The aroma of coffee
More than a thousand volatile compounds have been found in roasted coffee, yet a few dozen key odorants carry its smell. How they form in roasting, how they are lost, and how chemists find them with the human nose.
Across the chain: Science
Contents
In brief
- More than 1,000 volatile compounds have been identified in roasted coffee; only a small set of key odorants is known to shape its aroma.1,3
- Key odorants belong to a few classes – thiols, sulphides, aldehydes, pyrazines, diketones, phenols such as guaiacols, and furanones.1
- 2-Furfurylthiol is described as the only odorant that smells like coffee on its own, and only within a certain concentration range.1,2
- Gas chromatography–olfactometry (GC–O) separates the volatiles and lets a person smell each one as it leaves the column.1,7
- Aroma fades fast after brewing; in one study, a fresh brew lost 84% of its available 2-furfurylthiol within 60 minutes.6
What aroma is#
Aroma is what we smell. The molecules responsible are volatile: they evaporate easily and travel through the air to the smell receptors in the nose.9,1 Odour molecules reach those receptors by two routes. Breathing in through the nose is the orthonasal route. Breathing out from the back of the mouth is the retronasal route.9
Cuppers name the smells by when they meet them. Fragrance is the smell of the dry grounds, and aroma the smell of the brew; both are smelled through the nose.9 Flavour is the taste of the brew together with its retronasal smell, perceived as one impression while the coffee is in the mouth.9 The same molecules therefore shape fragrance, aroma and much of flavour.
Many compounds, few that matter#
According to a coffee chemistry textbook chapter, coffee is one of the beverages with the richest and most complex flavour content.1 More than 1,000 volatile compounds have been identified in roasted coffee.1,3 The same chapter describes most of them as odourless.1 It states that the number of compounds does not define an aroma.1 What counts is a much smaller set of key odorants – compounds shown to contribute to the overall aroma.1
A 2023 review reports, from an earlier study, 30 compounds that contribute significantly to the aroma of a coffee brew.8 A 2025 study of four arabica coffees identified 132 volatile compounds.3 By sniffing, the analysts detected 85 aroma compounds.3 Of the compounds they quantified, 25 were present above their odour threshold in water.3
None of these compounds smells like coffee by itself, with one exception. According to the textbook, only 2-furfurylthiol gives a coffee-like note, and only within a certain range of concentration.1,2 At high levels it smells rubbery.1 All the other odorants smell different from coffee.2
The main families of odorants#
The key odorants of roasted coffee fall into a few chemical families.1 The descriptions below are those reported in the sensory studies summarised in the textbook.
- Thiols and sulphides – sulphur compounds. 2-Furfurylthiol smells sulphury and roasty; methanethiol of sulphur and garlic; 3-mercapto-3-methylbutyl formate is described as catty and blackcurrant-like.1
- Aldehydes – methional smells of boiled potato; 2- and 3-methylbutanal are malty.1
- Diketones – 2,3-butanedione and 2,3-pentanedione smell buttery.1
- Pyrazines – nitrogen compounds that smell earthy and roasty, such as 2-ethyl-3,5-dimethylpyrazine.1
- Phenols – guaiacol smells smoky; 4-ethylguaiacol and 4-vinylguaiacol are spicy and clove-like.1
- Furanones – furaneol smells of caramel and fruit; sotolon of fenugreek and curry.1
Different reactions in roasting form them, and roast degree shifts the balance between them.1
Where aroma comes from and where it goes#
According to the textbook, green coffee smells very different from roasted coffee. Its aroma is described as green, hay-like and pea-like.1 The typical coffee aroma forms only during roasting, through a network of chemical reactions.1 The Maillard reaction between sugars and amino acids gives most of the families above. Chlorogenic acids break down into the smoky and spicy guaiacols.1
Aroma begins to fade as soon as roasting ends. A textbook chapter on freshness names three routes: aroma molecules escape, they react with other coffee components, or they oxidise and form new off-notes.2 All three run faster once the beans are ground and at higher storage temperatures.2 The gas that leaves fresh beans during degassing is mainly carbon dioxide.2
The losses continue in the cup. In a 2019 study, a fresh brew lost 84% of its available 2-furfurylthiol and 72% of its methanethiol within 60 minutes.6
Arabica and robusta#
C. arabica and C. canephora (robusta) form much the same aroma compounds in roasting. The differences lie in how much of each is formed.1 According to the textbook, robusta green coffee holds more free amino acids, which leads to more earthy, roasty pyrazines.1 Its higher chlorogenic acid content gives more phenols, and with them a smoky, earthy and phenolic profile.1
Arabica holds more sucrose, which the textbook links to more diketones, furfurals and furaneol after roasting.1 A 2020 review describes one comparison of the two species with the same pattern. Guaiacols were significantly higher in robusta, pyrazines slightly higher, and furans higher in arabica.4
Reference values#
Key odorants of roasted coffee as summarised in the coffee chemistry textbook from several dilution and quantification studies, with their odour qualities and the species in which they are more abundant.1 A = more abundant in arabica, R = more abundant in robusta, A/R = similar.
