Issue:
116
Page: 31-34
Summary of ‘Adulteration of Essential Oils’ Chapter from Handbook of Essential Oils, 2nd Edition
by Erich Schmidt
HerbalGram.
2017; American Botanical Council
Editor’s note: The ABC-AHP-NCNPR Botanical Adulterants
Program (BAP) is developing a series of peer-reviewed documents about the
confirmed adulteration of various commercially important essential oils (EOs)
in the global marketplace. These documents are available in the new “Essential
Oil Adulteration” section on the BAP homepage on the American Botanical Council
(ABC) website. The first item posted to this page is a chapter on the
“Adulteration of Essential Oils” by EO experts Erich Schmidt and Jürgen Wanner from the Handbook of
Essential Oils: Science, Technology, and Applications, 2nd edition (Taylor
& Francis, 2016),1 edited by K. Hüsnü Can Başer,
PhD, and Gerhard Buchbauer, PhD. The book received the ABC James A. Duke
Excellence in Botanical Literature Award for 2016. ABC and BAP are grateful to
Taylor & Francis for permission to digitally publish the chapter,
summarized below.
Introduction
EOs are concentrated liquids that contain volatile compounds
produced by specialized tissues in many medicinal and aromatic plants. These
compounds often give each respective plant its characteristic fragrance and
flavor. EO-containing materials have been used for thousands of years for
fragrances, incenses, and a variety of medicinal and other purposes. Because a
relatively large amount of plant biomass is typically required to produce a
small amount of EO, the cost of EOs can be very high. High prices for
commodities have often led to adulteration with undisclosed lower-cost
materials, at the economic benefit of the seller and frequent detriment to the
buyer or user.
The “Adulteration of Essential Oils” chapter from the Handbook
of Essential Oils spans 39 pages and contains 108 references. The chapter
provides, for the first time, a large amount of scientific information and
professional expert knowledge about the practice of EO adulteration.
“Essential oils are constituents of around 30,000 species of
plants around the world,” the chapter notes. However, only a small percentage
of EOs are used in aromatherapy and in flavor, cosmetic, animal-feed, and
pharmaceutical industries. A review of the product range of EO producers and
dealers indicates that about 250 to 300 EOs are available in the global market.
Among those, about 150 are considered commercially important oils, based on
their price and/or quantity traded. EOs are a global market commodity with
estimated annual sales of approximately $7 billion. The largest producer is
India, followed by Brazil and the United States. Europe, with about 40% of
global sales, dominates the EO market.2 This large market and the relatively
high prices of many EOs can tempt some producers and dealers to adulterate EOs.
The International Organization for Standardization (ISO)
defines an essential oil as a “product obtained from a natural raw material of
plant origin, by steam distillation, by mechanical processes …, or by dry
distillation, after separation of the aqueous phase — if any — by physical
processes.”3 The ISO has also published a standard that lists plants used for
the production of EOs.4 This document includes the following information:
botanical family, genus, and species of the plant, English and French common
names, reference for the ISO standard (if available), and plant part(s) used to
produce the EO.
History of EO Adulteration
The earliest cases of documented adulteration of EOs
occurred at the end of the 19th century, when knowledge about chemistry and the
availability of synthetic aromatic chemicals increased rapidly. Until the
beginning of the 20th century, analytical chemistry methods were unable to
detect added substances in EOs. In 1879, Schimmel & Co. (Leipzig, Germany)
became the first company to establish an industrial laboratory for EO
production. In 1909, Schimmel started publishing the famous series of Schimmel
Berichte (“Schimmel Reports”). These reports discussed progress in EO research,
the chemical composition of EOs, isolation of constituent chemicals in EOs,
cultivation of aromatic plants, and the adulteration of EOs.
During World War II, supplies of natural and synthetic raw
materials were severely limited, which prompted chemists to develop substitutes
for EOs. This resulted in the creation of several new synthetic aromatic
chemicals, which could be added to or serve as a substitute for EOs. Thus, the
corruption of EOs by EO producers increased rapidly.
Since the 1980s, ISO Technical Committee 54 (TC 54) has
established nearly 100 standards that include physical and chemical data as
well as chromatographic profiles of EOs.
Types and Examples of Adulteration
Accidental, or unintentional, adulteration is often based on
lack of knowledge, either of the botanical identity of the plant biomass or of
appropriate equipment and/or distillation procedures. Intentional adulteration
is based on the price and availability of plant material and/or the EO itself,
in addition to the demands of customers, and regulatory/safety requirements.
Adulteration can sometimes occur due to simple human error, but intentional
adulteration is fraudulent and driven by greed.
Currently, various governmental regulations require
processing interventions that maintain the “naturalness” of the EO product.
However, in some instances, processing changes are necessary to ensure safety.
Methyl eugenol, for example, is a component of damask rose (Rosa damascena,
Rosaceae) flower oil that is recognized to be able to induce cancer in mice.
