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

1. Schmidt E, Wanner J. Essential Oil Adulteration. In: Başer KHC, Buchbauer G. Handbook of Essential Oils. 2nd ed. New York, NY: Taylor & Francis; 2016.
2. 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.
3. International Organization for Standardization (ISO). Aromatic natural raw materials — Vocabulary. ISO/FDIS 9325. ISO, 2013.
4. International Organization for Standardization (ISO). Aromatic natural raw materials — Nomenclature. ISO/FDIS 4720. ISO, 2009.
5. de Groot AC, Schmidt E. Essential Oils: Contact Allergy and Chemical Composition. New York, NY: CRC Press; 2016.