{ "id": "endothermic-and-exothermic-phases", "url": "https://coffearium.org/entries/endothermic-and-exothermic-phases/", "license": "CC-BY-4.0", "title": "Endothermic and exothermic phases", "summary": "Why a roast first soaks up heat and later starts to release heat of its own – what calorimetry and models show, why nobody agrees where the switch happens, and what it means for heat control and cooling.", "domain": "roasting", "kind": "Concept", "stage": "roast", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-19T00:00:00.000Z", "updated": "2026-10-19T00:00:00.000Z", "added": "2026-10-19T00:00:00.000Z", "brief": [ "Early in the roast, heat goes mainly into warming the beans and evaporating water (endothermic); later the process becomes progressively more exothermic and needs much less heat input.1,2", "Where the switch happens is disputed; sources place it at first crack, before it, from 150 °C, near 200 °C or around 250 °C.1,13,7,6,9", "In a 2025 bean model, water evaporation affected bean temperature more than reaction heat; both terms were needed to fit the data.7", "Thermal analysis of dried, ground green coffee in argon showed mainly heat uptake between 100 and 245 °C and heat release only above 245 °C.10", "According to a roasting textbook chapter, the late, heat-releasing stage speeds up, so the roast must be stopped abruptly and cooled efficiently.1" ], "quickFacts": [ { "label": "Early roast", "value": "Net heat uptake – warming, evaporation, early reactions" }, { "label": "Late roast", "value": "Progressively more exothermic, much less heat input needed" }, { "label": "Onset reported", "value": "From about 150 °C to about 250 °C, or tied to first crack; disputed" }, { "label": "Reaction heat in models", "value": "232 kJ per kg dry matter (literature value used in two models)" }, { "label": "Measured by", "value": "Calorimetry (DSC), thermal analysis, fitted bean-temperature models" }, { "label": "Practical consequence", "value": "Heat input stepped down late; abrupt stop and fast cooling" } ], "aka": [ "Endothermic and exothermic roasting phases", "Exothermic phase of roasting" ], "linkTerms": [ "endothermic and exothermic phases", "switch to exothermic", "heat generation", "self-heating", "reaction heat", "heat of reaction" ], "related": [ "roasting", "first-crack", "second-crack", "drying-phase", "browning-phase", "development-time", "roast-profile", "rate-of-rise", "heat-transfer", "water-in-roasting", "cooling-and-quenching", "roast-defects", "maillard-reaction", "drum-roaster", "fluid-bed-roaster" ], "contested": [ "Where the roast turns exothermic: a roasting textbook chapter ties the switch to first crack1; a study of off-gas aroma compounds in a 15.6 kg drum roast placed it at 625 s, before first crack at about 730 s13; a 2025 model assumed reaction heat above 150 °C7; a heat-transfer model says exothermic reactions can begin near 200 °C6; a 2013 model study links exothermic reactions to roasting temperatures above 230 °C8; a 2009 model study puts them at about 250 °C9; thermal analysis of dried, ground green coffee found heat release only above 245 °C10; and calorimetry of six green coffees found a different switch temperature for each.11 The sources measure different things (bean core, modelled bean, ground sample, off-gas) under different conditions.", "Whether self-heating shows in roast curves: a roasting textbook chapter, citing a 1982 calorimetric study, states that the beans eventually start to generate heat on their own1; a doctoral thesis by the chapter's first author found that published bean temperature curves do not show a late rise that can be clearly attributed to exothermic reactions, and its own laboratory and industrial roasts showed no substantial additional rise either.4" ], "corrections": [ { "date": "2026-10-19T00:00:00.000Z", "change": "Removed the statement that 180–200 °C (start of reaction gases) is the range in which bean cracking is reported; the summary point on onset temperatures now cites the 2009 model study for about 250 °C instead of the 2013 study.", "reason": "Verification pass; Geiger et al. 2005 does not mention cracking, and the 250 °C value comes from Basile & Kikic 2009, not Alonso-Torres et al. 2013 (230 °C)." }, { "date": "2026-10-19T00:00:00.000Z", "change": "Strezov & Evans 2005 method corrected (silica tube; the dried sample showed no sign of