Therefore, it must be removed by fractioning the oil or by chemical reaction,
both of which will change the status of the naturalness of the original EO
material. Other examples of changes that are not considered forms of
adulteration include aging, which affects the composition of EO; inappropriate
storage (e.g., in extreme temperatures); and lack of protection against oxygen,
which alters EO color and frequently the aroma itself.
The materials used to adulterate EOs have changed during the
history of EO production. Water, for example, has been used as an EO
adulterant, and its detection was particularly challenging in certain
materials. EOs from plants in the family Pinaceae (e.g., pine [Pinus spp.], fir
[Abies spp.], and spruce [Picea spp.]) bind to water. At room temperature the
presence of water in these EOs is not easy to detect, but below 0°C, the water
and EO separate, making the water addition visible to the eye. In some cases of
adulteration, the quantity of water in an EO can reach up to 15%.
Ethanol is somewhat easier to use as an adulterant; it has
good solubility and, when properly added, is not detectable by the human nose.
Fatty oils were used as diluents at the beginning of 20th
century, but they can be easily detected by adding a drop of EO on blotting
paper: fatty oils leave a lasting grease spot. Proof of fatty oil addition also
could be obtained by the oil’s lack of miscibility in ethanol.
After the introduction of gas chromatography (GC) as an
analytical method, with which paraffins (a fatty oil) could be detected,
high-boiling-point glycols were used for adulteration. Due to their high
boiling points, these adulterants could be detected only after hours of GC
analysis running at high temperatures.
In addition, EOs from other parts of the same plant can be
used as adulterants. For example, clove (Syzygium aromaticum, Myrtaceae) bud
oil has been known to be adulterated with clove leaf oil, which has much higher
relative oil yields and is similar in composition to the bud oil. Therefore,
this type of adulteration is not easy to detect.
Related species can also be used to adulterate EO products.
Cananga (Cananga odorata var. macrophylla, Annonaceae) oil and ylang (also
known as ylang-ylang; C. odorata var. genuina) oil are obtained from varieties
of the same species, but they differ in terms of chemical profiles. Due to its
lower cost, cananga oil has been used to dilute ylang oil, which has a closely
related odor profile.
Often, EOs are fractionated to remove potentially
undesirable compounds, such as unwanted monoterpenes in EOs used as flavoring
agents. According to a study described in Essential Oils: Contact Allergy and
Chemical Composition (CRC Press, 2016), 91 (almost 90%) of the investigated EOs
showed (+)-limonene in a quantity between 0.02% and 95.7%.5 (+)-Limonene
comprises up to 97% of citrus oils. This monoterpene is desirable to improve
not only citrus oils but also many others. (–)-Limonene, the molecular mirror
image of (+)-limonene, occurs mainly in EOs derived from conifers.
Litsea (Litsea cubeba, Lauraceae) oil contains nearly 75%
citral, which is used in the flavor industry to impart a citrus odor to certain
products. Eugenol, citronellal, cedrol, menthol, and linalool are other
naturally occurring compounds in some EOs that are used to “improve” other oils
(e.g., to enhance their fragrance, flavor, or medicinal properties). For
transparency, EO sellers should note any addition of naturally derived
compounds on the certificate of analysis of the oil. The same chemicals can
also be produced by synthetic chemical reactions, but this process results in
impurities (byproducts) that can constitute up to 1.5% of these synthetically
produced compounds. However, these byproducts can be easily detected by gas
chromatography-mass spectrometry (GC-MS).
Another method to adulterate citrus oils is to steam-distill
the residues of the exhausted citrus peels. Unlike most EOs that are extracted
from their plant biomass via distillation, citrus oils (e.g., lemon [Citrus × limon,
Rutaceae), lime [C. × aurantifolia], orange [C. sinensis], and grapefruit [C. paradisi]
oils) are obtained by expression of the fruit peels, which are usually
considered a byproduct. Exhausted peels are treated with steam under pressure
to obtain the remaining hydrocarbons, including (+)-limonene.
Aromatic substances produced by fermentation (i.e., the
enzymatic reactions of micro-organisms such as yeasts) from any starting
materials are generally accepted as natural chemicals (although these
yeast-derived materials are not technically plant-derived). Only Japan and
China do not accept such products as “natural.” These “natural” chemical
materials are also frequently used to adulterate natural EOs, and the addition
of such can be detected by carbon-13 nuclear magnetic resonance (13C NMR)
spectroscopy. (Cinnamic aldehyde, a popular flavor and fragrance material found
in cinnamon [Cinnamomum spp., Lauraceae]
oils, is an example of an ingredient that can be produced by fermentation.)
Adulteration Detection Methods
With advanced laboratory analytical technologies, analysts
are able to describe components in EOs down to their molecular characteristics.
