Many of the compounds that determine the odour of essential oils, flowers and perfumes are not terpenoids: aliphatic aldehydes, esters, lactones, musks, amber compounds, thiols and pyrazines. The thresholds of these families span about nine orders of magnitude within a single compilation table. This part presents the structural rules of each family at the chemical level, without repeating the receptor profiles already given in Volume 3.
Note on units: the thresholds in this part come from three measurement systems that cannot be compared directly with one another: (a) measured in air (ppb by volume or ng/L); (b) the Leffingwell compilation table (ppb), which does not state the measurement medium and is used only to compare tendencies within the table itself; (c) measured in a wine matrix (ng/L or µg/L).
3.1 Straight-chain aldehydes: the threshold minimum at 8 carbons
Cometto-Muñiz & Abraham 2010 (doi:10.1093/chemse/bjq018) measured air thresholds for the aldehyde series with a vapour-delivery device whose concentrations were checked by GC, 16–18 people per compound Measured. The threshold falls from propanal (2.0 ppb) to octanal (0.17 ppb), then rises again at nonanal (0.53 ppb). The Leffingwell table (a different measurement system, for comparing tendencies only) also has octanal lower than hexanal, heptanal and nonanal, but lists decanal at 0.1–2 ppb, so the minimum at C8 is certain only within the C3–C9 series measured in air (Figure 3).
On nuance Industry: short-chain aldehydes are pungent and unpleasant; from C8 to C12 the odour shifts to waxy, floral, orange peel; by C14 only a faint odour remains (Cotton 2008, compiled from Arctander and Sell). In the Leffingwell table, a conjugated double bond at position 2 lowers the threshold about 10-fold at C9 (nonanal 1 versus (E)-2-nonenal 0.08–0.1 ppb), but not clearly at C10 Inference. A methyl branch at C-2 makes the odour stronger and more pleasant, as in 2-methylundecanal Industry. This is the family that creates the “aldehydic” note of many classic perfumes of the early 20th century.
In citrus oils, straight-chain aldehydes are trace constituents but carry the characteristic odour: ISO 3140:2019 for sweet orange specifies octanal 0.1–0.6% and decanal 0.1–0.7%, while limonene makes up 93–96% Standard.
3.2 Esters: the acid moiety determines potency
Short-chain esters give fruity odours: ethyl butyrate suggests fruit, isoamyl acetate banana, ethyl hexanoate apple; longer chains shift towards waxy, fatty Industry. In the Leffingwell table, ethyl butyrate has a threshold of 1 ppb, whereas ethyl isobutyrate has 0.1 ppb and ethyl 2-methylbutyrate 0.1–0.3 ppb. Adding a methyl branch to the acid moiety thus lowers the threshold about 3–10-fold Inference.
In flowers, esters are major constituents. Egyptian-grown jasmine Jasminum sambac has 23.7% benzyl acetate in the headspace over the concrete and 14.2% in the absolute; indole makes up about 13% in both; methyl anthranilate about 5% Measured (Edris et al. 2008, doi:10.1007/s00217-007-0623-y). Peer-reviewed data for J. grandiflorum have not been found (NR).
3.3 Lactones and coumarin
For γ-lactones (5-membered ring), the threshold falls by about 2 orders of magnitude as the chain grows from 6–7 to 8 or more carbons, then levels off: γ-hexalactone 1,600 ppb, γ-octalactone 7, γ-decalactone 11, γ-dodecalactone 7 (Leffingwell table). At C10, the δ-lactone (6-membered ring) is about 9 times weaker than the γ-lactone Inference. In Australian red wine, lactone thresholds range from 8 to 285 µg/L, and the two enantiomers differ significantly for every lactone except γ-decalactone Measured (Cooke et al. 2009, doi:10.1021/jf802866f; abstract only).
Massoia lactone, an unsaturated δ-lactone obtained from the tree Cryptocarya massoy, makes up 64.8% of the bark oil and 68.4% of the heartwood oil in samples from Papua New Guinea Measured (Rali et al. 2007, doi:10.3390/12020149).
Coumarin is an aromatic lactone with a hay, vanilla odour. Tonka beans contain 20.4–43.4 mg/g (Toma et al. 2025, doi:10.1007/s00217-024-04648-z; the same paper gives 29.1–43.4 in its conclusion). This paper cites literature stating that cassia bark can contain up to 76.1 g/kg coumarin, especially in Cinnamomum loureiroi and C. burmannii, whereas Sri Lankan cinnamon C. verum contains only traces. These figures were measured on bark, not on essential oil. EFSA sets a tolerable daily intake of 0.1 mg/kg body weight; in EU cosmetics, coumarin must be declared on the label when it exceeds 0.001% in leave-on products and 0.01% in rinse-off products Standard (as cited in Toma 2025). Part VI discusses the implications for Vietnamese cassia.
