Thoughts as of 2022 on the mysterious chemistry of Frankincense

parker25mv

Basenotes Dependent
Oct 12, 2016
Frankincense is one of the oldest smells of mankind. Yet despite that, the source of the actual smell of frankincense is still poorly understood. Over the years, I have been gathering research , coming up with theories, continually trying to better hone my understanding of where this smell actually comes from. The chemistry of frankincense is mysterious indeed.

I will present here an assembled collection of all my latest thoughts and research as of 2022. I still don't have the definitive certain answers but this is the best I have been able to come up with, and it is rather long and extensive.

The first thing, there is definitely at least a fair amount of limonene in the smell. This is probably where the orange and lemon facets of the smell are coming from, although it is possible there might be other molecules responsible for this as well (sesquiterpenes like bisabolene).

Olibanic acid (more specifically named octylcyclopropyl-1-carboxylic acid) has been discovered to be responsible for the exalting olfactory effect of frankincense, though not so much its explicit and discernable smell. It is somewhat "soapy" smelling in feel as well.

Cis- and trans-olibanic acids, triangular cyclopropane group attached to a carboxylate group on one point and an eight-length carbon chain on the other
(Cis-isocascarillic acid, with one less carbon length in the chain, was isolated from oranges)

C. Cerutti-Delasalle, M. Mehiri, C. Cagliero, P. Rubiolo, C. Bicchi, U.J. Meierhenrich and N. Baldovini, Angew. Chem. Int. Ed., 2016, 55, 13719 –13723 (key odorants in Frankincense)

There was a new study that found the chemical octylcyclopropyl-1-carboxylic acid to be very important in the overall smell of frankincense, very potent molecule in low concentration, "typical balsamic, old church-like endnote of frankincense".
(Nicolas Baldovini, from the Institute de Chimie de Nice in France)

This appears to be very similar to the captive "Mystikal" from Givaudan, 2-methyl-undecanoic acid, which is described to smell like "incense, olibanum".

Rutundone has also previously been described as contributing to the smell of frankincense.
Rutundone, a well known spicy, peppery odorant in wine, reportedly smells "woody, coniferous, incense like" near its detection threshold.
( The (+ +)-cis-and (+ +)-trans-Olibanic Acids: Key Odorants of Frankincense, Celine Cerutti-Delasalle, Angewandte Chemie)

Also, interestingly the above article makes a passing mention to someone finding incensole to be odorless. (Maybe incensole acetate and its derivatives do not actually contribute to the smell? More on that later below)


rotundone and mustakone were found as two highly potent odorants present at trace levels in frankincense.
Fragrant Sesquiterpene Ketones as Trace Constituents in Francincense Volatile Oil of Boswellia sacra, Johannes Niebler, Journal of Natural Products, Friedrich-Alexander-Universitat Erlangen-Nurnberg

literature data revealed that, at least for the sensitive subgroup of panelists, mustakone has an odor threshold in the same range as some of the most potent food-related odorants, like solotone (0.015 ng/L), E-Beta-damascenone (0.002-0.004 ng/L), and methional (0.1-0.2 ng/L). The panelists reported fast adaptation to it, resulting in the inability to perceive the smell after one to three seconds (anosmia).
The odor was described by panelists as spicy, woody, slightly fatty, meat-broth-like, and balsamic. Some panelists remarked that the smell resembled that of alpha-copaene, which has also previously been reported to contribute to the smell of frankincense and smells spicy, broth, woody.

Here is a general analysis of frankincense oil that includes everything identified near or above 1 percent, with a few notables highlighted below that amount.

α-Pinene 20.23%
Camphene 1.45%
Sabinene 4.27% (warm, oily-peppery, woody-herbaceous and spicy)
β-Pinene 1.67%
β-Myrcene 1.95%
δ-3-Carene 0.48%
p-Cymene 6.57%
Limonene 10.10% (presumably including both L and D enantiomers)
α-Terpineol 0.95%
Bornyl acetate 0.96%
α-Copaene 1.53%
β-Elemene 2.36% (main base note in champaca flower)
β-Eudesmene 1.06% (synonym: selinene; mild, woody-warm, herbaceous, slightly peppery odor; possibly distantly like valencene, valencia oranges; major component of celery seed and calamondin orange)
γ-Cadinene 0.93%
4-epi-Cubebol 1.09%
Cubebol 1.09% (very weak in smell, spicy, minty, cooling, present in Cubeb pepper at 10%)