| Compound | Class | Odour quality | More abundant in | Source |
|---|---|---|---|---|
| Methanethiol | Thiol | Sulphur, garlic | R | Poisson et al.1 |
| 2-Furfurylthiol | Thiol | Sulphury, roasty | R | Poisson et al.1 |
| 3-Mercapto-3-methylbutyl formate | Thiol ester | Catty, blackcurrant-like | A/R | Poisson et al.1 |
| Dimethyl trisulfide | Sulphide | Sulphur, cabbage | A/R | Poisson et al.1 |
| Methional | Aldehyde (Strecker) | Potato | A | Poisson et al.1 |
| 2-Methylbutanal | Aldehyde (Strecker) | Green, solvent, malty | R | Poisson et al.1 |
| 3-Methylbutanal | Aldehyde (Strecker) | Malty, cocoa | A/R | Poisson et al.1 |
| 2,3-Butanedione | Diketone | Buttery | A/R | Poisson et al.1 |
| 2,3-Pentanedione | Diketone | Buttery | A | Poisson et al.1 |
| 2-Ethyl-3,5-dimethylpyrazine | Alkylpyrazine | Earthy, roasty | R | Poisson et al.1 |
| 2,3-Diethyl-5-methylpyrazine | Alkylpyrazine | Earthy, roasty | R | Poisson et al.1 |
| 3-Isobutyl-2-methoxypyrazine | Methoxypyrazine | Pea, earthy | A | Poisson et al.1 |
| Guaiacol (2-methoxyphenol) | Phenol | Smoky | R | Poisson et al.1 |
| 4-Ethylguaiacol | Phenol | Spicy, clove-like | R | Poisson et al.1 |
| 4-Vinylguaiacol | Phenol | Spicy, clove-like | R | Poisson et al.1 |
| Sotolon | Furanone | Fenugreek, curry | A/R | Poisson et al.1 |
| Furaneol | Furanone | Caramel, fruity | A | Poisson et al.1 |
| Volatiles identified in roasted coffee | – | – | – | More than 1,0001,3 |
| Odour-active compounds in a filtered brew (AEDA, one study) | – | – | – | 405 |
| Compounds with OAV > 1, four arabica powders (one study) | – | – | – | 25 in total; 16–20 per coffee3 |
The textbook table names the class “dicarbonyls” for the diketones.1 Its abundance column compares the species in general terms; it gives no concentrations.1
From identification to key odorants#
Sensory-directed analysis. Coffee aroma research narrows the volatiles down in steps that combine instruments and the human nose. The textbook describes five: isolating the aroma; screening it by gas chromatography–olfactometry; identifying the most intense odorants by mass spectrometry and nuclear magnetic resonance; quantifying them; and testing a recombined model.1 Its authors call this the Sensomics concept. It was essential to identifying the rather limited number of aroma-relevant volatiles.1
Gas chromatography–olfactometry (GC–O). A gas chromatograph separates the volatile compounds of an extract in a column. In GC–O, the gas leaving the column is sniffed, so that the human nose acts as a detector that tells potent odorants from the odourless majority.1 In the 2025 study, a GC–MS instrument carried an olfactory detection port, so that each odour could be matched with a mass spectrum.3 A 2001 review groups GC–O methods into dilution analysis, detection frequency, posterior intensity and time-intensity methods.7 It shows that the value of GC–O data depends directly on the method chosen, as well as on sample preparation and analytical conditions.7
- Aroma extract. The volatile fraction isolated from the coffee is injected; in aroma extract dilution analysis, it is injected again in stepwise dilutions.1,3
- Column in the oven. Gas chromatography separates the compounds, which leave the column one after another.1
- Splitter. Our drawing divides the gas leaving the column between a detector and a sniffing port, as in a GC–MS instrument fitted with an olfactory detection port.3
- Mass spectrometer. Identifies the compounds by their mass spectra.1,3
- Sniffing port. An assessor smells the gas and records each odour and its time; the nose is the detector.1
- Detector trace. Shows the compounds the detector picks up; most volatiles are odourless.1
- Aromagram. Shows only the odour events; a potent odorant can sit where the detector shows almost nothing, because many important odorants occur in trace amounts.1
