Why No Two Ouds Are Alike: The Chemistry of Chromones, from Wounded Tree to Bottle

اقرأ المقال بالعربية

The chemistry of chromones and sesquiterpenes: from the wounded tree to the bottle

Take two pieces of oud from the same tree, or from two neighbouring trunks on the same plantation, and place each on the coal. The first gives you dark honey and leather; the second a dry woody sharpness with a hint of pepper and cold smoke. Distil them and the two oils will not match either. Oud lovers know this well, and usually explain it with a single word: "origin." Cambodian, Indian, Malaysian, Laotian...

But origin is half an answer at best. The better question is this: why does a tree whose healthy wood has almost no smell produce one of the most complex aromatic materials in the world? And why does it never produce it the same way twice? The answer lies in chemistry, specifically in two families of molecules: the sesquiterpenes and the 2-(2-phenylethyl)chromones. This article is an attempt to read oud from inside the wood, not from the label.

A tree without a scent: oud is a wound, not a wood

Oud is not a type of wood but a state the wood reaches. Trees of the genus Aquilaria (and to a lesser extent Gyrinops), in the family Thymelaeaceae, grow pale, light wood of almost no aromatic value. The transformation begins when the tree is injured: a wind break, a boring insect, lightning, an axe. Fungi then enter through the wound, and the tree mounts a defence, producing phytoalexin-type compounds that accumulate as resin in the wood tissue to contain the infection [1][2].

In other words, what we call oud is a chemical scar. The tree does not make this scent for us. It makes it to survive.

That explains its rarity. In natural forest, studies report that only 7 to 10% of trees contain agarwood [2]. The rest are ordinary wood, whatever their age or size. And because an "infected" tree is hard to recognise from the outside, many healthy trees were historically felled in search of oud that was not there. This was among the reasons Indonesia proposed listing the entire genera Aquilaria and Gyrinops in CITES Appendix II, a proposal adopted by vote at the 13th Conference of the Parties in 2004 (72 votes to 9) [3][4]. Aquilaria malaccensis alone had already been listed since 1995 [5].

Nor is there a single fungus behind oud. Research into artificial induction has used genera as varied as Fusarium, Lasiodiplodia, Trichoderma, Aspergillus and Penicillium [2]. Here lies the first source of difference: each attacker calls up a slightly different defence.

In the laboratory: two families that write the scent

A major 2019 review compiled the compounds isolated from agarwood of four main Aquilaria species and counted 367 known structures. Chromone derivatives made up 44.14% of them and sesquiterpenes 55.86% [6]. These two families are, in effect, the alphabet of oud.

Sesquiterpenes: the layer you smell first

Sesquiterpenes are 15-carbon molecules built from three isoprene units. What sets oud apart is not merely their presence but the variety of their skeletons: agarofurans, agarospiranes, eudesmanes, eremophilanes, guaianes, cadinanes and prezizaanes [6], with eudesmanes, guaianes and eremophilanes the most numerous.

Names that anyone reading oud chemistry will meet include agarospirol, jinkoh-eremol, kusunol and α-agarofuran [6]. "Jinkoh" itself comes from the Japanese word for agarwood (jinkō), a reminder that much of the classical chemistry of oud was written in Japan, where agarwood is bound to the ritual of kōdō, "the way of incense."

These compounds are volatile enough to appear in the distilled oil, and they account for much of what we describe as woody, spicy, resinous and faintly animalic in oud oil.

Chromones: the silent family

The second family is stranger. The 2-(2-phenylethyl)chromones consist of a chromone nucleus (a benzopyranone ring) linked to a phenylethyl chain. The simplest member, 2-(2-phenylethyl)chromone itself, has the formula C₁₇H₁₄O₂, a molar mass of about 250 g/mol and CAS number 61828-53-3 [7].

These molecules are relatively heavy, and many are practically non-volatile at room temperature, so they do not "diffuse" in the usual sense. Yet they matter enormously, for two reasons. First, they are an almost exclusive chemical marker of agarwood, which is why researchers propose them as quality and authenticity markers, being easier to separate and more stable [6]. Second, they turn into scent when heated, as we will see.

A recent (2025) review divides the family into structural classes, among them the flindersiachromone type (FDC), with more than 80 described derivatives; the agarotetrolchromone type (ATC), with more than 40; and the oxidoagarochromone type (OAC), with only three so far [1].

How does the tree make them?

