{ "id": "first-crack", "url": "https://coffearium.org/entries/first-crack/", "license": "CC-BY-4.0", "title": "First crack", "summary": "The first audible cracking of the beans during a roast – what breaks inside the bean, how it sounds, which temperatures are reported, whether heat is released, and why roasters time development and roast degree from 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": [ "First crack is the moment a popping sound is first heard during a roast; sensory studies time development from it.1", "Steam and gas pressure inside the bean make it crack and expel steam and other volatile compounds.1,6", "Bean cracking is reported at 180–200 °C, without saying whether bean or air temperature; a production probe's reading at that moment depends on the machine.4,2", "In a laboratory fluid bed, carbon dioxide and reaction water rose significantly only from a bean core temperature of 180–200 °C.3", "Whether first crack marks the switch to heat-releasing (exothermic) reactions is not settled; sources place it at first crack, near 200 °C, above 150 °C or differently for each coffee.2,5,10,12" ], "quickFacts": [ { "label": "How it is timed", "value": "By ear, when popping is first heard" }, { "label": "Temperature reported", "value": "180–200 °C (bean or air not stated)" }, { "label": "Cause", "value": "Steam and gas pressure; the bean fractures" }, { "label": "Sound", "value": "Louder, slower cracks than second crack (one study)" }, { "label": "Used for", "value": "Start of development time; a landmark on the roast log" }, { "label": "Mechanism", "value": "No detailed physical explanation established" } ], "aka": [ "1st crack", "First pop" ], "linkTerms": [ "first crack", "first cracking", "1st crack" ], "related": [ "roasting", "roast-profile", "preheating-and-charge", "drying-phase", "browning-phase", "development-time", "second-crack", "roast-degree-and-colour", "roast-defects", "roast-loss", "degassing", "maillard-reaction", "acids-in-coffee", "drum-roaster", "fluid-bed-roaster", "glass-rubber-transition", "porosity-and-cell-structure" ], "contested": [ "Exothermic at first crack: a roasting textbook chapter says first crack corresponds to the beans' switch from endothermic to exothermic2; another chapter of the same book describes a drying phase followed by a separate exothermic phase11; a heat-transfer model says exothermic reactions can begin at a bean temperature of nearly 200 °C5; a 2025 model study assumes exothermic heat above 150 °C10; and a calorimetry study of six green coffees found the switch at a different temperature for each.12 None of the sources opened measured heat release in a bean at the moment of first crack.", "Frequency content of first and second crack: in recordings from a small electric drum roaster, first crack carried more low-frequency energy and second crack more high-frequency energy14; on a much larger gas-heated roaster, first crack showed higher-frequency components than second crack.16 The first set of recordings is described in a patent application (the related 2014 journal article could not be opened for this entry); the second is a conference paper that does not state peer review.", "What first crack releases: one sensory paper describes it as a release of steam pressure6; a roasting textbook chapter describes a sudden microburst of a tiny quantity of gas2; an SCA handbook names both water vapour and carbon dioxide.8 No source opened measured the composition of the escaping gas at the moment of cracking." ], "corrections": [], "sources": [ { "id": "s1", "title": "Roasting Conditions and Coffee Flavor: A Multi-Study Empirical Investigation", "authors": "Münchow, M.; Alstrup, J.; Steen, I.; Giacalone, D.", "year": 2020, "publisher": "Beverages 6(2), 29", "url": "https://www.mdpi.com/2306-5710/6/2/29", "doi": "10.3390/beverages6020029", "tier": "A", "quote": "The first crack is an event between the onset of roasting and the termination of the roasting process where the accumulated steam pressure causes the beans to crack and expel steam along with other volatile compounds [18,29].", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s2", "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": "However, some minor structural break down and cracks occur during the final roasting stages, releasing a tiny quantity of gas in a sudden microburst and manifest in cracking and popping sounds.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s3", "title": "Carbon Dioxide Evolution and Moisture Evaporation During Roasting of Coffee Beans", "authors": "Geiger, R.; Perren, R.; Kuenzli, R.; Escher, F.", "year": 2005, "publisher": "Journal of Food Science 70(2), E124–E130", "url": "https://ift.onlinelibrary.wiley.com/doi/10.1111/j.1365-2621.2005.tb07084.x", "doi": "10.1111/j.1365-2621.2005.tb07084.x", "tier": "A", "quote": "Below these temperatures, no significant increase of chemical reaction water evaporation or carbon dioxide evolution was observed.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s4", "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": "s5", "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": "While a detailed physical explanation for First Crack has not yet been found a distinct popping sound can be heard, not unlike the sound of popping popcorn.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s6", "title": "The Effect of Roast Development Time Modulations on the Sensory Profile and Chemical Composition of the Coffee Brew as Measured by NMR and DHS-GC–MS", "authors": "Alstrup, J.; Petersen, M. A.; Larsen, F. H.; Münchow, M.", "year": 2020, "publisher": "Beverages 6(4), 70", "url": "https://www.mdpi.com/2306-5710/6/4/70", "doi": "10.3390/beverages6040070", "tier": "A", "quote": "This stage is defined as the time span from the popping sound of steam pressure release from the beans at ‘1st crack’ until the point of roast termination.