Aroma extract dilution analysis (AEDA). The extract is diluted stepwise and each dilution is sniffed by GC–O. The flavour dilution (FD) factor is the ratio of an odorant’s concentration in the original extract to its concentration in the most dilute extract in which it is still smelled.2 The higher the FD factor, the more potent the odorant in that extract. In the 2025 study, the extract was diluted three-fold at each step.3 The most intense odorants found this way are then identified and quantified.1
Odour activity values (OAV). The candidates are quantified, and each concentration is divided by the odour threshold of the compound in a defined matrix.1 According to the textbook, the method of choice for quantification is the stable isotope dilution assay, with isotope-labelled versions of the analytes as internal standards.1 The resulting OAV is a strong indicator of a compound’s relative importance.1 In the 2025 study, compounds above their threshold (OAV above 1) counted as key odour-active compounds.3
Recombination and omission. The last step mixes the identified odorants at their natural concentrations and compares the model with the real coffee.1 Leaving out one compound or a group shows its impact: those whose omission changes the profile significantly are the impact compounds.1
Published odour activity values#
In the 2025 study, four commercial washed, medium-roast arabica coffees were analysed as roasted powder: one from Colombia, two from Bench Maji and Yirgacheffe (Ethiopia) and one from Baoshan, China.3 Odour thresholds in water were taken from a published compilation.3 Of 132 volatile compounds identified, 25 had OAVs above 1; each coffee had 16–20 of them.3 4-Vinylguaiacol had the highest value in every coffee, 340–761, and furaneol reached 175–250.3
Data table
| Odour activity value (OAV) | Source | |
|---|---|---|
| 4-Vinylguaiacol | 340–761 (4 coffees) | 3 |
| Furaneol | 175–250 (4) | 3 |
| Linalool | 72–222 (3) | 3 |
| Isovaleric acid | 72–200 (4) | 3 |
| 2-Ethyl-3,5-dimethylpyrazine | 170 (1) | 3 |
| 3-Hydroxy-2-butanone | 94–137 (3) | 3 |
| 4-Ethylguaiacol | 12–79 (3) | 3 |
| 2-Ethyl-6-methylpyrazine | 45–72 (3) | 3 |
| Compound | OAV, lowest–highest (coffees with a value) | Source |
|---|---|---|
| 4-Vinylguaiacol | 340–761 (4) | Chen et al.3 |
| Furaneol | 175–250 (4) | Chen et al.3 |
| Linalool | 72–222 (3) | Chen et al.3 |
| Isovaleric acid | 72–200 (4) | Chen et al.3 |
| 2-Ethyl-3,5-dimethylpyrazine | 170 (1) | Chen et al.3 |
| 3-Hydroxy-2-butanone | 94–137 (3) | Chen et al.3 |
| 4-Ethylguaiacol | 12–79 (3) | Chen et al.3 |
| 2-Ethyl-6-methylpyrazine | 45–72 (3) | Chen et al.3 |
The values differ widely between the four coffees. Linalool, for example, had OAVs from 72 to 222 in three coffees and none reported in the fourth.3 In omission tests, the study identified guaiacol and furaneol as key odorants for the woody and caramel-like character of the Baoshan coffee.3 In that study the material was roasted powder, not a brew, and the thresholds came from other work.3
Brew versus instant. In a 2002 study, AEDA found 40 odour-active compounds in a filtered coffee brew.5 (E)-β-Damascenone, methional, 3-mercapto-3-methylbutyl formate, 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone, furaneol and several phenols were among the most intense.5 An instant coffee beverage held the same odorants, mostly at much lower FD factors. The largest differences were for some sulphur compounds, several methoxyphenols and vanillin.5
How roasting shapes the odorants#
Reactions. Strecker degradation within the Maillard network gives the malty 2- and 3-methylbutanal, potato-like methional and honey-like phenylacetaldehyde.1 It also yields the earthy, roasty alkylpyrazines.1 The sulphur amino acids cysteine and methionine react to thiols and sulphides. Some have low odour thresholds and act at trace levels, among them 3-mercapto-3-methylbutyl formate, 2-furfurylthiol and methanethiol.1 Ferulic acid from chlorogenic acid degradation breaks down further into guaiacol and 4-vinylguaiacol.1 According to the textbook, lipid oxidation gives potent aldehydes such as hexanal, but these do not belong to the key compounds of coffee aroma.1
