclic, or tricyclic), or aromatic Structure [3]. The most common and most chemical compounds are terpenes in essential oil. Terpenes are composed of isoprene units (5 carbon units) and the combination of two isoprene units is called a ”terpene unit.” Plant essential oils also include other compounds such as diterpenes (C20), triterpenes (C30) and tetraterpenes (C40) in low concentration [4, 13]. The second category of plant essential oils is oxygenated compounds including C, H and O. Oxygen compounds can be obtained from terpenes, in which case they are called terpenoids for example phenols, alcohols, aldehydes, ketones: esters, oxides, lactones, ethers [4, 6, 12]. The compounds in the essential oils cause varying smell or flavor [3]. Table 1 shows Some of the important plant essential oils and their major constituents [14].
Table 1. Some of the important plant essential oils and their major constituents