Alexis St-Gelais, M. Sc., chimiste

Boswellia serrata grows in India, as opposed to most other commonly sold frankincense species which rather grow in the Horn of Africa and the southern Arabic peninsula. Frankincense serrata is not only segregated from a geographical point of view, but also differs from the other frankincenses chemically. This is reflected in its essential oil. The information presented below was prepared from both our experience and a recent paper by Niebler & Buettner [1]. Their excellent paper should be read in full for our most experienced readers, but it should be noted that the percentages they report appear to significantly underestimate the content of monoterpenes compared to what we observe when injecting oils on our systems. This might be due to the facts that they use MS instead of FID for quantitation, and especially that they pooled data from both resins and oils using two sampling techniques.

Here are a few key constituents that usually are the ones catching our interest when looking at a sample labelled as being Boswellia serrata essential oil, along with their rough expected content. A typical chromatogram is also provided to have a visual idea of where to find them (figure 1).

Figure 1. Typical profile of a serrata frankincense on a DB-5 column. A: Toluene B: Hashishene C: α-Thujene D: α-Pinene E: β-Thujone F: Methylchavicol G: β-Bourbonene H: Methyleugenol I: Kessane J: Serratol

Toluene (A)

This constituent has an history of causing some unrest among our customers, but as far as we know, its occurrence is perfectly natural and expected in frankincenses. We indeed detect it in the vast majority of Boswellia sp. oils we receive, as well as in other oils (supposed to chiefly arise from resinous plant species) such as labdanum (Cistus ladaniferus), elemi (Canarium luzonicum) and pine oils (such as Pinus strobus, a tree from our geographical zone – we have confirmed the observation of toluene with local distillers). The origin of this molecule is unclear, but it has been reported formally in nature. For example, emissions of toluene by sunflowers and pines have been observed to be a response to environmental stress [2]. In frankincense serrata, toluene is encountered at levels below 0.10%, which makes it a very minor constituent. I wanted to discuss it especially to try to dissipate some concerns over its presence.

Hashishene (B)

Despite its name, which has been coined relatively recently due to the presence of this molecule in hashish volatiles (but not in dried Cannabis) [3], hashishene had been detected many years before in frankincenses [4]. And accordingly, we detect it in most batches of frankincense we analyse. In serrata-type frankincense, we usually observe hashishene (figure 2) at levels around 0.1-0.2%, and occasionally higher.

Figure 2. Structures of some noticeable constituents of frankincense serrata.

α-Thujene (C) and α-Pinene (D)

This is the most obvious key to determining whether a Boswellia oil is a serrata, or another. Indeed, serrata frankincense monoterpenic fraction is vastly dominated by α-thujene, whereas α-pinene is much more important in other species. We usually detect α-thujene in the 55-75% range for B. serrata.

β-Thujone (E)

Although it is not mentionned in Niebler and Buettner’s paper as a marker, by experience, that one is always present at relatively low levels in B. serrata oils. Although thujones are mildly neurotoxic, this oil is much less problematic than thujone-rich oils such as sage (Salvia officinalis) or arborvitae (Thuja occidentalis), because the thujone level is usually below 0.5%.

Methylchavicol (F) and Methyleugenol (H)

Once you have observed a high α-thujene content, methylchavicol (also called estragole) is the second compound to look for to confirm a serrata frankincense. It clearly stands out at the middle of the profile, often reaching 1.5-3.0% of the oil. Monoterpenes set aside, this is the most important constituent of serrata frankincense. It can be accompanied by other phenylpropanoids, such as methyleugenol (often present, although in minute amounts). Elemicin is also reported by Niebler and Buettner, but we do not detect it very often.

β-Bourbonene (G)

Other frankincenses have a more complex sesquiterpenic profile, but serrata-type frankincense is usually simpler in that part of the chromatogram. The main sesquiterpene, β-bourbonene, which is also a marker in spearmint (Mentha spicata) oil, is usually found in the 0.3-0.7% range.

Kessane (I)

Now this is a rare sight! In my practice as an oil analyst, I have almost only encountered kessane in frankincense serrata. This parallels the observations of Niebler and Buettner, and it likely makes kessane a very good biomarker for this oil. However, its content can be quite low. It happens that we do not even report it, since it is too faint, at most 0.3% but usually much lower. CO2 extracts seem to carry this constituent much better, and our current record-holder had 1.8% of kessane.

Serratol (J)

This one is more tricky, since it also can coelute with incensole, a common sight in Boswellia. Yet, in the case of serrata frankincense, incensole can be entirely missing, and in those cases, only serratol would be observed. The name of this molecule is thus relatively well chosen…

References

[1] Niebler, J., Buettner, A. (2016) Frankincense revisited, part I: Comparative analysis of volatiles in commercially relevant Boswellia species, Chem. Biodiversity, 13, 613-629, doi: 10.1002/cbdv.201500329

[2] Heiden, A. C., Kobel, K., Komenda, M., Koppmann, R., Shao, M. Wildt, J. (1999) Toluene emissions from plants, Geophys. Res. Let., 26(9), 1283-1286, doi: 10.1029/1999GL900220

[3] Marchini, M., Charvoz, C., Dujourdy, L., Baldovini, N., FIlippi, J.-J. (2014) Multidimensional analysis of cannabis volatile constituents: Identification of 5,5-dimethyl-1-vinylbicyclo[2.1.1]hexane as a volatile marker of hashish, the resin of Cannabis sativa L., J. Chromatogr. A, 1370, 200-215, doi: 10.1016/j.chroma.2014.10.045

[4] Basar, S. (2005) Phytochemical investigations on Boswellia species, Universität Hamburg (Thesis), p. 62-67. URL: https://www.chemie.uni-hamburg.de/bibliothek/2005/DissertationBasar.pdf

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