occluded water, as the authors state); Basile & Kikic 2009 described as comparing, not fitting, a model with published bean data, with the 190/290 °C versus 180/300 °C inconsistency of the source noted; the thesis's own finding of no substantial late temperature rise added.", "reason": "Verification pass against the full texts." } ], "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": "At one point during roasting, the energy balance of chemical reactions becomes autocatalytic (exothermic).", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s2", "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": "Conversely, at the end of the coffee roasting process, there is a much lower need for heat input, as the process itself becomes progressively more exothermic.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s3", "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": "Typically, the roast master starts the roast with close to maximum power output from the burner (heat source), and later performs stepwise reductions in the power output to slow down the speed of browning reactions as the roast progresses.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s4", "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": "At first, chemical reactions are endothermic, whereas a number of authors state an exothermic", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s5", "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": "As illustrated in Figure 9, the required product temperature to start the chemical reactions was 180°C to 200°C.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s6", "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": "Once the bean's temperature reaches nearly 200◦C, exothermic reactions in the bean, such as Maillard reactions, can begin.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s7", "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": "Endothermic heat due to water evaporation influences more strongly the model response.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s8", "title": "Modeling and Validation of Heat and Mass Transfer in Individual Coffee Beans during the Coffee Roasting Process Using Computational Fluid Dynamics (CFD)", "authors": "Alonso-Torres, B.; Hernández-Pérez, J. A.; Sierra-Espinoza, F.; Schenker, S.; Yeretzian, C.", "year": 2013, "publisher": "CHIMIA 67(4), 291–294", "url": "https://chimia.ch/chimia/article/download/2013_291/4675/15360", "doi": "10.2533/chimia.2013.291", "tier": "A", "quote": "Such major chemical transformations have not yet been considered within the model.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s9", "title": "A Lumped Specific Heat Capacity Approach for Predicting the Non-stationary Thermal Profile of Coffee During Roasting", "authors": "Basile, M.; Kikic, I.", "year": 2009, "publisher": "Chemical and Biochemical Engineering Quarterly 23(2), 167–177", "url": "https://hrcak.srce.hr/en/38328", "tier": "A", "quote": "However, this is technologically irrelevant since the industrial roasting process is normally stopped well before this threshold.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s10", "title": "Thermal Analysis of the Reactions and Kinetics of Green Coffee During Roasting", "authors": "Strezov, V.; Evans, T. J.", "year": 2005, "publisher": "International Journal of Food Properties 8, 101–111", "url": "https://informahealthcare.com/doi/full/10.1081/JFP-200048060", "doi": "10.1081/JFP-200048060", "tier": "A", "quote": "The estimated heats were −60 kJ/kg for the first exothermic reaction and −20 kJ/kg for the second.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s11", "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": "2026-10-19T00:00:00.000Z" }, { "id": "s12", "title": "Estudo termoanalítico (TG, DTG e DSC) dos cafés in natura e processados", "authors": "Pereira, C. A.; Schnitzler, E.; Carvalho Filho, M. A. S.", "year": 2005, "publisher": "Publicatio UEPG: Ciências Exatas e da Terra, Ciências Agrárias e Engenharias 11(1), 61–66", "url": "https://revistas.uepg.br/index.php/exatas/article/download/847/730/2520", "tier": "A", "quote": "Pela curva DSC do café in natura pode-se observar uma endoterma larga em correspondência com a desidratação verificada nas curvas TG e DTG, seguida de duas exotermas também em correspondência com as perdas de massa observadas.