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s7", "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": "s8", "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", "quote": "This force, caused by gas produced inside the structure of the bean, causes the coffee to expand in size and start cracking from the pressure buildup of both the water vapor and CO2.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s9", "title": "Evaluation of microstructural properties of coffee beans by synchrotron x-ray microtomography: a methodological approach", "authors": "Pittia, P.; Sacchetti, G.; Mancini, L.; Voltolini, M.; Sodini, N.; Tromba, G.; Zanini, F.", "year": 2011, "publisher": "Journal of Food Science 76(2), E222–E231", "url": "https://research.unite.it/handle/11575/16850", "tier": "A", "quote": "Roasting causes meaningful changes in the microstructure of the coffee bean tissue with the development in the entire bean of a porous structure with pores of different shape and size depending on the zone of the bean and cracks occurring mainly in the more external regions and between parenchyma and mucilage.", "abstractOnly": true, "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s10", "title": "Modeling and simulation of coffee bean heating during roasting: effect of heat generation", "authors": "Bustos-Vanegas, J. D.; Martins, M. A.; Corrêa, P. C.; Baptestini, F. M.; de Oliveira, G. H. H.", "year": 2025, "publisher": "Frontiers in Food Science and Technology 5, 1603783", "url": "https://www.frontiersin.org/journals/food-science-and-technology/articles/10.3389/frfst.2025.1603783/full", "doi": "10.3389/frfst.2025.1603783", "tier": "A", "quote": "the model overestimates the temperature during the exothermic reaction period, beginning at 150°C", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s11", "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": "A similar temperature profile applied on two roasters (laboratory scale fluidizing-bed roaster and traditional drum roaster) resulted in similar physical properties and aroma formation in the assessed coffees (Baggenstoss et al., 2008a).", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s12", "title": "Analysis of coffee thermophysical changes during roasting using differential scanning colorimetry", "authors": "Mutovkina, E. A.; Bredikhin, S. A.", "year": 2023, "publisher": "Food Science and Technology (Campinas) 43, e119722", "url": "https://new.scielo.br/j/cta/a/Ffv4BQ5nwkLLprXwNLvF8rP/?lang=en", "doi": "10.1590/fst.119722", "tier": "A", "quote": "Thermograms obtained for all samples (Figure 3) showed the following events: an initial endothermic phase followed by an exothermic phase.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s13", "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": "Bean expansion is caused by a very rapid pressure build-up due to rapid formation of water vapor and gas within the bean", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s14", "title": "Method and Apparatus for Controlling Coffee Bean Roasting (US 2016/0120211 A1)", "authors": "Wilson, P. S.; Wochner, M. S.", "year": 2016, "publisher": "United States Patent and Trademark Office (patent application)", "url": "https://patents.google.com/patent/US20160120211", "tier": "A/B", "tierNote": "A for how the apparatus is built; B for the acoustic measurements reported in its description, which are not peer-reviewed.", "quote": "The acoustic amplitudes were measured at 0.35 m range and scaled to 1 m assuming spherical spreading.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s15", "title": "Coffee Bean Roasting Acoustics (news release on Wilson, P. S., Coffee roasting acoustics, J. Acoust. Soc. Am., 2014)", "authors": "American Institute of Physics", "year": 2014, "publisher": "ScienceDaily (news release)", "url": "https://www.sciencedaily.com/releases/2014/05/140521162931.htm", "tier": "B", "quote": "Finally, the rate of cracks in the second crack chorus is higher than the rate in the first crack chorus.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s16", "title": "Control noise for coffee roasting", "authors": "Magrini, A.; Stallone, M.; Domenech, G.", "year": 2018, "publisher": "Euronoise 2018 Conference Proceedings, pp. 889–892", "url": "https://www.euronoise2018.eu/docs/papers/153_Euronoise2018.pdf", "tier": "B", "quote": "The second crack was not so evident and further measurements are needed to individuate it better.", "accessed": "2026-10-17T00:00:00.000Z" }, { "id": "s17", "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": "This roast defect has a temperature curve similar to the normal roast, but the roasting process was stopped about 4 min earlier, resulting in a shorter development time (Table 1).", "accessed": "2026-10-17T00:00:00.000Z" } ], "markdown": "## What first crack is\n\nFirst crack is the first audible cracking of the beans during a roast. In sensory studies it is timed by ear, as the moment a popping sound is first heard.1 The sound is often compared to popcorn popping.2,5\n\nIn a 2024 study, first crack ended the phase that began once the beans had turned from greenish yellow to yellowish brown.7 It is step 4 of 6 in Coffearium's roast steps. Previous step: the browning phase. Next step: development time.\n\nFirst crack is not a single pop but a series of cracks, which a news release on a 2014 acoustics study calls a “chorus”.15 According to a roasting textbook chapter, roasters notice at this point the first aromas of true coffee character and a smoother bean surface.2\n\n## Why the bean cracks\n\nA coffee bean is made of cells with thick walls. During roasting, water turns to steam inside the cells, and chemical reactions form carbon dioxide.1,8 The gas cannot leave quickly, so pressure builds up inside the bean.8,13\n\nAccording to a roasting textbook chapter, the cell walls stretch and the bean swells.2 At some point the structure gives way in places: small cracks open, release a little gas in a sudden burst, and this is what roasters hear.2 In images of roasted beans, cracks lie mainly in the outer regions of the bean.9\n\n
\n
\n

