Can We Identify the Origin of Anethole in Essential Oils?

Fennel, star anise and anise are different plants, but their essential oils all contain abundant anethole. Can we determine whether the anethole in an oil came from the plant named on the label, or was added from another plant or a synthetic source?

Brett J. Murphy and colleagues addressed this question in a study published in 2024 by combining the oil’s overall composition with characteristics of the atoms making up anethole. The aim was to examine analytical methods for checking consistency between essential-oil labels and contents, not to compare the dangers of synthetic fragrances with the health benefits of natural oils.

Fifteen natural references and five synthetic references

The researchers distilled five essential-oil samples each from fennel, star anise and anise materials in the laboratory, giving 15 samples in total. These served as natural-oil references of known origin. They also purchased five reference samples of synthetic anethole.

They collected a further 30 commercial oils from shops and online: 11 fennel, eight star anise and 11 anise samples. The 50 reference and commercial samples were examined using two analytical methods.

Origins of the reference plant materials included Egypt, Turkey, Spain, China, Vietnam, Europe and Tasmania. They did not, however, cover every growing region or cultivation condition worldwide.

First, look at the combination of other constituents

The first technique was GC/MS, which separates the constituents of an oil and identifies them. Looking at the associated constituents, rather than just the principal component anethole, provides clues to the plant species.

In these reference samples, fennel contained fenchone, while star anise showed a combination of linalool, α-terpineol and foeniculine. Constituents such as γ-himachalene provided clues for anise.

Finding one constituent alone does not establish origin. Linalool and α-terpineol, for example, occur in other oils too, so the presence of several constituents together and the absence of expected constituents need to be considered.

Some commercial samples contained substantial amounts of diluents such as triethyl citrate in addition to oil constituents. One sample sold as fennel lacked fennel markers and instead showed the constituent pattern of star anise together with a diluent.

From this compositional analysis, the authors judged six of the 30 samples to be adulterated or diluted. In some cases, however, composition alone could not establish whether added anethole came from another plant or a synthetic source.

The same molecule can carry clues in the ratios of atomic masses

The researchers therefore used stable isotope ratio analysis. Carbon and hydrogen each have atoms with slightly different masses. Carbon-12 and carbon-13, for example, are both carbon but differ in the number of neutrons in their nuclei.

The proportions of these atoms can vary with starting materials, the environment in which a plant grows and the production process. Even when anethole has the same molecular structure, measuring these ratios can offer clues to origin that the ingredient name alone does not reveal.

The study used GC/IRMS to separate anethole and measure its stable carbon and hydrogen isotope ratios. Each sample was analysed three times to check measurement repeatability. These three measurements do not represent three separate bottles of a commercial product.

Hydrogen ratios alone were difficult to interpret

When carbon and hydrogen ratios were considered together in the reference samples, the synthetic material and natural oils showed different distributions. The ranges for natural fennel and the other two oils nevertheless overlapped in part.

Applying these reference ranges directly to the commercial samples placed 83% outside the ranges on isotope data alone. The authors did not interpret this as meaning that 83% were fake. A major reason was that hydrogen isotope ratios extended beyond what the limited reference set captured.

Hydrogen ratios also relate to the source of the water used by plants, including rainfall and distance from the sea. Natural and synthetic ranges can overlap, leading the authors to conclude that a single hydrogen-ratio boundary is difficult to use as a definitive test of origin.

Combining composition with carbon ratios clarified the evidence

Stable carbon isotope ratios helped narrow down questions raised by composition. For example, in three anise samples where the source of the added material was unclear from composition alone, the authors considered the results to support the possible use of synthetic anethole.

Carbon ratios also suggested synthetic anethole addition in one star anise and one fennel sample whose composition alone did not readily reveal a problem. Their principal constituent may have been supplemented to increase volume while leaving the original plant’s characteristic minor constituents present.

Ultimately, the authors judged eight of the 30 samples, approximately 27%, to be adulterated or diluted. The problems included added diluents, use of other natural materials and addition of synthetic anethole, rather than a single type of alteration. This is a result for the 30 samples collected, not a percentage for the entire essential-oil market.

Building evidence rather than drawing a universal boundary

Carbon isotope ratios are not a universal test that perfectly separates natural and synthetic sources under all conditions. Earlier studies discussed in the paper also show overlap in carbon values between synthetic and natural materials. It is important to broaden the origins and manufacturing routes represented by reference samples and interpret the ratios alongside composition.

The study also inferred what had been added, but did not establish the proportion of adulteration. The authors called for future work using mixtures prepared in known proportions to assess whether mixing ratios can be estimated and how small an addition can be detected.

Authentic oils naturally vary with origin, variety or chemotype, cultivation and distillation conditions. Avoiding a conclusion of fraud solely because a sample falls outside a reference range, without knowing that natural range well enough, is important for more reliable analysis.

The research was funded by Young Living Essential Oils, and the authors were affiliated with the D. Gary Young Research Institute. The paper declared no conflicts of interest and stated that the funder had no role in study design, data collection or analysis, writing, or the decision to publish.

Summary

Investigating the origin of anethole in essential oils benefited from combining the surrounding constituent pattern with atomic isotope ratios, rather than relying on the main ingredient’s name alone. In these samples, adding carbon isotope measurements to compositional analysis revealed possible additions that could otherwise be missed.

Hydrogen isotope ratios alone, however, did not successfully distinguish natural variability from additions. Expanding the diversity of reference samples and testing mixtures of known composition are the next steps toward more reliable assessment of consistency between labelled materials and actual contents.

References

Murphy, B. J., Wilson, T. M., Ziebarth, E. A., Bowerbank, C. R., & Carlson, R. E. (2024). Authentication of Fennel, Star Anise, and Anise Essential Oils by Gas Chromatography (GC/MS) and Stable Isotope Ratio (GC/IRMS) Analyses. Plants, 13(2), 214. DOI: 10.3390/plants13020214 / Full original article. Original article license: CC BY 4.0.