β-Caryophyllene 0.15% (warm, dry, black pepper woody, incense-like)
50 (Z)-α-trans-Bergamotene 0.30%
Elemol 0.11% (smells like lemony dill pickles or thyme, very dry, scratchy texture, rosinous, incense-like)
Incensole and Serratol 0.43%

Caryophyllene oxide 6.24% (similar to Tobacarol)

Boswellia sacra Flueck. volatile oil, from Somalia, obtained by hydrodistillation of gum resin
addendum to study (Johannes Niebler) that was not actually published, GC-MS characterization

---
Caryophyllene seems to be more characteristic of B. sacra compared with other Boswellia species. Whereas estragole (sweet, phenolic, anise/licorice) was characteristic in B. serrata.
octyl acetate and octanol for B. papyrifera.
source: Frankincense Revisited, Part I: Comparative Analysis of Volatiles in Commercially Relevant Boswellia Species
Johannes Niebler, Chemistry & Biodiversity, March 2016


In 1978, De Rijke isolated traces of the monoterpene acid α-campholytic acid from olibanum oil. This acid was synthesised and showed a rather strong odor reminiscent of the oil. Thus in spite of being a trace constituent, it influences the olfactory character of the oil greatly.
D. de Rijke, P. C. Traas, R. ter Heide, H. Boelens and H. J. Takken, (1978), Acidic components in essential oils of costus root, patchouli and olibanum Phytochemistry, 17(9), 1664-1666.

Of course that was some time ago before the most modern analytic methods became available. It could easily be possible that their campholytic acid that was isolated contained traces (undetectable at the time) of olibanic acids which greatly affected the character of the smell.
On the other hand, in molecular structure campholytic acid does bear a resemblance to the aromachemical Firascone, which has saffron and damascone-like character (though very much weaker in odor strength than actual damascones). Smelling the residue of frankincense that has been left to evaporated for a year after having been heated, I do think I can recognise something that smells like Firascone in the smell. Being an acid, it would have a very long persistence.
(I am thinking campholytic acid may in smell be to Firascone as phenylacetic acid is to methyl phenylacetate, just to try to draw a parallel here)


I read from one source that incensole acetate does not have an odor. Apparently the molecule is a little too big to have a smell.
But several molecules very similar in structure to incensole acetate supposedly do have a smell, according to some references.
Serratol (also named Cembranol) is very similar to incensole except with one less oxygen atom bridging the ring as an ether. Normally this is not the type of thing that makes the difference between whether a molecule is volatile enough to have a smell or not. It might be possible this is the case here since serratol is already such a large molecule.

"The aroma profile of the frankincense used in Japanese incense showed that the important odor components were diterpenes [limonene], and incensole and its derivatives. These compounds have not been previously recognized as important odor compounds."
(Note: I have some doubts about whether this is actually correct, whether this single Japanese study can be relied on)
Structure and properties of constituents in hexane extract of frankincense, Toshio Hasegawa, Journal of Essential Oil Research, Volume 24, 2012, Issue 6.

Cembrene itself supposedly only has a faint waxy odor, which would lead one to believe Cembranol may likely be too big to have a smell.


Two other aromachemicals that may have some distant similarity to the feel of frankincense are orris butanal (Cetonal) and Aldehyde C12 MNA.

The molecule of Aldehyde C12 MNA (2-methylundecanal) meanwhile has some structural similarity to the 2-methyl position of 2-methyl-undecanoic acid (Givaudan's captive "Mystikal").
Good Scents describes it as fresh, amber, moss, citrus nuance, tuberose, metallic waxy, coumarin nuance, like other aldehydes, gassy, diffusive.

Arcadi Boix Camps wrote: "3-(5-methyl-2-furyl)-butanal ... The material is extremely strong and thus must be smelled at high dilution, under which conditions it smells of a combination of green violet and frankincense, tonalities also found in the marvellous Cetonal (Givaudan). I cannot say this strongly enough: it must be used with extreme care, at very small traces.
Cetonal: If Violiff and methyl furyl butanal and violently green violet, cetonal is delicately and velvety floral green. Cetonal has as methyl furyl butanal shades of frankincense. ... the material is impressive and one of my favorites.
alpha-Ionol: This is a great and ignored chemical. It is not far from cetonal, characteristically, but is weaker and less frankincense-like"
Perfumery: Techniques in Evolution - Second Edition, by Arcadi Boix Camps

Pell Wall describes the smell of Cetonal as "orris, leather-tobacco, woody, animal".


