Oakmoss is a natural fragrance material that has long supported chypre and fougère scents. Its complex mixture, however, also contains atranol and chloroatranol, which can cause contact allergy. Can these compounds be reduced while preserving as much of the material’s olfactory character as possible?
Hélène Bouges and colleagues, in a study published in 2018, developed a method that uses an enzyme to convert the target compounds into other substances and separate them from the fragrance material. Rather than replacing an entire natural mixture, the approach selectively processes particular constituents within it.
The scent to preserve and the compounds to reduce
Oakmoss comes from Evernia prunastri, a lichen: an organism formed through a symbiotic association involving fungi and algae or similar partners. Oakmoss absolute is the fragrance ingredient obtained by solvent extraction followed by removal of waxes and other material. Because it contains many compounds, reducing one constituent risks losing others that are important to the scent.
The study targeted atranol and chloroatranol. Its background was their contact allergenicity and the regulatory developments underway in 2018. It did not aim to remove every allergenic substance at once.
The authors considered existing separation and chemical treatments to have drawbacks, including changes in scent, process complexity, and consumption of resources and time. They therefore explored horseradish peroxidase, an enzyme abbreviated as HRP.
Changing compounds into water-soluble products before separation
An enzyme acts as a catalyst that promotes particular chemical reactions. The idea here was to use HRP and hydrogen peroxide to join atranol-derived molecules and convert them into substances that dissolve more readily in water. A product made from two joined molecular units is called a dimer.
Separating the mixture into aqueous and organic-solvent layers then leaves the more water-soluble products on the water side, apart from the material to be recovered as the fragrance ingredient. This is not simply filtering out the original atranol; its chemical properties are changed first.
The researchers first examined the reaction with pure atranol. The initial conditions produced no conversion, but under more alkaline conditions a water-soluble dimer was obtained through an intermediate substance. Its structure was investigated using nuclear magnetic resonance and mass spectrometry.
Control experiments showed that the initial oxidation could occur with hydrogen peroxide alone, while the enzyme was involved in the subsequent dimer formation. The mechanism was therefore not one in which the enzyme alone performed every step from beginning to end.
A substantial reduction in atranol in the absolute
Next, the researchers treated actual oakmoss absolute. Before treatment, the sample contained 43,000 ppm atranol, approximately 4.3%. The unit ppm means parts per million and here describes the concentration of a constituent in the fragrance ingredient.
Experiments varying the conditions with 100 mg samples reduced atranol to between 141 and 7 ppm. The lowest value, 7 ppm, was obtained with the largest amount of hydrogen peroxide tested and a reaction time of four hours. The residual concentration was not the same under every condition.
In an experiment scaled up to 1.2 g, the treated material contained 60 ppm atranol, and approximately 91% was recovered as fragrance material. The best small-scale result and the scaled-up result need to be considered separately.
Chloroatranol was not detected in the treated samples. However, non-detection means that it was not found within the analytical method’s detection capability; it is not proof that not a single molecule remained.
Were the compounds important to the scent retained?
The researchers compared composition before and after treatment using liquid chromatography, an analytical method that separates mixture components and examines their signals. The overall composition was largely retained, and methyl atratate, an important odor constituent, was unchanged under these conditions.
That does not mean every other compound remained completely unchanged. Some were removed during separation into water and organic-solvent layers, and some decreased slightly. It would be too strong to say that only the target compounds were completely removed and everything else was left untouched.
The researchers also tested whether people could smell a difference. Fifty-six people were each presented with three samples as 0.5% solutions in ethanol. Two were untreated and one was treated: a triangle test in which participants selected the different sample.
Twenty-two people selected the treated sample. With three choices, roughly one-third would be correct by chance. Taking this into account, the test did not establish statistically significant discrimination between the treated and untreated samples.
This supports the possibility of reducing the target compounds without substantially compromising olfactory quality. It does not prove that the two scents are completely identical, nor establish whether the result would be the same at higher concentrations, in a perfume formulation, or with different evaluators.
Reducing constituents and demonstrating safety are separate steps
This study did not apply samples to human skin to compare allergic reactions. A chemical finding that particular compounds have been reduced does not establish that the treated oakmoss is safe for everyone or cannot cause allergy.
The authors saw manufacturing advantages in the relatively mild conditions and straightforward separation after reaction. They positioned the method as a technology for continued use of a valuable natural fragrance material while responding to the regulations of the time, in 2018. These results alone do not determine the regulatory compliance of products today.
The process was demonstrated at milligram-to-gram scale. Industrial scale-up and assessment of the whole process, including solvent and water recovery, cost, and environmental burden, require further work. The paper’s sustainability framing was not established through a quantitative comparison of entire life cycles.
The work received support from French regional, university, and public research organizations, among others. IFF-LMR naturals supplied the oakmoss material. The authors declared no conflicts of interest.
Conclusion
Converting atranol-related compounds into more water-soluble substances with an enzyme substantially reduced their levels in oakmoss absolute. Important odor constituents remained, and the test with 56 people did not demonstrate clear discrimination between the treated and untreated samples.
The significance is a route toward selectively adjusting composition while retaining the complexity of a natural fragrance material. Examining both chemical selectivity and the scent people actually perceive can inform processing techniques that preserve an ingredient’s character. Skin safety and manufacturing scale remain separate questions requiring further investigation.
References
Bouges, H., Monchot, A., & Antoniotti, S.(2018). Enzyme-Catalysed Conversion of Atranol and Derivatives into Dimeric Hydrosoluble Materials: Application to the Preparation of a Low-Atranol Oakmoss Absolute. Cosmetics, 5(4), 69. DOI: 10.3390/cosmetics5040069. Full text (PDF). © 2018 by the authors. Licensee MDPI, Basel, Switzerland. CC BY 4.0.
Background on lichens: Natural History Museum (UK Natural History Museum), Nature and pollution: what lichens tell us about toxic air.
