{ "id": "second-crack", "url": "https://coffearium.org/entries/second-crack/", "license": "CC-BY-4.0", "title": "Second crack", "summary": "The second, snapping series of cracks heard when a roast is taken dark – what breaks in the bean, why oil reaches the surface, how flavour and chemistry change, and the smoke and gas that come with it.", "domain": "roasting", "kind": "Process", "stage": "roast", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-17T00:00:00.000Z", "updated": "2026-10-17T00:00:00.000Z", "added": "2026-10-17T00:00:00.000Z", "brief": [ "Second crack is a later, quieter and faster series of cracks than first crack, heard only when the roast continues until the beans are dark.2,7", "An SCA handbook explains it as gas pressure cracking the bean again once its structure has turned brittle; no peer-reviewed measurement of the mechanism was found.3,6", "Gas pressure pushes coffee oil through the cell walls to the surface; this runs faster in dark roasts, which is why they look oily.1,5", "By second crack, titratable acidity had fallen back to its green-coffee value in one study; smoky, burnt and bitter notes rise in darker roasts.4,8", "Roasting past second crack typically involves charring and much smoke; roasting and roasted coffee also release carbon monoxide.4,13,14" ], "quickFacts": [ { "label": "When", "value": "After development, only in dark roasts" }, { "label": "Sound", "value": "Quieter than first crack, with cracks in faster succession" }, { "label": "Bean temperature", "value": "Not established in the peer-reviewed sources found" }, { "label": "First to second crack", "value": "2.0–6.0 min across 13 profiles of three coffees (one study, 5 kg drum)" }, { "label": "Visible sign", "value": "Oil droplets on the surface, more with darker roasts" }, { "label": "Roast names", "value": "Vienna, French, Italian – trade names, not standards" } ], "aka": [ "2nd crack" ], "linkTerms": [ "second crack", "second cracking", "oily beans", "Vienna roast", "French roast", "Italian roast" ], "related": [ "development-time", "first-crack", "roast-profile", "roasting", "roast-degree-and-colour", "acids-in-coffee", "maillard-reaction", "degassing", "freshness-and-storage", "roast-defects", "roast-loss", "caffeine-and-roast-degree", "drum-roaster", "preheating-and-charge", "drying-phase", "browning-phase", "coffee-oil" ], "contested": [ "What causes second crack: the Specialty Coffee Association (SCA) freshness handbook explains it as renewed gas pressure cracking the bean once it has turned brittle3; a 2019 thesis calls the cause uncertain, naming carbon dioxide pressure and fracturing of the cell matrix as possibilities.6 No peer-reviewed measurement of the mechanism was found.", "Pitch of second crack: earlier recording work cited in a 2019 thesis reported second crack much higher in frequency than first crack6; the thesis itself, recording an air roaster, found the peak frequencies of both cracks in the same band and noted that this differs significantly from the earlier work.6 The first crack entry describes a third, opposite result on a much larger gas-heated roaster." ], "corrections": [], "sources": [ { "id": "s1", "title": "The Roast – Creating the Beans' Signature", "authors": "Schenker, S.; Rothgeb, T.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 11, pp. 245–271", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00011-6", "doi": "10.1016/B978-0-12-803520-7.00011-6", "tier": "B", "quote": "The oil migration process proceeds faster in dark roasted beans due to the stronger driving force.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s2", "title": "A heat and mass transfer study of coffee bean roasting", "authors": "Fadai, N. T.; Melrose, J.; Please, C. P.; Schulman, A.; Van Gorder, R. A.", "year": 2017, "publisher": "International Journal of Heat and Mass Transfer 104, 787–799", "url": "https://ora.ox.ac.uk/objects/uuid:c2433f0b-fc40-4d7c-a207-5453349f6f91/files/m2e0c0e8fb121aa744f0a35396a18ff60", "doi": "10.1016/j.ijheatmasstransfer.2016.08.083", "tier": "A", "quote": "For industrially-roasted coffee, the roasting process generally ends between First and Second Crack.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s3", "title": "The Coffee Freshness Handbook (ISBN 978-0-9995807-2-1)", "authors": "Smrke, S.; Sage, E.; Wellinger, M.; Yeretzian, C.", "year": 2018, "publisher": "Specialty Coffee Association, Santa Ana", "url": "https://sca.coffee/store/p/the-coffee-freshness-handbook-ld8kh", "tier": "B", "tierNote": "SCA handbook; used here only for explanations, not for targets or requirements, so Tier B.", "quote": "If the roast is continued, this build up in pressure will subsequently cause the structure of the coffee to crack again, releasing even more CO2.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s4", "title": "The effect of roast profiles on the dynamics of titratable acidity during coffee roasting", "authors": "Anokye-Bempah, L.; Styczynski, T.; de Andrade Teixeira Fernandes, N.; Gervay-Hague, J.; Ristenpart, W. D.; Donis-González, I. R.", "year": 2024, "publisher": "Scientific Reports 14, 8237", "url": "https://pmc.ncbi.nlm.nih.gov/articles/PMC11002029/", "doi": "10.1038/s41598-024-57256-y", "tier": "A", "quote": "Here we investigate roast profiles of the same total duration but very different dynamics inside a 5-kg commercial drum roaster, and we show that the TA invariably peaks during first crack and then decays to its original value by second crack.