3.4 Musks: five structural classes
| Class | Examples | Chemical characteristics | Notes |
|---|---|---|---|
| Macrocyclic ketones | muscone (15-membered ring), civetone (17-membered ring) | cycloalkanone, cycloalkenone | Ružička established the structure in 1926; Nobel Prize 1939 |
| Macrocyclic lactones | Exaltolide (16-membered ring), ambrettolide (17-membered ring), Habanolide | oxacycloalkan-2-one | Together with macrocyclic ketones, the direction the industry moved to when abandoning nitro and polycyclic musks |
| Nitro musks | musk ketone, musk xylene, musk ambrette, moskene, tibetene | aromatic ring bearing a tert-butyl group and 2–3 NO₂ groups | EU bans musk ambrette, moskene, tibetene; musk xylene phased out under REACH from 2014; musk ketone limited to 1.4% in fine fragrance |
| Polycyclic musks | Galaxolide (HHCB), Tonalide (AHTN) | indane, tetralin, isochroman skeletons; highly lipophilic (log Kow 5.7–5.9) | HHCB found in 97% of breast milk samples in Massachusetts (n = 39), mean 220 ng/g lipid |
| Acyclic and small-ring musks | Helvetolide, Romandolide | alkyl or cycloalkyl esters, no macrocycle | Cyclomusk 1975; Helvetolide 1990; Romandolide about 2000 |
The structural rules for musks are all only approximate Industry, taken from a secondary compilation (Moore 2005). Stoll’s rule (1936): rings of 14–19 atoms with at least one C=O or NH group; lactones and keto-ethers are strongest; with two or more oxygens in the ring the odour weakens. Molecular weight about 200–300 (Beets 1957); about 300 is the observed upper limit (Kraft & Cadalbert 2001). A pharmacophore model proposed in 1991 reproduced only 54–65% of the results when re-tested in 1995. Nitro musk thresholds can be very low: musk ketone 0.1 ng/L air (Hauser et al. 2020, doi:10.2533/chimia.2020.247).
Specific anosmia to musk has been reported in about 7% of white people in one study (Whissell-Buechy & Amoore 1973; both figures taken via the Koelega 1976 review) and in about 50% of adult men for cyclopentadecanolide in another study (Le Magnen); the large difference is probably due to different test compounds, criteria and populations. For product developers, this means that some customers may be unable to smell the musk note in a formulation.
3.5 Ambers, ionones and damascones
The odour of ambergris comes from oxidative degradation products of the triterpene ambrein. (−)-Ambrox has a threshold of 0.2 ng/L, while one of its stereoisomers is almost odourless (above 500 ng/L) Measured (patent US 10,294,211, a non-peer-reviewed source). Ambrox was discovered in the course of research on the oxidation of sclareol from clary sage Salvia sclarea; today an enzymatic route also exists. Ohloff proposed the “triaxial rule” for amber odour: on a trans-decalin skeleton, three substituents at positions 1, 2 and 4 all axial. The original text could not be read; Winter 1990 showed that many compounds satisfying the condition are still odourless, and added a criterion of the steric accessibility of the oxygen atom Industry.
Ionones and damascones are 13-carbon compounds formed by oxidative cleavage of carotenoids. The threshold of β-damascenone is given as 0.009 ppb in one source and 0.002 ppb in the Leffingwell table; β-ionone 0.007 ppb. This is why a constituent at 0.14% can dominate the odour of rose oil (Part I).
3.6 Thiols and pyrazines: the lowest thresholds
| Compound | Characteristic odour | Threshold | Matrix | Source |
|---|---|---|---|---|
| 1-p-Menthene-8-thiol | grapefruit | 0.000034 ng/L | air | Schoenauer & Schieberle 2016, doi:10.1021/acs.jafc.6b01645 (abstract) |
| 4-Mercapto-4-methylpentan-2-one (4MMP) | boxwood, blackcurrant | 0.8 ng/L / 3 ng/L | model solution / wine | Tominaga 1998; Capone 2010 |
| 3-Mercaptohexyl acetate (3MHA) | passion fruit | 4.2 ng/L | model solution | via Musumeci et al. 2015 |
| 2-Methoxy-3-isobutylpyrazine (IBMP) | bell pepper | 2 ppt | not stated (secondary citation) | Buttery 1969, via Harris et al. 1987 |
| (−)-Ambrox | amber | 0.2 ng/L | air | US 10,294,211 |
| Vetiver ketone (Part II) | vetiver | 29 pg/L | air | List et al. 2020 |
For 1-p-menthene-8-thiol, saturating the double bond or moving the SH group onto the ring both raise the threshold markedly (Schoenauer & Schieberle 2016; abstract only). The common motif of the “tropical” thiols is an SH group on a tertiary carbon, close to an unsaturated skeleton or a carbonyl group Inference. For pyrazines, a methoxy group combined with an alkyl branch at position 3 lowers the threshold by about 4.5–7 orders of magnitude compared with methylpyrazine Inference (Leffingwell table: 2-methylpyrazine 60–105,000 ppb, IBMP 0.002–0.016 ppb).
The choice of threshold matrix changes the OAV result: for 4MMP in New York Sauvignon blanc (27 ng/L), the OAV is about 34 using the model-solution threshold and about 9 using the threshold in wine Inference (Figure 1).
3.7 Nitrogen heterocycles in flowers
Indole (threshold 140 ppb, Leffingwell) makes up 13.4% of Egyptian J. sambac jasmine absolute. Methyl anthranilate is the character-impact compound of Concord grape; in Vitis labrusca varieties that accumulate large amounts of it, the regulatory region of the AMAT gene carries a deletion of 426 or 42 base pairs, confirmed in more than 1,300 samples Measured (Yang et al. 2020, Hortic Res 7:67). The methyl anthranilate content of orange blossom (neroli) oil has no source yet (NR).
A monograph compiled from 718 sourced facts collected under a common protocol and cross-checked independently. Items labelled Inference are the compilers’ own judgements. Diagrams are newly drawn from published data. Scientific reference material — not legal or medical advice, and no substitute for the text of the standards.