A · Before first crack

\n
    \n
  1. Cell walls. Thick walls of polysaccharides; depending on moisture and temperature they are rigid (glassy) or soft (rubbery).2
    2. \n
  2. Pressure in the cells. Water vapour and carbon dioxide formed in the bean cannot escape quickly and build pressure.8,13
    3. \n
  3. Swelling. Gas pressure drives the bean to expand while the walls hold against it; the swelling is steady, not a sudden burst.2
    4. \n
\n

B · At first crack

\n
    \n
  1. Fracture. The structure breaks in places; in imaged roasted beans, cracks lie mainly in the outer regions.2,9
    2. \n
  2. Release. Steam and other volatile compounds escape through the cracks.1,2
    3. \n
  3. Sound. The sudden release is heard as a crack or pop.2,5
    4. \n
\n
What happens inside a bean at first crack, as the sources cited describe it; no detailed physical explanation has been established.5 Schematic, not to scale.
\n
\n\nA complete physical explanation of first crack has not been found.5 The picture above is how the sources describe it, not a measured sequence.\n\n## What roasters hear and see\n\nRecordings show that first crack and second crack sound different. In one study, as reported in a news release, first crack was louder than second crack, and second crack had more cracks per unit of time.15 Second crack comes later, when the beans are much darker.5\n\nAccording to the textbook chapter, roasters rely on sight and smell during the roast. They watch colour and surface texture and smell the beans from the trier.2 A roasting textbook chapter recommends logging the time and temperature of first crack.2\n\n## At what temperature\n\nA study of roast colour reports bean cracking at 180–200 °C.4 It does not say whether this is the temperature of the beans or of the air.4\n\nIn a laboratory roaster with a probe inside a single bean, carbon dioxide and water from chemical reactions began to form in relevant amounts at a bean core temperature of 180–200 °C.3 This is the same range as the cracking temperature above.\n\nAccording to a roasting textbook chapter, production roasters usually cannot measure the bean core. Their probe sits in the bean mass and touches both beans and hot air, so it reads a mix of the two.2 The reading at first crack therefore depends on the machine and cannot simply be compared between roasters.2 The roast-profile entry explains the probes in detail.\n\n## Why roasters use it as a landmark\n\nBecause it can be heard, first crack serves as a marker on the roast log.1,2 Roasters divide the roast into the time to first crack and the time after it, the development time.1,6\n\nThe time after first crack matters for flavour. In eight sensory studies, roast colour predicted flavour best; at the same colour, development time mattered more than the time to first crack.1 Roast degree and colour and development time each have their own entry.\n\nAfter first crack, the bean is described as particularly sensitive to heat, because the escaping steam dries it out and browning moves faster.6 According to one textbook chapter, the senses become more crucial here, because the roaster must now decide when to drop the beans.2\n\n\n\n## Reference values\n\nValues reported in the sources. They describe particular coffees, machines and probes; they are not targets.\n\n| Parameter | Value reported | Source |\n|---|---|---|\n| Definition used in sensory studies | Time when a popping sound is first heard | Münchow et al.1 |\n| Bean cracking, temperature (bean or air not stated) | 180–200 °C | Bicho et al.4 |\n| Start of significant CO₂ and reaction-water release (bean core, laboratory fluid bed) | 180–200 °C | Geiger et al.3 |\n| Same study: roasting conditions | 100 g green coffee at 8.3 g/100 g (wet basis); air 260 °C for 170 s (HTST) or 228 °C for 720 s (LTLT) | Geiger et al.3 |\n| Same study: reaction water as share of all water evaporated | 41% (HTST), 36% (LTLT) | Geiger et al.3 |\n| Exothermic reactions can begin (heat-transfer model, bean temperature) | Nearly 200 °C | Fadai et al.5 |\n| Exothermic heat assumed in a drum model | Above 150 °C | Bustos-Vanegas et al.10 |\n| Time to first crack, eight sensory studies (1 kg drum; one 15 kg roaster) | 6:36–10:07 min in the sample table for studies 2–8 (the paper's text gives 7:21–10:07 min); see the table below | Münchow et al.1 |\n| Time to first crack, development-time study (1 kg drum) | About 570 s | Alstrup et al.6 |\n| Time to first crack, titratable-acidity study (5 kg drum, 16 min roasts) | About 8 min (fast start); 12 min (slow start) | Anokye-Bempah et al.7 |\n| Titratable acidity | Peaks during first crack in every profile tested | Anokye-Bempah et al.7 |\n| Earliest stop in a defect study | 10 s after first crack (“Light” roast) | Giacalone et al.17 |\n\n## Pressure, cell walls and fracture\n\n**Pressure.** Roasting forms a large amount of gas, mostly carbon dioxide, and much of it stays trapped in the bean; a roasting textbook chapter concludes that the trapped gas must cause high pressure inside the bean.2 A doctoral thesis on hot-air roasting traces bean expansion to a rapid pressure build-up from water vapour and gas formed inside the bean.13 The pressure values in these sources come from gas measurements and model calculations2,13; no direct measurement was found for this entry, so we give no figure here (see the roasting overview).\n\n**Cell walls.** The polysaccharides of the cell walls are glassy or rubbery depending on moisture and temperature. In principle the bean passes from glassy to rubbery and back to glassy during roasting, and it swells while rubbery.2 An SCA handbook places this glass transition around first crack, with the cells inflated by carbon dioxide.8 Swelling is continuous, not a sudden popcorn-like burst; the walls stretch rather than break.2\n\n**Fracture.** According to the same chapter, minor structural breakdowns and cracks occur during the final roasting stages and release a tiny quantity of gas in a sudden microburst, heard as cracking and popping.2 Synchrotron microtomography of roasted beans found cracks mainly in the outer regions of the bean.9 A heat-transfer study states plainly that a detailed physical explanation for first crack has not yet been found.5\n\n**Older descriptions.** A literature review in the thesis notes that earlier authors described bean expansion as progressive but including a “popping phase”, and that it is unclear whether that term referred only to the sounds.13\n\n## Water, carbon dioxide and mass loss\n\nIn a laboratory fluid bed, the exhaust air was analysed continuously for carbon dioxide and water vapour, and a fine thermocouple recorded the bean core temperature.3 Below a core temperature of 180–200 °C there was no significant increase in carbon dioxide or in water formed by chemical reactions.3 Reaction water made up 41% of all water evaporated in the fast roast and 36% in the slow roast.3\n\nThe two ranges coincide: bean cracking is reported at the same temperatures at which reaction water and carbon dioxide begin to form in relevant amounts.3,4 According to an SCA handbook, the Maillard reaction contributes most of the carbon dioxide up to and around first crack, and pyrolysis adds more only in darker roasts.8 Degassing after the roast is covered in its own entry.\n\nTwo sensory papers call first crack a release of steam pressure that expels steam with other volatile compounds.1,6 No source opened for this entry measured how much mass a bean loses at the moment of cracking. Roast loss over the whole roast is covered in its own entry.\n\n**Chemistry peaks around first crack.** In a 5 kg drum, titratable acidity peaked during first crack in every profile and coffee tested and fell back by second crack.7 The acids in coffee entry and the Maillard reaction entry describe what forms and breaks down around this point.\n\n## Heat release around first crack\n\nEarly in the roast, energy goes into heating the beans and evaporating water: the roast is endothermic.1,2 Later it becomes progressively more exothermic and needs much less heat input.1 Where the switch lies, and whether it coincides with first crack, is not agreed.