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s5", "title": "Use of colour parameters for roasted coffee assessment", "authors": "Bicho, N. C.; Leitão, A. E.; Ramalho, J. C.; Lidon, F. C.", "year": 2012, "publisher": "Ciência e Tecnologia de Alimentos 32(3), 436–442", "url": "https://www.redalyc.org/pdf/3959/395940113004.pdf", "doi": "10.1590/S0101-20612012005000068", "tier": "A", "quote": "At 180-200 °C, with the disruption of the endosperm, bean cracking occurs", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s6", "title": "Coffee Roasting Process Monitoring with In Situ NIR Spectroscopy (MSc thesis)", "authors": "Yergenson, N.", "year": 2019, "publisher": "University of Idaho (master's thesis, chemical engineering)", "url": "https://objects.lib.uidaho.edu/etd/pdf/Yergenson_idaho_0089N_11534.pdf", "tier": "B", "quote": "Two sets of popping sounds occur during roasting, referred to as first crack and second crack.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s7", "title": "Coffee Bean Acoustics (news release on Wilson, P. S., Coffee roasting acoustics, J. Acoust. Soc. Am., 2014)", "authors": "Acoustical Society of America", "year": 2014, "publisher": "Newswise (news release)", "url": "https://www.newswise.com/articles/coffee-bean-acoustics", "tier": "B", "tierNote": "News release of the Acoustical Society of America on a peer-reviewed paper (J. Acoust. Soc. Am. 135(6), 2014, doi:10.1121/1.4874355); the paper itself could not be opened.", "quote": "Near the end of the roasting process, sounds known as \"first crack\" exhibit higher acoustic amplitudes than the \"second crack\" sounds that are emitted later.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s8", "title": "Common roasting defects in coffee: Aroma composition, sensory characterization and consumer perception", "authors": "Giacalone, D.; Degn, T. K.; Yang, N.; Liu, C.; Fisk, I.; Münchow, M.", "year": 2019, "publisher": "Food Quality and Preference 71, 463–474", "url": "https://doi.org/10.1016/j.foodqual.2018.03.009", "doi": "10.1016/j.foodqual.2018.03.009", "tier": "A", "quote": "The differences in acidity could be attributed to additional acid degradation associated with the longer roasting time for the Dark sample.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s9", "title": "The Chemistry of Roasting – Decoding Flavor Formation", "authors": "Poisson, L.; Blank, I.; Dunkel, A.; Hofmann, T.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 12, pp. 273–309", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00012-8", "doi": "10.1016/B978-0-12-803520-7.00012-8", "tier": "B", "quote": "Volatile phenols from the class of guaiacols such as 2-methoxyphenol (guaiacol), 4-ethyl-2-methoxyphenol (4-ethyl guaiacol), and 4-vinyl-2-methoxyphenol (4-vinyl guaiacol) provide the typical smoky, woody, and ashy characteristics of dark roasted coffees.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s10", "title": "Real-time monitoring of 4-vinylguaiacol, guaiacol and phenol during coffee roasting by resonant laser ionisation time-of-flight mass spectrometry", "authors": "Dorfner, R.; Ferge, T.; Kettrup, A.; Zimmermann, R.; Yeretzian, C.", "year": 2003, "publisher": "Journal of Agricultural and Food Chemistry 51(19), 5768–5773", "url": "https://digitalcollection.zhaw.ch/handle/11475/5424", "doi": "10.1021/jf0341767", "tier": "A", "quote": "A model is proposed, based on two connected reaction channels.", "abstractOnly": true, "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s11", "title": "Investigations on the Hot Air Roasting of Coffee Beans (Diss. ETH No. 13620)", "authors": "Schenker, S.", "year": 2000, "publisher": "ETH Zürich (doctoral thesis)", "url": "https://library.sweetmarias.com/wp-content/uploads/2020/08/roasting_hot_air_doctorate.pdf", "tier": "B", "quote": "Darker roasted beans tend to a more severe oil migration.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s12", "title": "Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee", "authors": "Smrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C.", "year": 2018, "publisher": "Journal of Agricultural and Food Chemistry 66(21), 5293–5300", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/12175/2/2017_Smrke-etal_Roasted-coffe-degassing-time-resolved-gravimetry.pdf", "doi": "10.1021/acs.jafc.7b03310", "tier": "A", "quote": "The darker the roast and the shorter the roast time (for fixed roast degree), the larger the amount of gas which is released and at a higher rate.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s13", "title": "Carbon monoxide emission rates from roasted whole bean and ground coffee", "authors": "LeBouf, R. F.; Aldridge, M.", "year": 2019, "publisher": "Journal of the Air & Waste Management Association 69(1), 89–96", "url": "https://stacks.cdc.gov/view/cdc/76991", "doi": "10.1080/10962247.2018.1515125", "tier": "A", "quote": "For ground coffee, medium-dark and dark roasts had significantly higher emissions than medium and light roasts.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s14", "title": "Evaluation of exposures and respiratory health at a coffee roasting and packaging facility (Health Hazard Evaluation Report 2016-0144-3305)", "authors": "Harvey, R. R.; Hawley, B.; Martin, S. B.", "year": 2018, "publisher": "National Institute for Occupational Safety and Health (NIOSH), CDC, U.S. Department of Health and Human Services", "url": "https://www.cdc.gov/niosh/hhe/reports/pdfs/2016-0144-3305.pdf", "tier": "A", "quote": "Carbon monoxide levels were higher when workers were performing tasks near the roasters and grinders.