Caryophyllene oxide is very similar to the aromachemical known as Tobacarol. The only difference is that in carophyllene oxide, one of the methyl groups is connected by a double bond, rather than a single bond. That typically adds a pine wood sawdust textured dimension to the smell.
Both caryophyllene oxide and Tobacarol are classified as epoxides (where an oxygen atom is included in a triangular ring).

My past research through the literature has led me to believe that Tobacarol is the same exact molecule as one of the two most important substances in the smell of spikenard.

According to one source, the main chemical components in spikenard oil are "bornyl acetate, valeranone, jonon [ionone], tetramenthyloxatricylodecanol [probably meant tetramethyloxatricylodecanol], menthylthymyl-ether and 1,8-cineol".
https://essentialoils.co.za/essential-oils/spikenard.htm#Chemical composition (on the site of Esoteric Oils of South Africa, though no source is cited, this information probably originated from The Oil Apothecary: A complete introduction, Porche Berry, 2019 )

I did plenty of cross searching and the only closest thing I could find to "tetramethyloxatricylodecanol" was " 4,9,12,12-Tetramethyl-5-oxatricyclo[8.2.0.04,6]dodecane ", which I believe is probably the same molecule as Tobacarol).

This is not specifically related to frankincense, but could have some small possibility of being relevant.
While investigating the complex fragrance of agarwood (oud), they found one molecule, (-)-guaia-1(10),11-dien-15-al, that had a "pleasant Beta-damascenone-like woody and floral note with a slight cooling side note". (This molecule has a shape very similar to rotundone, except doesn't have a ketone group, and the methyl group on the heptagonal ring is replaced with an aldehyde group).
"The Characteristic Fragrant Sesquiterpenes and 2-(2-Phenylethyl)chromones in Wild and Cultivated 'Qi-Nan' Agarwood", Li Yang, Molecules (journal), January 2021
 

Alex F.

Super Member
Nov 29, 2019
Olibanum seems to be a mystery still. I love its odour, but only if it's burnt. I'm not a big fan of the biting, citric-resinous smell you get when you merely heat it, so not a fan of the essential oil, either, at least not of the type I have which captures the non-burnt odour. So for me, pyrolysis is key.

Here are two more sources that, even though they're more than 30 years old, may be of interest:

(1) I've read some of his work and was somewhat disappointed, but he used to be an authority in the field of odour research - so here are the short notes provided by Günther Ohloff in Riechstoffe und Geruchssinn, die molekulare Welt der Düfte, Berlin/Heidelberg 1990, p. 184, https://books.google.com/books?id=8yitBgAAQBAJ&pg=PA184:
The main product in "Aden"-oil is α-pinene (43%), whereas n-octyl acetate (52%) and n-octanal (8%) dominate in the "Erythrea"-quality. According to P. Maupetit, by the end of 1984, more than 250 constituents and products of pyrolysis of olibanum were known, yet no compound has been found that could be said to be responsible for its characteristic odour. Only that a number of monoterpenic acids in traces support its resinous-smelling character. Among the 66 sesquiterpenoid compounds, viridiflorol is the main component. This diastereoisomeric aromadendrol is said to be one of the main sources for the characteristic note of the Italian peppermint oil (Mentha piperita L. Italo-Mitchen). Several odourless cembranoid diterpenes 415-417 have been found in olibanum resin, as well as pentacyclic triterpenes, whose unknown products of pyrolysis should be found in the smoke-aroma of the [crude] drug. Whereas the diterpene cembrene A 415 was first identified in conifer resins, incensole 416 as well as its 1-hydroxy-derivate 417 are found exclusively in olibanum.

and (2), mentioned by Ohloff: P. Maupetit, New Constituents in Olibanum Resinoid and Essential Oil, 1984/1985, PDF available in full via Perfumer & Flavorist, https://www.perfumerflavorist.com/f...tuents-in-olibanum-resinoid-and-essential-oil
 
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ScentAle

Super Member
Oct 26, 2021
Olibanum seems to be a mystery still. I love its odour, but only if it's burnt. I'm not a big fan of the biting, citric-resinous smell you get when you merely heat it, so not a fan of the essential oil, either, at least not of the type I have which captures the non-burnt odour. So for me, pyrolysis is key.