\n\n- **At first crack.** A roasting textbook chapter says the first crack corresponds to the beans' switch from endothermic to exothermic, so a roaster may adjust the flame at that point.2\n- **A separate phase after drying.** A second chapter of the same book describes roasting as an endothermic drying phase followed by an exothermic phase in which most flavour components form.11\n- **Nearly 200 °C.** A heat-transfer model states that exothermic reactions such as Maillard reactions can begin once the bean reaches nearly 200 °C.5\n- **Above 150 °C.** A 2025 single-bean model of a drum roaster assumed exothermic heat above 150 °C, taken from earlier literature.10\n- **Different for each coffee.** Differential scanning calorimetry of six green coffees showed an endothermic phase followed by an exothermic one, but the change happened at a different temperature for each sample; the authors relate it to the beans' physical properties.12\n\n
\n
\n
Temperatures reported for bean cracking, for the start of carbon dioxide and reaction-water release, and for the onset of exothermic reactions. The sources measure or assume different things: bean core, bean model or an unstated basis.3,4,5,10
\n
\n\nNone of the sources opened measured heat release in a bean at the moment of first crack. In the sources, the cracks are described as structural breakdowns2, and the heat as coming from chemical reactions such as Maillard reactions.5 On the roast log, the rate of rise often changes around first crack; the roast-profile entry discusses the practitioner terms “crash” and “flick” and how little is known about them. In one 2024 study, a sudden increase in the rate of rise after first crack was described as typically associated with the onset of exothermic reactions.7\n\n## The sound of first crack\n\n**Two choruses.** A 2014 acoustics study recorded the cracks in a small electric drum roaster.14,16 As reported in the news release on the study, first crack had higher acoustic amplitudes than second crack, and second crack had a higher rate of cracks.15 The related patent application describes the same kind of recordings: first crack carried more low-frequency energy, and second crack a flatter spectrum, lower in amplitude, with its spectral peak at a much higher frequency.14\n\n**Not the same on every roaster.** On a much larger gas-heated roaster, a 2018 conference paper found first crack dominated by higher-frequency components and second crack by lower ones – the opposite pattern.16 Its authors found second crack hard to identify and called for further measurements.16 The two set-ups differed in size and heating: a small consumer electric drum roaster and a much larger gas-heated one.14,16\n\n**Automatic detection.** The patent application describes a roasting system with a microphone that records crack sounds, processes them and uses them to control the roast.14 The 2018 study, by an Italian university group working with two companies, recorded crack sounds on the larger roaster.16 No peer-reviewed study of automatic first-crack detection with a stated accuracy was found for this entry.\n\n## Variation between coffees and roasts\n\n**Time to first crack.** In eight sensory studies, airflow was constant and the charge temperature was the same within each study, except in Study 1.1 The times listed in the paper's Table 2 show how much the timing can vary.\n\n| Study | Coffee (washed arabica) | Roaster · charge air | Time to first crack (min) | Source |\n|---|---|---|---|---|\n| 2 | Colombia | 1 kg drum · 200 °C | 8:45–9:35 | Münchow et al.1 |\n| 3 | Colombia | 1 kg drum · 200 °C | 6:36–8:37 (varied by design) | Münchow et al.1 |\n| 4 | Colombia | 1 kg drum · 200 °C | 8:28–8:33 | Münchow et al.1 |\n| 5 | Ethiopia | 1 kg drum · 200 °C | 8:50–9:52 (varied by design) | Münchow et al.1 |\n| 6 | Ethiopia | 15 kg roaster · 160 °C | 8:25–9:25 | Münchow et al.1 |\n| 7 | Colombia | 1 kg drum · 200 °C | 9:19–10:07 | Münchow et al.1 |\n| 8 | Colombia | 1 kg drum · 200 °C | 8:52–9:49 | Münchow et al.1 |\n\nIn the defect study (Study 1, a Kenyan coffee), the charge air ranged from 135 to 275 °C. The “Scorched” roast, charged at 275 °C, reached first crack after about 6 min; the “Underdeveloped” roast, charged at 135 °C, after about 18 min.1,17 The times we derived from the defect paper (total roast time minus development time; our calculation) differ from the multi-study table by up to 80 s, so we give rounded values.1,17\n\n**Density, moisture, origin and processing.