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s15", "title": "AP-42, Fifth Edition, Section 9.13.2: Coffee Roasting", "authors": "U.S. Environmental Protection Agency", "year": 1995, "publisher": "U.S. EPA, Compilation of Air Pollutant Emission Factors", "url": "https://www.epa.gov/sites/default/files/2020-10/documents/c9s13-2.pdf", "tier": "A", "quote": "Gaseous emissions from roasting operations are typically ducted to a thermal oxidizer or thermal catalytic oxidizer", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s16", "title": "We Consumers – Tastes, Rituals, and Waves", "authors": "Morris, J.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 19", "url": "https://www.sciencedirect.com/book/9780128035207/the-craft-and-science-of-coffee", "tier": "B", "quote": "requiring a “French roast,” i.e., a high roast, to counteract the bitterness through caramelization.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s17", "title": "Vienna Roast (Coffee Library glossary)", "authors": "Sweet Maria's", "publisher": "Sweet Maria's Coffee Library", "url": "https://library.sweetmarias.com/glossary/vienna-roast/", "tier": "C", "quote": "Vienna roast occurs at the beginning of second crack.", "accessed": "2026-10-17T00:00:00.000Z" } ], "markdown": "## What second crack is\n\nSecond crack is a second series of cracking sounds late in a roast. It comes after first crack and the development time that follows it.2,6 It happens only if the roast continues until the beans are dark.2 Industrially roasted coffee is generally taken out of the roaster between first and second crack.2\n\nRoasters use second crack as a landmark. According to a 2019 thesis, its start marks the transition to a dark roast.6 Coffee roasted past second crack is considered burnt.2,6 Even dark roasts typically end somewhere in the middle of second crack.6\n\nThis is the sixth step in the roast. **Previous step:** development time.\n\n## What breaks inside the bean\n\nDuring roasting, the bean forms gas, mostly carbon dioxide (CO₂), together with water vapour.1,3 The pressure makes the bean swell and, at first crack, crack for the first time.3 Late in the roast, gas formation rises steeply.3\n\nThe SCA freshness handbook explains second crack as follows. The bean has turned brittle, the gas cannot escape, and the pressure cracks the structure a second time, releasing more CO₂.3 A roasting textbook chapter describes the same kind of event: small structural breakdowns that release gas in sudden bursts, heard as cracking and popping.1\n\nThe cause is not settled. A 2019 thesis calls it uncertain and names two possibilities: gas pressure, or fracturing of the cell matrix itself.6 No peer-reviewed measurement of the mechanism was found for this entry (see Contested).\n\n## How it sounds\n\nFirst crack sounds like popcorn popping; second crack is a snapping sound.1,2 A news release on a 2014 acoustics study compares second crack with the crackle of puffed rice cereal.7\n\nTwo recording studies, reported in a news release and a thesis, found two differences. First crack was louder, and the cracks of second crack followed each other at a higher rate.6,7 In the 2019 thesis, second crack also had more pops.6 Whether second crack is higher in pitch differs between studies (see Contested).\n\n## Temperature: what is and is not known\n\nNo peer-reviewed study opened for this entry reports the bean temperature at which second crack starts. The peer-reviewed sources describe it only by sound and bean colour.2,4 A practitioner glossary gives a temperature for it, without naming the probe or roaster, so it is not repeated here.17\n\nAny temperature would carry the usual probe caveat. According to a roasting textbook chapter, a roaster's probe reads a mix of bean surface and hot air, specific to the machine, so readings from different roasters cannot be compared directly.1 The roast profiles entry explains why.\n\nWhat can be compared is time. In one study on a 5 kg drum, second crack was heard 2.0–6.0 min after first crack, depending on the profile and coffee.4\n\n
\n
\n
Time from first to second crack, and from the end of second crack to the drop. 13 profiles of three coffees, each roasted three times for 16 min, 5 kg drum.4
\n
\n\n## Oil on the surface\n\nAccording to a roasting textbook chapter, coffee oil sits in small oil bodies along the cell walls of the green bean.1 Roasting breaks these up and frees the oil.1 Gas pressure inside the bean then pushes it through tiny channels in the cell walls to the surface.1\n\nThe oil first appears as many small droplets. These can join and cover the whole bean with a shiny film.1 Oil migration runs faster in dark roasts, because the driving force is stronger.1,11 A colour study notes that the outer layer of the beans becomes more oily as roast intensity rises.5\n\nTiming differs with roast degree. In extremely dark roasts, oil may already appear in the last stages of roasting.1 A 2019 thesis notes that beans often turn oily soon after second crack starts.6 Otherwise the oil reaches the surface during storage, as the gas leaves the beans.1,11\n\n
\n
\n

