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