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s13", "title": "Online monitoring of coffee roasting by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS): towards a real-time process control for a consistent roast profile", "authors": "Wieland, F.; Gloess, A. N.; Keller, M.; Wetzel, A.; Schenker, S.; Yeretzian, C.", "year": 2012, "publisher": "Analytical and Bioanalytical Chemistry 402(8), 2531–2543", "url": "https://digitalcollection.zhaw.ch/bitstream/11475/5554/2/2012_Flurin-etal_Online-monitoring-of-coffee-roasting-by-PTR-ToF-MS.pdf", "doi": "10.1007/s00216-011-5401-9", "tier": "A", "quote": "Roast time of 625 s marks the transition from the initial endothermic drying process of the green beans", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s14", "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": "A “flick” is a sudden increase in the RoR sometime after first crack, typically associated with the onset of exothermic reactions.", "accessed": "2026-10-19T00:00:00.000Z" }, { "id": "s15", "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": "Roasting can be described as a dry heating food process starting with a drying phase", "accessed": "2026-10-19T00:00:00.000Z" } ], "markdown": "## What the two words mean\n\nEndothermic means that heat is taken up; exothermic means that heat is released.1 In a roast, both kinds of process run side by side, and the balance between them shifts as the roast goes on.1,7\n\nAt the start, a large energy input is needed to evaporate water and to set chemical reactions going. A roasting textbook chapter calls this the endothermic phase.1 Towards the end, much less heat input is needed, because the process itself becomes progressively more exothermic.2\n\nA second chapter of the same textbook describes the roast in the same way: an endothermic drying phase, then an exothermic phase in which most flavour compounds form, then cooling.15 The glossary terms endothermic reaction and exothermic reaction give the short definitions.\n\n## Heat in, heat out\n\nDuring a roast, heat flows into the bean from hot air and hot surfaces. The heat transfer entry describes those paths. Inside the bean, the heat has three main uses or sources:\n\n- **Warming.** Heat raises the temperature of the solid bean and the water in it.2\n- **Evaporation.** Turning water into vapour takes up a large amount of heat. One model uses 2.3 million joules per kilogram of water.6 In a 2025 model, this evaporative cooling starts at 100 °C.7\n- **Reactions.** Some reactions take up heat and others release it. According to the textbook chapter, the beans eventually start to generate heat on their own.1\n\nEarly on, heating the water dominates, so the roast is primarily endothermic.2 Later, the process becomes progressively more exothermic, and much less heat input is needed.2 In a 2025 single-bean model, evaporation still had the larger effect on bean temperature than reaction heat.7\n\n
\n
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The heat balance of a roast in four steps: heat uptake early, a disputed transition, heat release late, and an abrupt stop.1,2,3,4,7,9,13 Schematic, not to scale; the arrows show direction, not amounts.
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\n\n## Where the balance turns\n\nThe sources do not agree on when the roast turns exothermic. A roasting textbook chapter ties the switch to first crack.1 A study of aroma compounds in the exhaust of a production drum roaster placed it at 625 s, before first crack at about 730 s.13\n\nModel studies give temperatures instead. One assumes reaction heat from 150 °C.7 Another says exothermic reactions can begin once the bean nears 200 °C.6 A 2013 study links them to roasting temperatures above 230 °C, and a 2009 study places them at about 250 °C.8,9\n\nLaboratory heat measurements disagree too. In one, ground green coffee mainly took up heat up to 245 °C.10 In another, six green coffees each switched at a different temperature.11 Part of the spread comes from method: the studies measure a whole bean, a model, a ground sample or the exhaust gas.\n\n
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Temperatures reported for the start of reaction gases and of heat-releasing reactions, by method. Values are not comparable one to one: they come from a bean core, models, a dried ground sample and different roasting temperatures.5,6,7,8,9,10
