Plant Based Repellents - Green Mosquito Control

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11.2

PLANT ESSENTIAL OILS - COMPOSITION AND EXTRACTION

Essential oils are volatile natural complex secondary metabolites characterized by a

strong odor and have a generally lower density than that of water [11]. Many chemical

compounds can be found in essential oils, but in general essential oils consist of chem-

ical compounds which have hydrogen, carbon, and oxygen. The volatile components of

essential oils can be classified into four main groups: terpenes, benzene derivatives, hydro-

carbons and other various compounds. Terpenes and terpenoids are the main components

found in essential oils and are present either as hemiterpenes, monoterpenes or sesquiter-

penes and as their derivates. Most essential oils contain monoterpenes at around 90%,

which allow a great variety of structures with diverse functions [12]. There are different

hydrocarbons or their related compounds such as: alcohols (citronellol, linalool, geran-

iol, terpeniol, menthol, borneol), phenoles (eugenol, thymol, carvacrol), aldehydes (citral,

citronellal, neral, cuminic aldehyde), ketones (camphor, menthone, jasmone, carvone, thu-

jone) [13]. There are also some other groups of compounds that may occur in certain

essential oils: acids (alantic acid, benzoic acid, phenyl acetic acid, and anisic acid), oxides

(ascaridol, bisabolol oxide, bisabolone oxide, and cineol) [14]. Here are some examples for

common constituents in some essential oils having mosquito repellent activity: 1,8-cineole,

the major constituent of oils from rosemary (Rosmarinus officinale)and eucalyptus (Euca-

lyptus globus); eugenol from clove oil (Syzygium aromaticum); thymol from garden thyme

(Thymus vulgaris); and menthol from various species of mint (Mentha species) [15].

There are a variety of methods employed in extracting essential oils from aromatic

plants. The processes of extraction must result in a minimal chemical change of the com-

pounds present in the oil, in order to maintain its natural aroma. The economy of the pro-

cess and the yield and recovery of active components are important. Essential oils can be

extracted from the plant materials by simple expression as is the case in most of the citrus

oils including lemon and bergamot. The process of distillation, steam and water distillation

is the main method for extracting the aromatic parts of plants. Hydrodiffusion and water

infussion as well as use water or steam as solvents. Other methods use alcoholic tinctures,

extraction of oleoresins, using organic solvents such as methanol, ethanol, isopropanol,

ethyl acetate or acetone. However, the use of non-toxic solvents during the extraction pro-

cess is an important issue because of regulatory restrictions. New techniques using super-

critical or subcritical CO2 have been developed with significant advantages compared to

the use of organic solvents. CO2 extraction is a relatively recent development over the last

few decades [16]. It produces oils that are very pure and with a unique quality, such that

they can differ greatly from steam distilled essential oils. Other advantages include the

fact that CO2 is inert which means that it does not react chemically with the oil being ex-

tracted, it is non-toxic, colorless and odorless, temperatures are kept very low, so thermally

labile compounds do not suffer damage. Methods such as gas-and high-performance liq-

uid chromatography, mass spectrometry and nuclear magnetic resonance have been used

to determine the composition of oils, the quantities present in the extracts, as well as the

nature of the oils components [17].