The biosynthetic pathway is still being uncovered, but the outline is clearer now. Construction starts from intermediates derived from the amino acid phenylalanine; a type III polyketide synthase then carries out successive condensations using malonyl-CoA units. The enzyme PECPS (PEC precursor synthase) is described as the "gatekeeper" of the pathway. Further enzymes, including O-methyltransferases (OMTs), cytochrome P450s and glycosyltransferases, then diversify the core scaffold into dozens of derivatives [1].

Scientific honesty requires noting what the review's own authors say: direct experimental evidence for some steps of this pathway is still lacking [1]. What we have today is a reasonable map, not a complete one.

Molecule cards

Name	CAS	Formula	Role in oud
2-(2-phenylethyl)chromone	61828-53-3	C17H14O2	Core of the chromone family and a chemical marker of agarwood [7]
Agarotetrol	—	—	A major chromone in ordinary agarwood; source of aromatic compounds on heating [8][9]
Benzylacetone	2550-26-7	C10H12O	Product of heating agarwood; sweet, floral, balsamic [9]
Benzaldehyde	100-52-7	C7H6O	Thermal breakdown product of agarotetrol; bitter almond [9]
p-Anisaldehyde	123-11-5	C8H8O2	Light aromatic compound in agarwood smoke [9]
Agarospirol	—	C15H26O	Sesquiterpene alcohol of the agarospirane skeleton [6]
Jinkoh-eremol	—	—	Sesquiterpene of the eremophilane skeleton [6]

Note: fields marked (—) were left empty where we could not verify the value from a reliable source.

The secret of incense: why oud changes when it burns

Here lies the paradox that explains much of what oud lovers notice without being able to account for: the scent of a piece of oud on the coal is not the same scent that is stored in the wood. A substantial part of it is made at the moment of heating.

In a Japanese study published in 2018 in the Journal of Natural Medicines, researchers followed what happens to agarotetrol, a non-volatile chromone and one of the most abundant in agarwood, when it is heated. They found that it breaks down into small, volatile aromatic compounds, chief among them benzylacetone and benzaldehyde [8][9]. The "silent" molecule in the wood becomes, through heat, "speaking" molecules.

In a later study (2021) from the same research group, the aroma of two agarwood samples heated to about 120 °C was analysed, revealing low-molecular-weight aromatic compounds including benzylacetone, benzaldehyde, p-anisaldehyde, p-vinylanisole, acetanisole and anisylacetone. The authors suggest these are pyrolysis products of other agarwood constituents, mainly lignin and the non-volatile chromones [9].

What does this mean in practice?

• Oud oil and oud incense are two different chemistries. Distilled oil carries mainly what is volatile and survives distillation, above all the sesquiterpenes. Incense adds what heat generates from the breakdown of chromones and lignin. So you may love a piece on the coal and not its oil, or the reverse.
• Temperature is part of the formula. If breakdown products change with temperature, then the way oud is heated (direct coal, a lower-temperature electric burner, or a mica plate as in the Japanese ritual) becomes a variable of the scent itself, not just a way of releasing it. This is a logical inference from the chemistry rather than a measured result for each method.
• Chromones are a reservoir of deferred scent. A chromone-rich piece may seem quiet in the hand, then open up on the fire.

In the same study, the researchers tested the effect of inhaling these compounds on mice. Each one alone reduced locomotor activity by up to 58–67% compared with controls. The surprise was that mixing them all together did the opposite: movement increased by about 20% [9]. The authors proposed an antagonistic-like interaction, possibly through competition for the same olfactory receptors. This is a result in mice and cannot be transferred directly to humans, but it is an important reminder for every perfumer: a blend is not the sum of its parts.

Why no two ouds are alike: four variables

1. The botanical species

Oud is not one species. The CITES proposal document mentions more than 15 agarwood-producing species [3]. The 2019 review states plainly that "specific species produce specific chemical components which are quite different from one another" [6]. For instance, about 130 chromones have been reported from A. sinensis agarwood, more than 30 from A. malaccensis, and few from A. crassna [6]. Part of this gap may reflect how intensively the Chinese species has been studied rather than nature itself, but the chemical difference between species is well established in the literature.

A common confusion needs correcting here: "Cambodian" and "Indian" are geographic and trade names, not botanical species. The same species can grow in several countries, and one country's oud can be sold under another's name. Geographic origin gives a probability, not a guarantee.

