Biomedical Science and Research Journals | Detection of Essential Oils Adulteration: A Quick Overview and Current Challenges
Detection of Essential Oils Adulteration: A Quick Overview and Current Challenges
Opinion
Essential oils are highly appreciated raw materials due to a
variety of their intrinsic characteristics and properties. They are
complex mixtures of volatile compounds produced by plants as
secondary metabolites; communicational and defence roles were
attributed to constituents of essential oils [1]. Over the years,
many researchers have reported a great variety of properties of
essential oils including antimicrobial and antioxidant activities, as
well as, some other effects of biological interest that could be used
in human and veterinary medicine [1]. Therefore, not only wellknown
essential oils are more demanded in the market, but also
enormous efforts are focused on the extraction and characterization
of alternative sources of these products. From an economic point
of view, this fact can significate the opening of opportunities for
developing countries in South Asia, Africa, and Latin America since
many of them have great biodiversity within their territories and
essential oils prices tend to increase year by year. Moreover, the
technologies for the extraction of essential oils (steam distillation,
hydro distillation, cold expression, and supercritical fluid
extraction) are well known and available at reasonable costs.
Only considering the high demand and the highly appreciated
properties of some essential oils, for example, those extracted
from lavender or frankincense, there is no wonder that counterfeit
and adulterated essential oils can be offered in the market. The
formulation of medicinal products with counterfeit and adulterated
essential oils could be very dangerous and put in serious risk the
health of consumers. As an example, clove essential oil is used in
dental care, in fact, the characteristic odour of a dentistry office is
due to eugenol, the main component of this essential oil. In earlier
work, we reported the occurrence in the market of adulterated
clove essential oil, and the adulterant was benzyl alcohol [3]. It is
not difficult to imagine the adverse effects on the health of a person
treated with a product based on clove essential oil with benzyl
alcohol as adulterant. It is out of question that strict controls must
be carried out, not only by governmental institution but also by
companies working with essential oils as raw materials. One of
the aims of using essential oils instead of synthetic substances in
cosmetic and pharma products (among others) is to obtain natural
products, which have fewer secondary effects and high acceptance
by the customers. Clearly, the adulteration by the illegal addition of
cheaper substances to essential oils modifies the quality of the final
product and goes against this aim.
Currently, the technique considered the gold standard for the
quality control of essential oils is gas chromatography (GC), ideally,
coupled to mass spectrometry (MS). By methods based on this
technique, the detection of volatile compounds present in essential
oils is possible and even the confirmation of the identity of such
compounds can easily be achieved. To deal with the adulteration
based on the addition of synthetic versions of compounds present
in the essential oils, isotope-ratio mass spectrometry and chiral
GC analysis have demonstrated great efficiency [4,5]. In cases in
which the adulteration with a non-volatile substance is suspected,
the use of liquid chromatography (i.e., high performance liquid
chromatography, HPLC) could be useful [6]. However, this technique
is not routinely used for quality control in industries and methods
based on GC are still predominant with the consequent limitations.
The adulteration of essential oils by the addition of nonvolatile
substances is a very concerning issue that seems to be
underestimated. In an earlier study on clove essential oils [3],
we
found that a commercial sample was adulterated with a vegetable
oil. While (in this work) it was relatively easy to detect the
adulterant
benzyl alcohol by GC/MS/MS, the vegetable oil was not detected by
mean of this technique. Only volatile and semi volatile compounds
can be detected with a GC system because this technique requires the
vaporization of analyte for its introduction into the column.
Basically, in a GC analysis, any non-volatile substance (in this
case, the adulterant) is retained in the injection port and the
chromatogram shows peaks only of the volatile constituents, i.e.,
the expected constituents of the essential oil. Changes in the areas
of the peaks could be a clue of adulteration, however, such changes
could also be attributed to regional and seasonal variations. Earlier
studies have demonstrated that the composition of an essential oil
shows certain variations due to the location where the plants were
grown and the season [7]. Therefore, the detection of adulterated
essential oils by GC methods has serious limitations when it comes
to deal with non-volatile adulterants. In such cases, the application
of spectroscopic techniques may be more appropriate. In fact,
the application of Raman spectroscopy to the quality control of
essential oils has some advantages; the Raman spectra of essential
oils consist in sharp bands that can easily be assigned to specific
compounds [8] and the presence of adulterants can be noticeable
by the occurrence of new bands and/or changes in the shape of the
bands. Even slight changes in the spectra may be observable by data
processing with chemometric methods (e.g., Principal Components
Analysis, PCA). Moreover, since portable and handheld Raman
spectrometers are already commercially available, the possibility
of performing in situ measurements provides more versatility to
quality control activities.
As mentioned before, essential oils have already important
applications in human and veterinary medicine, and clearly, they will
gain in importance in years ahead. Many authors have considered
the possible substitution of some antibiotics by essential oils or
individual components they contain [9,10]. This substitution may
start in activities related to animal production and then, perhaps,
also find its way to the treatment of bacterial infections in humans.
The emergence of multidrug-resistant bacteria forces not only to
promote the diversification of antimicrobial substances used in
veterinary and human medicine, but also the substitution of some
generations of antibiotics that should be kept as the last resource
against “superbugs”. Only by ensuring the quality of essential oils
by mean of the most appropriate analytical strategies would be
possible to obtain the best benefits of these products.
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