What green coffee brings. Green coffee contains volatiles in much lower concentrations than roasted coffee. Some, such as 3-isobutyl-2-methoxypyrazine, formed by enzymes in the green bean, pass through roasting unchanged.1
Roast degree. Diketones, furfurals and 4-vinylguaiacol form early in the roast, alongside the use of free sugars and amino acids, and peak at a medium roast degree.1 Excessive roasting lowers them, and the blackcurrant-like 3-mercapto-3-methylbutyl formate can disappear completely in a dark roast.1 Alkylpyrazines also form early but stay almost constant in the final stage.1 2-Furfurylthiol, pyridine, N-methylpyrrole and dimethyl trisulfide kept increasing with roasting time in the study the textbook cites.1 According to the textbook, sensory and instrumental analysis support the roasters’ account: sweet, fruity, floral, bread and nutty notes in light roasts give way to cocoa, spicy, phenolic and ashy notes in dark ones.1
How aroma is lost#
After roasting. According to the freshness chapter, many aroma-active compounds are reactive, carrying thiol, carbonyl or enolone groups. Depending on oxygen, moisture and temperature, they change the aroma over time.2 Loss of freshness is best described as a progressive imbalance in the aroma profile, not simply a loss.2 Methanethiol decreases strongly soon after roasting, and its oxidation forms dimethyl disulfide, which is why the ratio of the two serves as a freshness index.2 Hexanal from lipid oxidation is at best an early marker of fading aroma; it does not explain the loss of freshness.2
After grinding. The chapter reports a study of ground-coffee headspace in which several aldehydes and diketones – 2- and 3-methylbutanal, methylpropanal, 2,3-butanedione and 2,3-pentanedione – fell sharply shortly after grinding.2
In the cup. In a 2019 study of a fresh brew kept for 60 minutes, available 2-furfurylthiol fell by 84%, methanethiol by 72% and 3-methyl-1H-pyrrole by 68%; 2-pentylfuran rose by 65%.6 The freshness chapter cites mechanistic work in which 2-furfurylthiol, which carries the sulphury-roasty note of a brew, decreased considerably during storage.2 A 2020 review reports that 2-furfurylthiol declines by bonding to melanoidins, lowering burnt, sulphurous and roasty notes in the brew.4
Arabica and robusta in detail#
According to the textbook, aroma formation during roasting is quite similar in the two species; concentration differences decide the final character.1 Lower isoleucine and leucine in arabica green coffee give less 2-methylbutanal and 3-methylbutanal after roasting.1 More tryptophan in robusta gives more of the animal-smelling skatole (3-methylindole), which is almost absent from arabica.1 Robusta’s higher amino acid and chlorogenic acid contents lead to more pyrazines and phenols.1 Arabica’s higher sucrose leads to more diketones, furfurals and furaneol.1
A 2020 review describes a headspace study of arabica and robusta from six countries. Guaiacols such as 2-methoxyphenol were significantly higher in robusta, pyrazines slightly higher, and furans such as 2-acetylfuran and furfural higher in arabica.4 The analysis differentiated the arabica samples (Costa Rica, El Salvador, Brazil) from the robusta samples (India, Togo, Vietnam), with an interaction between species and country.4 Aroma formation also varies within a species, as reported for arabica coffees from Colombia, Guatemala and Ethiopia.1
Open questions#
- From powder to cup. Most odour activity data are for roasted powder or extracts. How the balance of key odorants in the brew, at drinking temperature and retronasally, maps onto cupping descriptors is not established by the sources opened for this entry.1,3
- Thresholds and matrix. OAVs depend on thresholds measured in a defined matrix, often water; values from different studies may not be comparable.1,3
- Interactions. The textbook presents recombination and omission tests as the step that links quantitative data to the sensory character of a coffee.1 No source opened for this entry gives a model that predicts the aroma of a brew from its key odorants.