A · At second crack

\n
    \n
  1. Gas-filled cells. Gas from the roast reactions builds pressure inside the bean.3
    2. \n
  2. Brittle structure. Late in the roast the bean is brittle, and gas cannot escape.3
    3. \n
  3. Mobilised oil. Roasting breaks up the oil bodies along the cell walls and frees the oil.1
    4. \n
  4. Crack and gas burst. Pressure cracks the structure again and more carbon dioxide escapes.3 The mechanism is not settled.6
    5. \n
  5. Oil to the surface. Gas pressure pushes oil through tiny channels in the cell walls; small droplets form, sooner in dark roasts.1,11
    6. \n
  6. Smoke. Roasting past second crack typically brings charring and much smoke.4
    7. \n
\n

B · After roasting

\n
    \n
  1. Oil film. The droplets can join into a shiny film over the whole bean.1
    2. \n
  2. Gas still leaving. Darker roasts release more gas, and faster.12 Oil migration and gas release are linked.3
    3. \n
\n
A dark-roasted bean cut across: at second crack (A) and after roasting (B). No sizes or values are implied. Schematic, not to scale.
\n
\n\n## Flavour and chemistry\n\nAcidity falls as the roast goes on. In a 5 kg drum, titratable acidity peaked during first crack and had fallen back to its green-coffee value by second crack.4 The acids in coffee entry covers which acids form and break down.\n\nPerceived bitterness rises with roast degree, according to a roasting textbook chapter.1 In one sensory study, a dark roast was rated more bitter than a normal one.8 The textbook chapter attributes a lingering harsh bitterness in dark roasts to phenylindanes, breakdown products of chlorogenic acid lactones.1\n\nSmoky, woody and ashy notes come from volatile phenols called guaiacols.9 They can form when breakdown products of chlorogenic acids degrade further.9 According to the roasting textbook chapter, most important aroma compounds, such as most pyrazines, decay late in the roast, while a few, such as guaiacol, keep forming.1\n\nThe same chapter says that in a roast continued into second crack, sugar caramelisation stops and roasted notes are added instead.1 The browning chemistry behind these changes is covered under the Maillard reaction.\n\n## Dark roast names\n\nAccording to a roasting textbook chapter, baristas use names such as “French roast”, while industrial roasters and scientists prefer to measure colour.1 The roast degree and colour entry shows how loosely names map to numbers.\n\nSecond crack is the reference point for several names. In practitioner usage, “Vienna” names a roast at the beginning of second crack.17 A coffee history chapter describes “French roast” as a high roast.16 The SCA freshness handbook equates a dark roast in one of its studies with an “Italian espresso roast”.3\n\n\n\n## Reference values\n\nValues reported in the sources. They describe particular coffees, roasters and methods; they are not targets.\n\n| Parameter | Value reported | Source |\n|---|---|---|\n| How second crack is identified | By ear: the second round of popping sounds | Anokye-Bempah et al.4 |\n| First crack to second crack | 2.0–6.0 min across 13 profiles of three coffees (each roasted three times; 16 min roasts, 5 kg drum) | Anokye-Bempah et al.4 |\n| Mean rate of rise, first to second crack | 2.13–5.52 °C per 30 s, same profiles | Anokye-Bempah et al.4 |\n| End of second crack to drop | 1.0–4.5 min, same profiles | Anokye-Bempah et al.4 |\n| Titratable acidity at second crack | 2.5–2.8 mL NaOH per 40 mL brew (three profiles, African washed coffee); green coffee 2.2 ± 0.3 | Anokye-Bempah et al.4 |\n| Bean temperature at second crack | Not established in the peer-reviewed sources found | – |\n| Acoustics, first versus second crack | First crack louder; second crack cracks at a higher rate | News release on Wilson 20147; Yergenson6 |\n| Dark roast defect versus normal roast (one arabica coffee) | Air at charge 220 versus 210 °C; development 4:45 versus 2:40 min; total 13:45 versus 11:25 min; Agtron 45.7 versus 74.4 | Giacalone et al.8 |\n| Carbon monoxide at roasters (one facility) | 3.1–14.5 ppm (real-time, at the roasters); two of three roaster operators with maxima of 203 and 267 ppm | NIOSH14 |\n| Carbon monoxide limits (US) | NIOSH REL 35 ppm (TWA), ceiling 200 ppm; OSHA PEL 50 ppm | NIOSH14 |\n\n## Physics of the second crack\n\n**Pressure in a brittle bean.