Summary of ‘Adulteration of Essential Oils’ Chapter from Handbook of Essential Oils, 2nd Edition
By Erich Schmidt
Editor’s Note: Essential oils (EOs) are volatile materials
produced by specialized tissues in many medicinal and aromatic plants, often
giving each respective plant its characteristic fragrance and flavor. EOs-containing
materials have been used for thousands of years for fragrances, incenses, and a
variety of medicinal and other uses. Because it often requires a relatively
large amount of plant biomass to produce a small amount of EO, the cost of many
of these EOs has traditionally been 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 ABC-AHP-NCNPR
Botanical Adulterants Program (BAP) is developing a series of peer-reviewed
documents dealing with the confirmed adulteration of various commercially
important EOs in the global marketplace. As part of this new series, BAP has
created an Essential Oil
Adulteration section on
the BAP homepage on the American Botanical Council (ABC) website. The first
item to be posted to this page is a chapter on Adulteration of Essential Oils, summarized
below, by EO experts Erich Schmidt and Jürgen Wanner, published in 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,
which received the ABC James A. Duke
Excellence in Botanical Literature Award for 2016. (The second item published on this page is the
Botanical Adulterants Bulletin on Tea Tree Oil, posted in August 2017.)
ABC and BAP are deeply
grateful to Professors Başer and Buchbauer for their cooperation and assistance
in this matter, to Taylor & Francis for their permission to digitally
publish the chapter, and to Erich Schmidt for writing a summary of the chapter,
included below.
Introduction
The “Adulteration of Essential Oils” chapter, summarized
below, 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 field of essential oil adulteration.
“Essential oils [EOs] are constituents of around
30,000 species of plants around the world,” the chapter notes. However, only a
few EOs are used in aromatherapy and in today’s 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 to be commercially important oils based
on their price and/or quantity traded. EOs are a global market commodity with
annual sales estimated at around $7 billion. The largest producer is India,
followed by Brazil and the United States. Europe, with around 40% of global
sales, dominates the EO market.2 This large market and the
relatively high prices of many EOs often 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” (Draft International Standard [DIS] ISO
9235:2013 Aromatic natural raw materials — Vocabulary).3 The ISO has
also published a standard that lists plants used for the production of EOs (ISO
4720:2009 Essential oils — Nomenclature).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 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 were rapidly increasing. Until
the beginning of the 19th century, analytical chemistry methods were unable to
detect added substances in EOs. In 1879, Schimmel & Co. 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. Nearly all raw material
resources were used for military efforts, which forced chemists to develop
substitutes for EOs. This resulted in the creation of several new synthetic aroma
chemicals, which could be used to extend or serve as a substitute for an EO.
Thus, the ingenuity of adulteration and corruption of EOs by EO producers
increased rapidly.
Since the 1980s, ISO Technical Committee 54 (TC
54) has established nearly one hundred standards that contain 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 the 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 of 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 EOs,
inappropriate storage (e.g., in excess temperatures), and lack of protection
against air/oxygen, which alters EO color and frequently the aroma itself.
The materials used to adulterate EOs have
changed during their history of 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.], spruce [Picea spp.], et al.) possess the
property of binding water. At room temperature the presence of water in these
EOs is not easy to detect. 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 have been 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 could be obtained also by the lack of miscibility in ethanol of
90%.
After the introduction of gas chromatography
(GC) as an analytical method, with which paraffins could be detected, high
boiling point glycols were used for adulteration. The boiling points of such
glycols were extremely high and 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 to adulterate. 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; Cananga 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.
Very often, EOs will be fractionated to remove
potentially undesirable compounds, such as unwanted monoterpenes for use in
flavors. 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 is the dominant component in citrus oils up to 97%. This
monoterpene is desirable to improve not only citrus oils but also many others.
(-)-Limonene occurs mainly in EOs coming from conifers and can only be used for
this type of EO.
Litsea (Litsea cubeba, Lauraceae) oil
contains nearly 75% citral and this is used in the flavor industry because it
is a natural way to impart a citrus odor. Weak qualities of lemon also will
profit from the addition of citral from EO of Litsea cubeba.
Eugenol, citronellal, cedrol, menthol, and
linalool are also naturally occurring compounds in some EOs used to “improve”
other oils (i.e., to extend their fragrance, flavor, or medicinal properties).
Such addition of naturally-derived compounds should be noted on the certificate
of analysis of the oil provided by the seller in order ensure transparency to
the buyer.) The same chemicals can also be produced by synthetic chemical
reactions, but this process results in impurities as byproducts that can
constitute up to 1.5% of these synthetically produced compounds. However, these
byproducts these can be easily detected by gas chromatography-mass spectroscopy
(GC-MS) analysis.
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 [Citrus × aurantifolia],
orange [Citrus sinensis], and grapefruit [Citrus × paradisi]
oils) are obtained by expression of the fruit peels, usually a byproduct of the
juice or other food use of the fruit. Exhausted peels are treated with steam
under pressure to obtain the remaining hydrocarbons, including (+)-limonene.
Odorous 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.” Of course, such “natural”
chemical materials are also frequently used to adulterate natural EOs and
addition of such can be discovered once by carbon-13 nuclear magnetic resonance
(13C NMR) spectroscopic analysis. (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.) Isotopic ratio is nowadays mainly determined by MS, not NMR.
Adulteration Detection Methods
The chapter also deals with chemical analytical
methodology in determining the authenticity and detecting potential adulterants
in EOs. [Editor’s note: The book has an entire separate chapter dedicated to
explaining analytical methods that are appropriate for EOs.] The following is a
brief summary of the chapter’s analytical section.
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 was produced with the use of
exogenous aromatic compounds; however, an experienced human nose cannot detect
adulteration using fatty oils, ethanol, glycols, et al.) 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 spp.,
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 a 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 applied in a closed heating
block and 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 burnt
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 4
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 coupled and working with two columns
(polar and unpolar) is considered to be today’s analytical standard for EO
analysis. But even with this sophisticated and highly 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 here.
References
- Schmidt E, Wanner J. Essential Oil Adulteration. In:
Baser 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 Standards Organization (ISO). Aromatic
natural raw materials — Vocabulary. ISO/FDIS 9325. ISO, 2013.
- International Standards Organization (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.