\n
\n\n## What it means at the roaster\n\n**Heat input goes down.** A 2020 study describes the usual practice: start near maximum burner output, then step it down as the roast goes on.3 The stated aim is to slow the browning reactions.3 A roasting textbook chapter adds that a roaster may adjust the flame at first crack, linking this to the switch to exothermic.1\n\n**The roast gains momentum.** According to the textbook chapter, the final stages speed up and gradually approach the conditions of a combustion process.1 A few seconds can then separate the intended roast degree from an over-roasted coffee.1\n\n**The stop must be abrupt.** For this reason, the chapter calls for an abrupt stop with efficient precooling or cooling.1 A doctoral thesis also states that the roast must be stopped by rapid cooling.4 The cooling and quenching entry describes how.\n\n**Fire.** According to the same chapter, beans can catch fire if the roast runs on uncontrolled.1 A 2009 model study notes that exothermic reactions near 250 °C lead the beans to burn, but that industrial roasts normally stop well before that point.9\n\n\n\n## Reference values\n\nValues reported in the sources. They come from different methods, samples and bases, and are not targets.\n\n| Parameter | Value reported | Method and basis | Source |\n|---|---|---|---|\n| Reaction CO₂ and water begin | 180–200 °C | Thermocouple in the bean core; laboratory fluid bed, 100 g batches | Geiger et al. 20055 |\n| Exothermic reactions can begin | Nearly 200 °C | Statement in a heat-and-mass-transfer model study | Fadai et al. 20176 |\n| Reaction heat switched on | Above 150 °C | Lumped single-bean model; threshold taken from a 2002 model | Bustos-Vanegas et al. 20257 |\n| Total reaction heat in models | 232 kJ per kg dry matter | Literature parameter used in two model studies (per kg dry matter in the 2025 paper; the 2013 paper gives kJ/kg) | Bustos-Vanegas et al. 20257; Alonso-Torres et al. 20138 |\n| Exothermic reactions occur | At roasting temperatures above 230 °C (roasting, not bean, temperature) | Single-bean CFD study; reaction heat not in its model | Alonso-Torres et al. 20138 |\n| Exothermic reactions, beans burn | About 250 °C | Lumped model compared with published bean data; better at 190 °C than at 290 °C air (values in the text; the figure captions give 180 and 300 °C) | Basile & Kikic 20099 |\n| Net heat uptake | 100–245 °C; about 100 kJ/kg | Thermal analysis, 2 g dried, ground green coffee, argon, 20 °C/min | Strezov & Evans 200510 |\n| Heat release | From 245 °C; about −60 and −20 kJ/kg in two reactions | Same study, up to 500 °C | Strezov & Evans 200510 |\n| Endothermic then exothermic, switch differs by coffee | Six green coffees, 20–250 °C | DSC, 10 mg cross-sections, nitrogen, 10 °C/min | Mutovkina & Bredikhin 202311 |\n| Transition endothermic to exothermic | 625 s (first crack about 730 s) | Off-gas aroma compounds (real-time PCA), one dark roast of 15.6 kg, gas-heated drum | Wieland et al. 201213 |\n| Latent heat of water in models | 2.3 × 10⁶ J/kg | Model parameter | Fadai et al. 20176 |\n| Water evaporated during roasting | 10.2 g (fast) and 11.4 g (slow) per 100 g green coffee | Exhaust gas analysis, laboratory fluid bed | Geiger et al. 20055 |\n\n## Thermodynamics of the roast\n\n**Three terms.** A single-bean energy balance has a heat-transfer term from the surroundings, an endothermic evaporation term proportional to the loss of moisture, and an exothermic reaction term.7 In a 2025 model, the reaction term was switched on above 150 °C. Its rate followed an Arrhenius law and stopped once the total heat of reaction was reached.7 That total was 232 kJ per kg dry matter, a value both this study and a 2013 study take from a 2002 model.7,8 The two papers print different values for the activation parameter of that model (5500 K and 500 K).7,8\n\n**Evaporation against reaction heat.** In the 2025 model, leaving out evaporation made the model overestimate bean temperature during the exothermic period from 150 °C. Leaving out reaction heat made it underestimate.7 Evaporation had the stronger effect, but both terms were needed for the best fit.7\n\n**Orders of magnitude.** In a fluid-bed study, 10.2–11.4 g water per 100 g green coffee evaporated during roasting.5 At 2.3 × 10⁶ J/kg6, that is about 235–262 kJ per kg green coffee (our calculation). This is the same order as the 232 kJ per kg dry matter of reaction heat used in the models.7 Part of the evaporated water is itself formed by reactions, so the comparison is rough.5\n\n**What the reactions are.** According to the textbook chapter, Maillard reaction, Strecker degradation, pyrolysis and caramelisation are among the main reactions of carbohydrates during roasting.1 The off-gas study links the exothermic phase to Maillard reactions forming coffee aroma compounds.13 The 2025 model describes the heat as coming from oxidation reactions inside the grain.7 No source opened for this entry measured the heat of single roasting reactions inside a bean.\n\n