** The SCA handbook ties first crack to pressure from water vapour and CO₂ and a glass transition around that time, when the cell material softens and the cells inflate.3 Later, gas formation increases exponentially; the bean turns brittle, the gas can no longer escape, and continued roasting cracks the structure again.3 In the roasting textbook chapter, the thick cell walls stretch rather than break, and only minor structural breakdowns release gas in microbursts, heard as cracking.1\n\n**What is missing.** Neither account rests on a direct measurement of second crack. A 2019 thesis lists CO₂ pressure and fracturing of the cell matrix as candidate causes.6 Even for first crack, a 2017 heat and mass transfer study states that no detailed physical explanation has been found.2 Estimates of the gas pressure inside the bean disagree; the roasting overview gives the details.\n\n**Structure and pyrolysis.** Structural carbohydrates of the intercellular matrix are degraded, partly by pyrolysis.2 According to the SCA handbook, pyrolysis forms CO₂ during the late, high-temperature stages, and CO₂ from pyrolysis appears only in darker roasts.3 According to the textbook chapter, the darker the roast, the more volume and porosity the bean develops, although swelling levels off at some point.1\n\n**Heat release.** According to the textbook chapter, at some point in the roast, the reactions become exothermic and the beans generate heat of their own.1 The chapter adds that the final stages approach the conditions of a combustion process, and a few seconds separate the intended roast degree from an over-roasted product.1 Where the switch to exothermic happens is disputed (see the roasting overview).\n\n## Acoustics\n\n**What was measured.** The news release on a 2014 study reports that first crack had higher acoustic amplitudes than second crack, and that the cracks in second crack came at a higher rate.7 A 2019 thesis using an air roaster confirmed both: a higher amplitude for first crack and a higher rate for second crack.6 The number of pops rose with roasting temperature for both cracks.6\n\n**Where results differ.** The thesis found the peak frequencies of both cracks in one band, unlike earlier work it cites, where second crack was much higher in frequency.6 Frequency results thus differ between studies (see Contested). The first crack entry compares the published recordings, including a much larger gas-heated roaster with the opposite pattern.\n\n**Use in practice.** The same thesis used the cracks as reference points: it predicted the start and end of first and second crack from in-line near-infrared spectra, and the model for the start of second crack fitted best; the model for its end did not meet the thesis's minimum for quantification.6\n\n## Chemistry near and after second crack\n\n**Acids.** Titratable acidity rose from the start of the roast, peaked during first crack and had decayed to its initial value by second crack, in every profile tested.4 At second crack, three profiles of the African washed coffee measured 2.5–2.8 mL NaOH per 40 mL, against 2.2 ± 0.3 mL for the green coffee.4 In a defect study, the dark roast was rated less acidic than the normal one; the authors attribute this to further acid breakdown during the longer roast.8\n\n**Phenols.** Chlorogenic acids break down to quinic acid and hydroxycinnamic acids, which can degrade further to volatile and non-volatile phenols.9 A 2003 real-time study followed 4-vinylguaiacol, guaiacol and phenol during roasting. It proposed two connected reaction channels; according to its abstract, a second channel opens only at higher bean temperatures and forms guaiacol and then phenol.10 The defect study reports that the dark roast had more of the phenolic compounds 4-ethyl-2-methoxyphenol and phenol than the normal roast.8 Robusta has more of these odorants than arabica, because it has more chlorogenic acids.9\n\n**Bitterness.** According to the roasting textbook chapter, caffeine contributes only part of coffee's bitterness; most bitter compounds form during roasting.1 Chlorogenic acid lactones are among the main contributors, and phenylindanes, formed from them, give dark roasts a lingering harsh bitterness.1 What happens to caffeine itself in dark roasts is covered under caffeine and roast degree.\n\n**Sensory profile of a dark roast.** In the 2019 defect study, one arabica coffee was roasted to a normal profile and to a dark one that ran about 2 min longer.8 A trained panel rated the dark roast significantly more burnt, bitter and tobacco-like, and less citrus, sweet, acidic in the aftertaste and complex.8 The paper does not state whether the dark roast reached second crack.\n\n