** According to a roasting textbook chapter, green bean density, moisture content, screen size and shape all factor into the roast plan.2 In a calorimetry study, six green coffees differing in density and moisture (basis not stated) switched from endothermic to exothermic at different temperatures, and the authors conclude that denser, moister samples needed more heat.12 No study opened for this entry measured how density, moisture, origin or processing method shift the timing or temperature of first crack itself. The sensory evidence on first crack and development comes from washed arabica only.1\n\n**Fast and slow roasts.** Fast roasting raises gas formation and makes the beans expand faster; at the same colour, fast-roasted beans have much greater volume and porosity.2 How this changes the cracking itself has not been measured in the sources found.\n\n## Fluid bed and drum\n\nAccording to a roasting textbook chapter, convection dominates heat transfer in a fluidised-bed roaster; in a drum roaster, a substantial share of heat may be transferred by conduction from the hot walls.2 The detailed measurements of carbon dioxide and reaction water around 180–200 °C come from a laboratory fluid bed, roasting fast (170 s) and slow (720 s).3 The sensory studies that time first crack by ear used drum roasters and one convection roaster.1,6,7\n\nAccording to a second chapter of the same textbook, a similar temperature profile applied in a laboratory fluid bed and a drum roaster gave coffees with similar physical properties and aroma formation.11 No study opened for this entry compared the sound or timing of first crack between fluid beds and drums. The fluid-bed roaster and drum roaster entries describe the machines.\n\n## Risks and defects\n\n- **Stopping too soon after first crack.** In a defect study, a “Light” roast followed the normal curve but was stopped 10 s after first crack. It was much lighter in colour and the sample consumers liked least.17 See roast defects.\n- **Losing heat during first crack.** One profile in a 2024 study stopped the roast energy during first crack, to mimic an undesired loss of gas flow; it was assessed for titratable acidity only.7\n- **Mistaking a probe value for first crack.** Probe readings at first crack are machine-specific2; a temperature copied from another roaster does not mark first crack on one's own machine.\n- **Running on after first crack.** According to a roasting textbook chapter, the roast speeds up in the later, exothermic stages, and a few seconds can separate the desired roast degree from an over-roasted product.2\n\n## Open questions\n\n- **Mechanism.** A detailed physical explanation of first crack has not been found.5 No direct measurement of pressure inside a bean was found for this entry; the values in the sources opened come from gas measurements and calculations.2,13\n- **Heat at first crack.** Whether first crack coincides with the switch to exothermic reactions is disputed (see Contested).2,5,10,12\n- **Acoustics.** The two recordings found disagree on which crack carries more high-frequency energy; one is described in a patent application, the other in a conference paper without stated peer review.14,16\n- **Green coffee properties.** How density, moisture, origin and processing shift first crack has not been measured in the sources found.12\n- **Bean versus probe.** Studies report the bean core temperature in laboratory roasters3, an unstated basis4, or a production probe that reads a mix of bean and air.2\n\n## Key studies\n\n
\n