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Heat quantities reported or derived, on their own bases: heat uptake and release in thermal analysis of dried, ground green coffee; reaction heat used in models; and heat to evaporate the water lost in two fluid-bed roasts (our calculation).5,6,7,10
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\n\n## Measuring heat release\n\n**Differential scanning calorimetry (DSC).** A DSC measures the heat flow into a small sample against an empty reference while both are heated at a set rate. A 2023 study heated 10 mg cross-sections of six green coffees from 20 to 250 °C at 10 °C/min under nitrogen.11 All showed an endothermic phase followed by an exothermic one, at a different temperature for each sample.11 The authors conclude that denser, moister coffees needed more heat.11 The paper's table and text label the same values differently, so we do not give its peak temperatures here.11\n\n**Thermal analysis of ground coffee.** A 2005 study heated 2 g of ground green coffee, dried under vacuum, in a silica tube under argon at 20 °C/min.10 It found mainly endothermic reactions between 100 and 245 °C, then exothermic decomposition with two reactions up to 430 °C.10 The heat taken up was estimated at 100 kJ/kg; the two exotherms released about 60 and 20 kJ/kg.10 Because the sample was dried first, the thermograms showed no sign of the bean's own water.10\n\n**Atmosphere matters.** In a 2005 thermoanalytical study in synthetic air, green coffee showed a broad endotherm from dehydration, followed by two exotherms.12 In freeze-dried samples, a strong exotherm above 360 °C was attributed to heat generated by the sample in an oxidising atmosphere.12 A roaster's air is neither pure air nor inert gas, so these results transfer only in part.\n\n**Calorimetry of self-heating.** The textbook chapter bases its statement on heat release on a 1982 calorimetric study of self-heating in coffee and chicory.1 That study could not be opened for this entry. A doctoral thesis lists earlier calorimetric and textbook values for the switch to exothermic, which disagree with one another.4\n\n**Fitting bean temperatures.** Model studies compare a heat balance with measured bean temperatures; reaction heat is either fitted as a term or invoked to explain where the model deviates.7,9 In a 2009 study, a lumped model matched published bean data better at 190 °C air than at 290 °C. The authors attribute the deviation to exothermic reactions at about 250 °C.9\n\n**Off-gas signals.** In a production drum, aroma compounds in the exhaust changed abruptly at 625 s and again at 730 s, with first crack.13 The authors read the first change as the transition to the exothermic phase. It was a change in the gas profile, not a heat measurement.13\n\n**Energy balance on a roaster.** No study opened for this entry measured reaction heat from an energy balance over a production roaster.\n\n## How models handle reaction heat\n\nModel studies treat reaction heat in three ways:\n\n- **Included as a source term.** The 2025 single-bean model adds reaction heat above 150 °C and fits the data best with both evaporation and reaction heat.7\n- **Left out.** A 2013 CFD study of single beans lists the reaction-heat parameters but states that the major chemical transformations were not yet in its model.8 A 2017 heat-and-mass-transfer study treated its reaction-energy term as small and did not use it in a simplified model.6\n- **Lumped into heat capacity.** A 2009 model folds evaporation and other thermal effects into one apparent specific heat that depends on temperature and initial moisture.9\n\n## Relation to first and second crack\n\n**First crack.