\n
\n
Mean trained-panel ratings for a normal roast (Agtron 74.4) and a dark roast (Agtron 45.7) of the same arabica coffee; the attributes shown differed significantly between the two.8
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\n\n## Smoke and carbon monoxide\n\nThis section describes what authorities and studies report. It does not give safety advice.\n\n**Smoke.** A 2024 study calls the phase after second crack “post second crack” and notes that it typically involves charring and blackening of the beans and much smoke production.4 The US Environmental Protection Agency (EPA) lists particulate matter, volatile organic compounds, organic acids and combustion products among the emissions of coffee roasting; the roaster is the main source of gaseous pollutants.15 Gaseous roaster emissions are typically ducted to a thermal or thermal catalytic oxidiser.15\n\n**Carbon monoxide.** Carbon monoxide is produced by reactions during roasting itself, and natural gas-fired roasters are also expected to emit it.14,15 In a National Institute for Occupational Safety and Health (NIOSH) evaluation of one roasting facility, carbon monoxide was higher when workers performed tasks near the roasters and grinders.14 Real-time levels at the roasters were 3.1–14.5 ppm, and two of three roaster operators had maxima of 203 and 267 ppm, above the NIOSH ceiling of 200 ppm.14 The NIOSH recommended exposure limit is 35 ppm and the Occupational Safety and Health Administration (OSHA) permissible exposure limit 50 ppm, both as time-weighted averages; none of the average area or personal levels in that evaluation exceeded them.14\n\n**After roasting.** Roasted coffee keeps releasing carbon monoxide. In a 2019 study of commercial coffees, roast degree did not significantly change emissions from whole beans, but ground medium-dark and dark roasts emitted significantly more than medium and light ones.13 The degassing entry describes these measurements and the authorities' recommendations in full.\n\n## How dark roasts age\n\nDark roasts change faster after roasting. In a gravimetric study, darker roasts released more gas and released it faster.12 According to the SCA handbook, degassing is linked to the migration of coffee oils to the surface, and an oily surface exposed to oxygen forms stale and rancid off-flavours.3 The handbook also describes rancidity as the result of lipid degradation, chemical oxidation or pyrolysis of fats.3\n\nThe oil itself is a sign of age as well as of roast degree. In cryo-scanning electron micrographs reproduced in the textbook chapter, a bean surface was smooth just after roasting and covered with small oil droplets after one day of storage.1 In earlier work reviewed in a doctoral thesis, fast-roasted coffees showed more oil sweating, which was regarded as a sensory risk.11 Packaging and storage are covered under freshness and storage.\n\n## Risks and defects\n\n- **Over-roasting.** Past second crack the beans typically char and blacken, with much smoke.4 Coffee roasted past second crack is considered burnt.2,6\n- **Loss of origin character.** Practitioners describe roast flavour beginning to cover the coffee's own character at the start of second crack17; a textbook chapter says the roasted notes gained in second crack are not inherent in the coffee.1 Peer-reviewed sensory work shows dark roasts rated less complex and less citrus.8\n- **Rancidity.** Oily surfaces exposed to oxygen form rancid off-flavours.3\n\n## Open questions\n\n- **Mechanism.** Whether second crack is driven by gas pressure, by fracture of a weakened cell matrix, or both, has not been measured in the sources found.3,6\n- **Temperature.** No peer-reviewed bean temperature for the onset of second crack was found; published research describes it by sound.4\n- **Acoustic signature.** Frequency results disagree between roasters and studies.6\n- **Sensory studies at second crack.** The dark-roast sensory data found do not state whether the roast reached second crack.8\n\n## Key studies\n\n
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Anokye-Bempah et al., 2024 – Acidity up to and past second crack. Sampled beans every minute of 16-minute roasts of three coffees on a 5 kg drum, with up to seven profiles (each roasted three times) that all ran past second crack. Titratable acidity peaked during first crack and was back at its starting value by second crack; the study also reports how long each phase lasted.