Key studies#
Chen et al., 2025 – Key aroma compounds in four arabica coffees. Ground washed, medium-roast arabica coffees from Colombia, Ethiopia and Baoshan, China, were analysed by GC–O with aroma extract dilution and by quantification. Of 132 identified volatiles, 25 exceeded their odour thresholds, with 4-vinylguaiacol and furaneol reaching the highest odour activity values.
Sanz et al., 2002 – Filtered brew versus instant coffee. Aroma extract dilution analysis found 40 odour-active compounds in a filtered brew. The instant beverage contained the same odorants at much lower dilution factors, with the largest losses among sulphur compounds and methoxyphenols.
Sun et al., 2019 – How fast a brew loses 2-furfurylthiol. A fresh coffee brew was held for 60 minutes and its volatiles followed over time. The available 2-furfurylthiol fell by 84% and methanethiol by 72%, while 2-pentylfuran rose.
- How many odorants matter: a coffee chemistry textbook chapter speaks of a rather limited number of aroma-relevant volatiles1; a 2023 review reports, from an earlier study, 30 compounds, 2-furfurylthiol among them, that contribute significantly to the aroma of a brew8; a 2025 study of four arabica coffees found 25 compounds with an odour activity value above 1, between 16 and 20 in each coffee.3 The counts depend on the coffee, the sample (powder or brew) and the method.
- Which odorants lead: the classic dilution studies summarised in the textbook give the highest ranks to sulphur compounds, pyrazines, furanones and phenols, with 2-furfurylthiol as the coffee-like note1,2; in the 2025 odour-activity study, 4-vinylguaiacol had the highest value in every coffee and furaneol reached the second-highest maximum, and no thiol was among the compounds it quantified.3 According to the textbook, many important odorants occur only in trace amounts or are reactive and unstable, which makes coffee aroma hard to characterise.1
Coffearium (2026). The aroma of coffee. Coffearium – The open coffee compendium. Retrieved 22 October 2026, from https://coffearium.org/entries/the-aroma-of-coffee/
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author = {{Coffearium}},
title = {{The aroma of coffee}},
year = {2026},
howpublished = {Coffearium -- The open coffee compendium},
url = {https://coffearium.org/entries/the-aroma-of-coffee/},
urldate = {2026-10-22},
note = {Retrieved 22 October 2026}
}Entries are updated when the evidence changes; the retrieval date tells your readers which version you used.
Sources
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- BYeretzian, C.; Blank, I.; Wyser, Y. (2017). Protecting the Flavors – Freshness as a Key to Quality (chapter 14). In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 14, pp. 329–353. doi:10.1016/B978-0-12-803520-7.00014-1 · other copies
- AChen, X.; Wu, P.; Wang, S.; Sun, J.; Chen, H. (2025). Identification of Key Aroma-Active Compounds in Commercial Coffee Using GC-O/AEDA and OAV Analysis. Foods 14(18), 3192. doi:10.3390/foods14183192 · other copies
- ASeninde, D. R.; Chambers, E., IV (2020). Coffee Flavor: A Review. Beverages 6(3), 44. doi:10.3390/beverages6030044 · other copies
- ASanz, C.; Czerny, M.; Cid, C.; Schieberle, P. (2002). Comparison of potent odorants in a filtered coffee brew and in an instant coffee beverage by aroma extract dilution analysis (AEDA). European Food Research and Technology 214(4), 299–302. doi:10.1007/s00217-001-0459-9 · other copies
- ASun, Z.; Yang, N.; Liu, C.; Linforth, R. S. T.; Zhang, X.; Fisk, I. D. (2019). Aroma binding and stability in brewed coffee: A case study of 2-furfurylthiol. Food Chemistry 295, 449–455. doi:10.1016/j.foodchem.2019.05.175 · other copies
- Avan Ruth, S. M. (2001). Methods for gas chromatography-olfactometry: A review. Biomolecular Engineering 17(4–5), 121–128. doi:10.1016/S1389-0344(01)00070-3 · other copies
- AZhang, G.; Xiao, P.; Yuan, M.; Li, Y.; Xu, Y.; Li, H.; Sun, J.; Sun, B. (2023). Roles of sulfur-containing compounds in fermented beverages with 2-furfurylthiol as a case example. Frontiers in Nutrition 10, 1196816. doi:10.3389/fnut.2023.1196816 · other copies
- A/BSpecialty Coffee Association (2024). SCA Standard 102-2024 Coffee Value Assessment: Sample Preparation and Tasting Mechanics. Specialty Coffee Association.
Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.
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