2. The wound and the attacker

In a study of 48 cultivated A. sinensis agarwood samples, researchers compared four induction methods: axe wounding, burning-chisel drilling, chemical inducers and biological inoculation. Sesquiterpenes dominated in axe-wounded agarwood, while chromones made up more than 60% of the components in agarwood produced by chemical inducers and biological inoculation [10]. The types of chromone differed too: axe samples were lower in tetrahydro-chromones (THPECs) and higher in epoxy-chromones (EPECs/DEPECs) [10].

In other words: the kind of wound sketches the scent. The same tree, wounded two different ways, writes two different perfumes.

3. Time

The same study's authors noted that some of the differences may come from formation time: the axe samples were typically over 18 months old, while others were 9 to 16 months [10]. Chemical accumulation in the wood is a continuing process, and the recent review notes that chromone and sesquiterpene accumulation is coordinated over time [1]. Oud that has formed longer inside the tree differs chemically, which is something quite different from oud that has been stored for years after cutting.

4. The tree itself: the case of kynam

The strongest evidence that genetics plays a role is the oud known as Chi-Nan or kynam, legendary in Asian markets for its rarity and price. In a 2021 study, researchers compared agarwood induced from Chi-Nan germplasm of A. sinensis with ordinary agarwood of the same species. The results were striking: average 2-(2-phenylethyl)chromone content was about 170 times higher in Chi-Nan, and 2-[2-(4-methoxyphenyl)ethyl]chromone about 420 times higher. Conversely, the dihydro-chromones (including agarotetrol) were present at very low levels, while guaiane- and eudesmane-type sesquiterpenes were higher [11].

Later studies went as far as treating Chi-Nan germplasm as a distinct chemotype within the species [12]. Two trees bearing the same scientific name may therefore produce oud of radically different chemistry, for genetic reasons unrelated to soil or country.

Then comes distillation

Even if the wood were identical, what happens in the still adds another layer of difference. In a Malaysian study of agarwood hydrodistillation, soaking in water before distillation gave an average yield of 0.27%, while soaking with the fungus Phanerochaete chrysosporium raised it to 0.58%, and the proportions of identified components shifted with it [13]. Figures like these remind us that oil is not a "copy" of the wood but a translation of it. And every translation chooses and omits.

Wild versus induced: what science actually says

This is one of the most sensitive questions in the oud market, and the one most prone to categorical answers. The honest scientific answer: the literature is divided. The authors of the 2019 study themselves note that some research found agarwood from non-conventional methods "comparable" to wild agarwood, while other research found clear chemical differences [10].

What can be said with confidence:

• Artificially induced oud is chemically real oud: the tree itself produces the same chromones and sesquiterpenes as a defence response. It is not an "imitation."
• But the induction method and its duration shift the balance between the two families and the types of derivatives within them [10]. The recent review notes that artificial induction often yields inconsistent chromone profiles [1].
• Wild oud usually formed over much longer periods and under multiple, successive stresses. That makes greater complexity a reasonable expectation, but an expectation, not a measured law.

With CITES restrictions and the scarcity of wild trees, cultivated and induced oud has become most of the market. The useful question is no longer only "wild or cultivated?" but: what does the nose say?

Common myths

"Origin determines the scent"

Origin is one factor among many. Species, germplasm, the nature of the wound, the attacker, formation time and distillation method all write into the scent [6][10][11][13]. The country name on the label may be the weakest signal of them all.

"Oil smells the same as the incense"

We have seen that part of the incense scent is generated by heat from molecules that do not pass into the oil in the same form [8][9]. Oil and incense are two different experiences of the same raw material.

"The more expensive, the better"

Price in the oud market reflects rarity, demand, the reputation of an origin and the absence of unified standards, as much as olfactory quality. There is still no single agreed chemical formula linking "value" to a particular scent. Some studies propose markers such as agarotetrol for evaluating agarwood as a raw material [6][8], but these are markers of authenticity and content, not of beauty.

"Darker oud is always better"

Dark colour is associated with resin accumulation, but it is a visual measure, not a chemical one. Colour does not tell you which family dominates a piece or which chromones it carries. Two pieces of the same colour can carry different chemistries, as the induction experiments show [10].

