Botanical Adulterants Monitor

Authentication of Australian Tea Tree Oil by GC-FID Using Enantiomeric Ratios of Terpinen-4-ol and α-Terpineol

Reviewed: Wong YF, Davies NW, Chin S-T, Larkman T, Marriott PJ. Enantiomeric distribution of selected terpenes for authenticity assessment of Australian Melaleuca alternifolia oil. Ind Crops Prod. 2015;67:475-483.

The essential oil from the leaves of Melaleuca alternifolia (Myrtaceae), a tree native to Australia, is commonly known as tea tree oil (TTO). The oil is a popular ingredient due to its antibacterial, antiviral, antifungal, and anti-inflammatory activities. Adulteration of the oil, where cheaper synthetic material or volatile compounds obtained from other plants are added intentionally, has become a significant problem for the producers of authentic TTO. Current quality standards, such as those established by the International Organization for Standardization (ISO), are insufficient to detect adulteration of TTO, therefore, the authors have developed a new method basing the authenticity of the oil on the specific ratios of the (+)-terpinen-4-ol/(-)-terpinen-4-ol (see Figure 1) and the (+)-α-terpineol/(-)-α-terpineol enantiomers (enantiomers are two molecules that differ only in the spatial arrangement of the substituents at one carbon atom, representing mirror images of each other).

The authors analyzed 57 authentic TTOs from plantations in Australia and determined the average enantiomeric ratios by gas chromatography-flame ionization detection (GC-FID) as 68.5:31.5 for terpinen-4-ol and 74.2:25.8 for α-terpineol, with the (+)-enantiomer being the predominant isomer in both cases. In addition, 43 commercial samples obtained from North America, the European Union, Asia, Australasia, and South Africa were analyzed using the same GC-FID method. Results indicated that over 50% of the samples had significantly different enantiomeric ratios than authentic Australian TTO, suggesting widespread adulteration of the ingredient. Although certain processing steps can alter the terpene profiles, the steam-distilled authentic Australian TTO samples, which were collected over a five-year period, had very consistent enantiomeric ratios. In addition, TTOs from South Africa and New Zealand also showed the same enantiomeric ratios for terpinen-4-ol; however, the samples from New Zealand had a lower α-terpineol ratio with 66.9 ± 0.2%.

Comment: This is the most comprehensive study on the composition of M. alternifolia oils to date. The combination of the GC-FID fingerprint and the enantiomeric ratio of terpinen-4-ol and α-terpineol provides a basis to determine the authenticity of Australian TTOs and detect adulteration with other oils. The authors included essential oils from two other Melaleuca species, M. linariifolia and M. dissitiflora, but the limited sample size (n=1) of these species makes it difficult to make definitive conclusions about the compositional similarity of the various Melaleuca essential oils. As the authors indicate, “[A] more thorough study covering additional representative samples (same species from different geographical locations) and further specific and/or unique chiral terpene markers is warranted in order to validate the feasibility and reliability of stereoselective based techniques for differentiation of TTO from dissimilar Melaleuca species.”