Here are two more sources that, even though they're more than 30 years old, may be of interest:

(1) I've read some of his work and was somewhat disappointed, but he used to be an authority in the field of odour research - so here are the short notes provided by Günther Ohloff in Riechstoffe und Geruchssinn, die molekulare Welt der Düfte, Berlin/Heidelberg 1990, p. 184, https://books.google.com/books?id=8yitBgAAQBAJ&pg=PA184:


and (2), mentioned by Ohloff: P. Maupetit, New Constituents in Olibanum Resinoid and Essential Oil, 1984/1985, PDF available in full via Perfumer & Flavorist, https://www.perfumerflavorist.com/f...tuents-in-olibanum-resinoid-and-essential-oil
Burned pleasable deep olibanum smell, maybe one of the hardest things to make.
I bought every type of Olibanum in the world, for understand then, that all the best deep smoky burned scents, have a minimum part of Frank.
Anyway I love darker types like res and res Pyrogenated, Serrata co2 and abs, Neglecta black that is the smokiest, and Rivae that is the most aromatic.
 

parker25mv

Basenotes Dependent
Oct 12, 2016
It's very unlikely this is the explanation, but two things to add.
I wonder if the high level of heat over hot charcoal might perhaps break down some of the heavier molecules into floral terpenes. Perhaps when a drop of melted resin oozes and suddenly falls on the very hot part (before the heavier molecule gets hot enough to be vaporised away).
β-Myrcene is produced industrially by the pyrolysis of β-pinene.

Orcinol dimethyl ether has also been found as a pyrolysis product of frankincense resin. It was not emitted until the third fraction that was taken, after more prolonged heating.
"Frankincense essential oil prepared from hydrodistillation of Boswellia sacra gum resins induces human pancreatic cancer cell death in cultures and in a xenograft murine model", Kar-ming Fung, BMC Complementary and Alternative Medicine, 2012

Orcinol dimethyl ether may help shape the fragrance of some varieties of Chinese roses.
Orcinol dimethyl ether source? Chinese rose chemical.
The phenolic methyl ether 3,5-dimethoxytoluene is the major scent compound of many rose varieties, supposedly.
Role of petal-specific orcinol O-methyltransferases in the evolution of rose scent, Gabriel Scalliet, Plant Physiol. 2006 Jan; 140(1):18-29
I do not really know how it smells, but 3-methoxytoluene smells "naphthyl, camphoreous, phenolic and woody with a salicylate nuance". I would guess that an additional methoxy group might make it smell even sweeter, and give a deeper slightly fixative effect.
(for additional comparison, 1,2-dimethoxybenzene smells "sweet creamy vanilla phenolic musty", while 1,4-dimethoxybenzene smells somewhat floral grey herbaceous musty mowed grass, phenolic, tiny fennel nuance)

I wonder if perhaps tiny traces of something like cresol might be affecting the smell of burned frankincense. (ortho-cresol is said to smell "musty phenolic plastic medicinal herbal leathery", while 4-methyl guaiacol smells "spicy clove vanilla phenolic medicinal leathery woody smoky burnt" )

I doubt any of these are the explanation for the transformation in frankincense after it is burnt, but perhaps they are worth mentioning.
 

parker25mv

Basenotes Dependent
Oct 12, 2016
de Rijke et al. (from Naarden, the Nederlands) also underlined the importance of the olfactory contribution of the acidic fraction to the typical frankincense odor. Their analysis was more detailed as they characterized α-campholytic acid as one of the important contributors of frankincense smell, described as possessing “a rather strong smell reminiscent of the oil”

J. Niebler and A. Büttner published a series of articles on detailed gas chromatography-olfactometry (GC-O) investigations of Boswellia sacra. ... a total of 17 identified odorants out of the 23 detected. The three most potent compounds (with FD ≥ 1024) were α-pinene 15 (considered to be responsible for the typical rosiny head note of frankincense), and two sesquiterpenic ketones: mustakone 14 (broth, meat, spicy) and rotundone 16 (coniferous, woody, peppery). The identification of 14 and 16 required detailed fractionation procedures and was established without ambiguity with the help of pure reference compounds obtained from the fragrance industry (16) or isolated from Cyperus scariosus in the case of 14. The olfactory descriptors associated with the 20 other odorants covered a wide range of odor qualities: geranium (17), fecal (18), fresh (19), eucalyptus (20), savory, spicy (21) etc. but interestingly, only two odorants were reported as possessing an "incense" odor: an unknown compound at RIFFAP = 2513 (incense, geranium), and serratol 32 (woody, rosiny, incense) which had probably a very limited contribution to the overall odor due to its low FD factor.