In the past, organoleptic analysis (e.g., appearance, color, odor, etc.) was
the primary tool used to detect adulteration. (This is true if the adulteration
involved the use of exogenous aromatic compounds; however, even an experienced
human nose cannot detect adulteration with certain compounds, such as fatty
oils, ethanol, glycols, and others.) Physical-chemical methods, such as those
used to determine density, refraction index, optical rotation, acid value,
water content, and peroxide value to name a few, aided in the discovery of
certain unusual substances (e.g., synthetic aromatic chemicals that do not
appear in nature) and still are basic criteria of ISO standards.
Another method of detection of adulteration is the
calculation of the relationship coefficient. The ratio between specific EO
components was measured by GC-MS and proved to be an appropriate criterion for
authentication of specific EOs like French lavender (Lavandula stoechas,
Lamiaceae). This method can also be used with other EOs.
A forerunner of GC was thin-layer chromatography (TLC),
which has the disadvantage of low resolution. The introduction of GC instruments
was an analytical revolution, and the GC instrument’s columns provided much
better separation than TLC.
GC relies on a flow of nitrogen and helium through columns
with a stationary phase to which the volatile molecules have various affinity.
Microliters of diluted EO are inserted into a closed heating block, the stream
of gas presses the molecules to the end of the column, and — in the case of the
commonly used flame ionization detector (FID) — are burned in a
helium/synthetic air flame. The resistance measurement gives a signal that
allows the molecule to be detected, and comparison with authentic chemical
standards of known concentration allows the molecule to be quantified.
The first GC columns were metal and between four and 20
meters in length. To separate chemicals, packed stationary phases were used. A
milestone was the development of capillary glass columns with a length of 60
meters (approximately 200 feet) or more. These columns used a liquid stationary
phase and the inner diameter was reduced continously.
The next important analytical step was the development of
mass spectrometry (MS), which allows the identity of the molecules to be
confirmed. This is possible because each compound has a unique
mass-spectrometric fragmentation pattern (mass spectrum) according to their
differing mass and charge ratios. The fragmentation patterns can be compared to
mass spectra from reference standards to determine the identity of the
compound. Correction and retention factors ensure proper detection.
GC-MS with two columns (polar and non-polar) is today’s
analytical standard for EO analysis. But even with this sophisticated and
efficient analytical technique, the naturalness of the analyzed molecule cannot
be ensured. Many EOs contain substances with asymmetric carbon atoms (chiral
compounds) that exist in two molecular forms that are structurally mirror
images of each other (enantiomers). The ratio of one enantiomer to the other
(i.e., the enantiomeric distribution) provides a solution to the challenge of
detecting more sophisticated types of adulteration. It is currently used, for
example, to detect adulteration of tea tree (Melaleuca alternifolia, Myrtaceae)
oil.
The complexity of mixtures like fragrances and flavors is
another problem. Co-elution (in which two components come off the column at the
same time) with other analytes or sample matrix elements causes problems in
detection and quantitation. These problems may be solved by two-dimensional
GC-MS. After a preliminary separation on the first column, the sought-after
substances are then resolved on a second GC column. For example, regular GC-MS
will show that lemon oil has around 75 components, but two-dimensional GC-MS
will show a plot of more than 300 compounds.
The most efficient, but also most expensive, method to
determine the identity of an EO is to take advantage of the magnetic resonance
properties of the 13C nuclei through 13C NMR. This type of spectroscopy is a
particularly helpful technique and is regularly used to elucidate the structure
of individual substances.
Adulteration Concerns for Individual EOs
The second and largest part of this chapter reviews 77
individual EOs and their potential adulterants. The information contained in
this section is based on published scientific literature and the authors’ own
findings. EOs from ambrette (Abelmoschus moschatus, Malvaceae) seed oil to
ylang oils are described, and information about available ISO standards and
values of enantiomeric distribution of many chiral molecules also are included.
The PDF version of the entire chapter summarized in this
article is available on ABC’s website.
References
- Schmidt E,
Wanner J. Essential Oil Adulteration. In: Başer
KHC, Buchbauer G. Handbook of Essential Oils. 2nd ed. New York, NY: Taylor
& Francis; 2016.
- World
Essential Oil Market Expected to Reach $11.5 billion by 2022. Perfumer &
Flavorist. November, 22, 2016. Available at: www.perfumerflavorist.com/fragrance/research/World-Essential-Oil-Market-Expected-to-Reach-115-billion-by-2022-402514105.html.
Accessed September 7, 2017.
- International
Organization for Standardization (ISO). Aromatic natural raw materials —
Vocabulary. ISO/FDIS 9325. ISO, 2013.
- International
Organization for Standardization (ISO). Aromatic natural raw materials —
Nomenclature. ISO/FDIS 4720. ISO, 2009.
- de Groot
AC, Schmidt E. Essential Oils: Contact Allergy and Chemical Composition. New
York, NY: CRC Press; 2016.
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