** In a fluid-bed study, reaction gases rose significantly only from a bean core temperature of 180–200 °C.5 The first crack entry covers the cracking itself and the temperatures reported for it. A textbook chapter ties the switch to exothermic to first crack.1 The off-gas study placed the transition about 105 s before first crack (our calculation from 625 s and 730 s).13 No source opened measured heat release in a bean at the moment of first crack.\n\n**Rate of rise.** A 2024 study describes the practitioner term “flick”, a sudden increase in the rate of rise after first crack, as typically associated with the onset of exothermic reactions.14 In that study, the flick profile was made by changing the roaster's gas and airflow, so it does not separate burner heat from reaction heat.14 The thesis found that published temperature curves do not show a late rise clearly attributable to exothermic reactions, and its own laboratory and industrial roasts showed no substantial additional rise either.4\n\n**Second crack.** No source opened for this entry links second crack to a measured change in heat release. The strongest heat release in thermal analysis began at 245 °C.10 In one drum study, medium roasts ended at 191.1–204.1 °C (measuring point not stated).3\n\n## Risks\n\n- **Overshoot.** In the late, exothermic stage, a few seconds can turn the intended roast degree into an over-roasted product.1 See roast defects.\n- **Burning and fire.** Beans can catch fire in an uncontrolled roast, according to the textbook chapter1; a model study places burning at about 250 °C.9\n- **Late stop.** The roast must be stopped abruptly and cooled efficiently, or it runs on after the drop.1,4\n- **Copied set points.** Temperatures for the switch come from different methods and bases; none is a probe value for a given roaster.5,7,10\n\n## Open questions\n\n- **Onset of net heat release.** Reported positions range from 625 s in one drum roast to about 250 °C in a model (see Contested).13,9\n- **Size of reaction heat in a whole bean.** The models use one literature value7; the thermal analysis opened measured a dried, ground sample.10\n- **Self-heating in roast curves.** Whether the late roast shows a temperature rise from reactions alone is not settled.1,4\n- **Coffee to coffee.** One DSC study found the switch at different temperatures for six coffees11; how origin, processing and density shift it is not established.\n\n## Key studies\n\n
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Bustos-Vanegas et al., 2025 – Effect of heat generation. Single arabica beans were held in a gas-fired drum at 200–280 °C and their temperature fitted with a lumped model. The model included evaporation and a reaction-heat term from 150 °C; evaporation mattered more, but both were needed for the best fit.

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Frontiers in Food Science and Technology 5, 1603783 · doi:10.3389/frfst.2025.1603783

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Strezov & Evans, 2005 – Thermal analysis of green coffee. Dried, ground green coffee was heated under argon to 500 °C while heat flow was measured. Reactions were mainly endothermic from 100 to 245 °C, and two exothermic reactions followed at higher temperatures.

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International Journal of Food Properties 8(1), 101–111 · doi:10.1081/JFP-200048060

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Mutovkina & Bredikhin, 2023 – DSC of six green coffees. Cross-sections of six green coffees from four countries were scanned from 20 to 250 °C under nitrogen. All showed an endothermic phase followed by an exothermic one, at a different temperature for each coffee.

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Food Science and Technology (Campinas) 43, e119722 · doi:10.1590/fst.119722

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Wieland et al., 2012 – Off-gas monitoring of a drum roast. Aroma compounds in the exhaust of a 15.6 kg gas-heated drum roast were followed in real time. Two abrupt changes appeared, at 625 s and with first crack at 730 s; the authors read the first as the change from endothermic drying to the exothermic phase.

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Analytical and Bioanalytical Chemistry 402(8), 2531–2543 · doi:10.1007/s00216-011-5401-9

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