What we don't know yet

Oud is one of the few materials whose alphabet science is still discovering, not just its grammar. Open questions include:

• The full biosynthetic pathway of the chromones. We know the key enzyme and the broad outline, but some steps still lack direct evidence [1].
• Which molecules actually "make" the smell of oud? Most published research has focused on structure and pharmacological activity rather than odour. The 2019 review itself notes that research is moving towards pharmacologically active substances "rather than only fragrance components" [6]. We still lack a precise olfactory map (using techniques such as GC-O) linking each molecule to its effect on the overall scent.
• Why do germplasms like Chi-Nan differ so radically? The finding that they form a distinct chemotype [12] opens the genetic question without fully answering it.
• Interaction between components. The mouse result in which the mixture reversed the effect [9] hints that the scent of oud may be a "collective" phenomenon that no single-molecule analysis can explain.

Conclusion: the nose is the final judge

The deeper the chemistry of oud goes, the harder it becomes to reduce it to a single word on a label. It is the sum of a species, a germplasm, a wound, an attacker, time, and a fire or a still. No origin, price or colour can compress that whole chain.

Which is why the old rule remains truer than any table: the nose is the final judge. The value of an aromatic material cannot be reduced to its origin, rarity or price, but lies in what it does when you smell it once, return to it a second time, and discover what you missed the first time. Chemistry does not overturn this rule. It explains why it holds: because no two ouds are truly alike.

Glossary

English	Français	العربية
agarwood / oud	bois d'agar / oud	العود
sesquiterpenes	sesquiterpènes	سيسكيتربينات
2-(2-phenylethyl)chromones (PECs)	2-(2-phényléthyl)chromones	كرومونات
phytoalexins	phytoalexines	فيتوأليكسينات
pyrolysis	pyrolyse	تحلل حراري
chemotype	chimiotype	نمط كيميائي
inoculation	inoculation	تلقيح (تحفيز فطري)
hydrodistillation	hydrodistillation	تقطير مائي
polyketide synthase (PKS)	polykétide synthase	سينثاز البوليكيتيد
GC-olfactometry (GC-O)	CPG-olfactométrie	كروماتوغرافيا غازية مع الشم

Sources

1. Wu, W. et al. (2025). Biosynthesis and Its Regulatory Mechanisms of 2-(2-Phenylethyl)chromones in Agarwood. Plants, 14(7), 1012. doi.org/10.3390/plants14071012
2. Jalil, A.-M. et al. (2022). Assessment of the Effects of Artificial Fungi Inoculations on Agarwood Formation and Sap Flow Rate of Aquilaria malaccensis Lam. Forests, 13, 1731. mdpi.com
3. CITES (2004). CoP13 Prop. 49: Inclusion of Aquilaria spp. and Gyrinops spp. in Appendix II (proponent: Indonesia). cites.org
4. Earth Negotiations Bulletin (IISD). CITES CoP13 Daily Report, 13 October 2004. enb.iisd.org
5. FAO. Non-Wood News 8: Agarwood. fao.org
6. Gao, M. et al. (2019). Overview of sesquiterpenes and chromones of agarwood originating from four main species of the genus Aquilaria. RSC Advances. pubs.rsc.org
7. ChEBI (EMBL-EBI). CHEBI:5092: 2-(2-phenylethyl)chromone. ebi.ac.uk
8. Takamatsu, S.; Ito, M. (2018). Agarotetrol: a source compound for low molecular weight aromatic compounds from agarwood heating. Journal of Natural Medicines. PubMed 29464509
9. Castro, K.P.; Ito, M. (2021). Individual and Combined Inhalational Sedative Effects in Mice of Low Molecular Weight Aromatic Compounds Found in Agarwood Aroma. Molecules, 26(5), 1320. doi.org/10.3390/molecules26051320
10. Yan, T. et al. (2019). Chemical Profiles of Cultivated Agarwood Induced by Different Techniques. Molecules, 24(10), 1990. doi.org/10.3390/molecules24101990
11. Yu, M. et al. (2021). Remarkable Phytochemical Characteristics of Chi-Nan Agarwood Induced from New-Found Chi-Nan Germplasm of Aquilaria sinensis Compared with Ordinary Agarwood. International Journal of Analytical Chemistry. doi.org/10.1155/2021/5593730
12. Chi-Nan agarwood germplasms constitute a new chemotype of Aquilaria sinensis (Lour.) Spreng. AGRIS (FAO). agris.fao.org
13. Production of Agarwood Essential Oil: Study on Effectiveness Pre-Treatment Technique of Hydrodistillation Extraction (2015). Borneo Journal of Resource Science and Technology, UNIMAS. publisher.unimas.my

— Souheyb Zine
Founder, Perfumer & Creative Director