Extraction and processing of EOs
EOs are complex mixtures of volatile compounds
extracted from plants. These low molecular mass compounds
are insoluble in water. However, the exact defi-
nition of EOs is still debated in the scientific community
[22]. The majority accept the definition of the International
Standards Organization (ISO), which limits the
extraction methods used to obtain EOs. According to
the ISO 9235.2 specifications, EO is defined as ‘A product
obtained from vegetable raw material — either by distillation
with water or steam or — from the epicarp of Citrus
fruits by a mechanical process, or — by dry distillation’
[2
,9
]. The regulation also states that steam distillation
can be conducted in the presence or absence of water in
the distillery, whereas dry distillation does not permit the
presence of water or water vapors in the still. In addition,
various possible methods of processing raw EO such as
redistillation, rectification, and aeration are highlighted.
Aromatic plants used in EO extractions are ubiquitous.
Some aromatic plants were collected from wild flora (wild
collection) growing beneficially in natural conditions.
However, most of the aromatic herbs are harvested from
systematic crops, which are cultivated under ecological
conditions.
More than 3000 types of EOs are currently known, of
which only 300 are of commercial interest [1,5].
After being synthesized in different parts of herbs, EOs
are released as an aroma through the epidermal cells of
petals, or they are accumulated and stored in different
anatomical parts of the plant, such as intracellularly
secreting cells, glandular trichomes, or secretory canals
and pockets [9
,10].
Whole aromatic herbs or parts such as leaves, flowers,
buds,seeds,fruits, roots, wood or bark are harvested during
Ava plant-specific stages of maturity. Then, they are stored
under controlled conditions of light, temperature, and
humidity, and then subjected to different extraction methods
[10].
Systemized extraction techniques in terms of required
quantities, process complexity, and field of application are
presented in Figure 1. Thus, large amounts of EOs for
commercial use can be obtained via classical methods
such as distillation, organic solvent extraction, and cold
pressing.
Distillation is the oldest and simplest method of extracting
EOs, applicable in several technologies. The main
technological difference underlying methods such as
hydrodistillation, vapor-hydrodistillation, and steam distillation
is the presence or absence of water in contact
with vegetal material. However, these methods have
some disadvantages such as high energy consumption,
long extraction time (4–6 hours), simultaneous extraction
of other polar components (coumarins and plant pigments),
degradation of temperature-sensitive compounds,
and environment pollution [9
,11].
Citrus EOs are obtained by cold pressing, wherein oil
glands localized in the external part of the mesocarp are
pressed to release EOs, which are further separated by
centrifugation.
New extraction technologies that eliminate some of
the disadvantages of the classical methods have been
developed over the past years. Some methods such as
ultrasound-assisted extraction [12] or microwave-assisted
extraction [13–15] employ alternative sources of energy,
whereas others such as supercritical fluid extraction [16–
19] or subcritical water extraction [9
,20] enhance the
solvent characteristics.
CO2 is mostly used in supercritical fluid extraction for its
unique properties such as decreased values of critical
parameters (Tcr = 31.1 8C and Pcr = 7.4 MPa), low chemical
reactivity, low toxicity, and reasonable price. In the
supercritical phase (31–55 8C and 0.5–7.4 MPa), CO2
behaves as a nonpolar liquid with high diffusivity, which
allows the extraction of nonpolar components from the
targeted material.
Subcritical water extraction (SWE), also known as pressurized
hot water extraction (PHWE) or pressurized lowpolarity
water extraction (PLPWE), is a novel method of
EO extraction [21]. Here, superheated water is used at
temperatures ranging between 100 and 375 8C (critical
temperature) at high pressures (>20 bar). Under these
conditions, the water polarity decreases with the decrease
in dielectric constant until 14.86 (350 8C and 250 bar),
which ensures that nonpolar components are solubilized
and extracted from plant materials [9
,11]. In general,
SWE is mainly used at the laboratory and pilot plant
scales to produce reduced quantities of EOs. These new
techniques have the following important advantages over
traditional methods: reduced power consumption and
extraction time, extraction of a much higher number of