In addition to these GC-O studies on solvent assisted flavor evaporation (SAFE) extracts, Niebler and Büttner applied also the original technique of Pyrolysis-GC-MS-Olfactometry to B. sacra gum resin samples.10 This approach is interesting as it mimics the burning process by which frankincense resin is used for perfuming purposes (i.e. burned on glowing charcoal), mostly in the Middle East. It may then help to identify odoriferous pyrolysis products and indeed, 18 additional odorant compounds could be identified in the analysis of B. sacra gum resin fractions with this technique. By fractionation of the gum resin, a hydrosoluble part (gum), a non-volatile resin and a volatile fraction were obtained and submitted separately to pyrolysis-GC-MS-O. Odorants 18, 28, 33-43 were produced by the pyrolysis of the gum part, while 18-19, 31 and 44-46 were identified in the pyrolyzed resin. The pyrolysis of the volatile part generated (probably by a simple physical volatilization) 14-16, 18-19, 24, 26, 31 but also the tentatively identified 47-48. It is worth to mention that surprisingly, some odorants were detected in several pyrolyzed fractions, suggesting that they are not simple constituents, but maybe chemically bound to larger molecules or generated by a complex pyrolysis reaction. Hence, compounds 19 and 31 were detected by pyrolysis of the volatile and the resin fractions, 49 and 50 in the pyrolyzed gum and resin, and 18, 47 and 48 were emitted by the pyrolysis of each of the three fractions (gum, resin and volatile). Interestingly, 17 and 20 were perceived by pyrolysis-GC-MS-O of the entire gum resin, but not in any pyrolyzed fraction, indicating that the chemical interactions between the different molecular families also play a role in the odor development of burned frankincense.

Uncommon Boswellia Species: New Ingredients for Perfumery? July 30, 2019. Ahmed Al-Harrasi, Ph.D., University of Nizwa, Oman and Nicolas Baldovini, Ph.D., Université Côte d'Azur, France, Perfumer & Flavorist

18 is p-cresol, 28 is 2-methoxytoluene (ortho-methyl anisole)
44 is 2-acetyl furan (odor: sweet almondy nutty, brown and toasted with a milky lactonic undernote)
39 and 40 are indole and 3-methyl indole
19 is linalool, 34 is methyl maltol
17 and 20, nothing too interesting, myrcene and cineole
24 is thymoquinone (also found in cumin seed and incense cedar)
43 is 2-methylcyclopentenone (which is related to Jasmone but without the longer tail)
other than that there did not seem to be any interesting ones not already mentioned.


"Among the key odorants found were a-pinene, b-myrcene, linalool, p-cresol and two unidentified sesquiterpenoids."
source: Identification of odorants in frankincense (Boswellia sacra Flueck.) by aroma extract dilution analysis and two-dimensional gas chromatography–mass spectrometry/olfactometry Johannes Niebler, Friedrich-Alexander-Universität Erlangen-Nürnberg

Thymoquinone supposedly has a characteristic intense smell of pepper.
source: Thymoquinone (2-Isopropyl-5-methyl-1, 4-benzoquinone) as a chemopreventive/anticancer agent: Chemistry and biological effects, Anas Ahmad, Saudi Pharmaceutical Journal Volume 27, Issue 8, December 2019, p 1113.
Another source describes naturally occurring thymoquinone as smelling "pencil-like" (presumably akin to virginiana cedar).
"in our recent study on incense cedar wood, thymoquinone was found to have an even higher odour potency, possibly being one of its key odorants."
source: Resolving the smell of wood - identification of odour-active compounds in Scots pine (Pinus sylvestris L.), Linda Schreiner, Scientific Reports volume 8, Article number: 8294 (2018)
(Though it might be a little more complex than that because a closely related molecule, 4-oxoisophorone, has a smell of brewed black sweet tea)
 
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