Geiger et al., 2005 – Carbon dioxide and water during roasting. Roasted 100 g batches fast and slow in a laboratory fluid bed and analysed the exhaust air continuously, with a thermocouple in the bean core. Carbon dioxide and reaction water rose significantly only from a core temperature of 180–200 °C, the range in which beans are reported to crack.

\n

Journal of Food Science 70(2), E124–E130 · doi:10.1111/j.1365-2621.2005.tb07084.x

\n
\n\n
\n

Münchow et al., 2020 – Time to first crack versus development. Pools eight sensory studies of washed arabica, timing first crack by ear. Colour predicted flavour best; at constant colour, development time after first crack mattered more than the time to first crack.

\n

Beverages 6(2), 29 · doi:10.3390/beverages6020029

\n
\n\n
\n

Anokye-Bempah et al., 2024 – Acidity peaks at first crack. Sampled beans every minute of 16-minute roasts on a 5 kg drum, with up to seven profiles and three coffees. Titratable acidity always peaked during first crack and fell back by second crack.

\n

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

\n
\n\n
\n

Pittia et al., 2011 – Inside the roasted bean. Imaged green and roasted beans by synchrotron X-ray microtomography. Roasting created a porous structure throughout the bean, with cracks mainly in the outer regions.

\n

Journal of Food Science 76(2), E222–E231 · repository record

\n
\n\n
\n

Fadai et al., 2017 – A model of the roasting bean. Models heat and moisture transport, carbon dioxide formation and changing porosity in a single bean. Its background section notes that no detailed physical explanation of first crack has been found and places the start of exothermic reactions at nearly 200 °C.

\n

International Journal of Heat and Mass Transfer 104, 787–799 · doi:10.1016/j.ijheatmasstransfer.2016.08.083

\n
" }