Summary of ‘Adulteration of Essential Oils’ Chapter from Handbook of Essential Oils, 2nd Edition
By Erich Schmdit
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) oiland 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.