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Scientific Reports 14, 8237 · doi:10.1038/s41598-024-57256-y

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Giacalone et al., 2019 – Roasting defects. Roasted one arabica coffee to a normal profile and five defect profiles, including a dark roast at Agtron 45.7, and studied them by GC–MS, trained panel and consumers. The dark roast carried more phenolic compounds and was rated more burnt and bitter, less acidic and less complex.

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Food Quality and Preference 71, 463–474 · doi:10.1016/j.foodqual.2018.03.009

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Dorfner et al., 2003 – Phenols in real time. Followed 4-vinylguaiacol, guaiacol and phenol in real time during roasting by laser ionisation mass spectrometry. Proposes two linked reaction channels, the second opening only at higher bean temperatures.

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Journal of Agricultural and Food Chemistry 51(19), 5768–5773 · doi:10.1021/jf0341767

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Wilson, 2014 – Coffee roasting acoustics. Recorded and analysed the cracking sounds of coffee during roasting. As reported in the publisher's news release, first crack was louder and the cracks of second crack came at a higher rate.

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Journal of the Acoustical Society of America 135(6) · doi:10.1121/1.4874355

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Smrke et al., 2018 – Gravimetric degassing. Weighed roasted coffee continuously at 35 °C: whole beans of one arabica coffee from El Salvador roasted to nine profiles, and ground arabica and robusta. For whole beans, roast degree had a stronger effect than roast speed; darker roasts released more gas, and faster.

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Journal of Agricultural and Food Chemistry 66(21), 5293–5300 · doi:10.1021/acs.jafc.7b03310

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