{ "id": "roast-gases-and-co2", "url": "https://coffearium.org/entries/roast-gases-and-co2/", "license": "CC-BY-4.0", "title": "Roast gases and CO₂", "summary": "The gases a coffee bean forms while it roasts – mostly carbon dioxide, plus water vapour, carbon monoxide and aroma compounds – which reactions make them, how much leaves in the roaster and how much stays trapped.", "domain": "roasting", "kind": "Concept", "stage": "roast", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-20T00:00:00.000Z", "updated": "2026-10-20T00:00:00.000Z", "added": "2026-10-20T00:00:00.000Z", "brief": [ "Green coffee holds no roast gas. Roasting forms mainly carbon dioxide, with carbon monoxide, nitrogen and volatile organic compounds; water vapour leaves as well.2,3,12", "Carbon dioxide comes from Strecker degradation of amino acids and, in darker roasts, from pyrolysis of carbohydrates.2,4", "In a laboratory fluid bed, carbon dioxide formed in relevant amounts only from a bean core temperature of 180–200 °C.1", "Only part leaves in the roaster. In the same study, about a third of the carbon dioxide measured left during roasting and cooling, two thirds in 63 days of storage (our calculation).1", "Darker roasts form and hold more gas. Whether faster roasts form more in total is disputed.1,3,5,6" ], "quickFacts": [ { "label": "Main gas formed", "value": "Carbon dioxide; also CO, N₂ and volatile organic compounds" }, { "label": "Main reactions", "value": "Strecker degradation; pyrolysis of carbohydrates in darker roasts" }, { "label": "Onset (one study)", "value": "Bean core 180–200 °C" }, { "label": "Left in the roaster (one study)", "value": "0.4–0.5 g CO₂ per 100 g green coffee" }, { "label": "Left in 63 days of storage (same study)", "value": "0.83–0.99 g CO₂ per 100 g green coffee" }, { "label": "Pressure in the bean", "value": "Only calculated values in the sources opened" }, { "label": "After the roast", "value": "See degassing" } ], "aka": [ "Roasting gases", "Carbon dioxide in roasting", "CO₂ formation in roasting" ], "linkTerms": [ "roast gases", "roasting gases", "roast gas", "CO₂ formation", "carbon dioxide formation" ], "related": [ "degassing", "first-crack", "second-crack", "water-in-roasting", "roast-loss", "roaster-emissions", "packaging", "cooling-and-quenching", "roast-degree-and-colour", "maillard-reaction", "strecker-degradation", "chlorogenic-acids", "caramelisation", "coffee-oil", "porosity-and-cell-structure", "glass-rubber-transition" ], "contested": [ "Total gas from fast and slow roasts: a roasting textbook chapter states that, at the same roast degree, fast-roasted beans generate larger gas quantities.3 In a fluid-bed study, the fast roast released less carbon dioxide in the roaster and more in 63 days of storage; the authors read the totals as almost equal.1 A second study found no significant difference in residual carbon dioxide at the same roast degree.6 A weighing study found faster roasts releasing more gas after roasting in medium and dark roasts, but not in light roasts.5 The studies measure different things: gas in the exhaust, gas left in the beans, and gas released afterwards.", "Pressure inside the bean: a roasting textbook chapter and two doctoral theses cite calculated values, from gas measurements and models.3,9,10 The values differ between authors, and none is a direct measurement or a peer-reviewed value, so no figure is given here.", "Share of carbon monoxide: the degassing entry explains why no figure is given for the share of carbon monoxide in the gas from roasted coffee." ], "corrections": [ { "date": "2026-10-20T00:00:00.000Z", "change": "The SCA handbook is now reported as saying that Strecker degradation and pyrolysis contribute to carbon dioxide formation, not that they form most of it; the quick fact on bean pressure now says only calculated values were found, instead of \"never measured directly\".", "reason": "Verification pass; the handbook says the two reactions \"contribute to the formation of CO2\", and the sources opened report calculated values without stating that no measurement exists." } ], "sources": [ { "id": "s1", "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": "A substantial part of this quantity is trapped in the coffee beans and is released only during storage.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s2", "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": "Strecker degradation is a chemical reaction that occurs in parallel with the Maillard reaction and involves the reactions of amino acids, transforming them into aldehydes – of which a byproduct is the creation of CO2.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s3", "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 predominant gas formed upon roasting is carbon dioxide (CO2).", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s4", "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": "Basically, it is a deamination and oxidative decarboxylation of the amino acid resulting in the Strecker aldehyde", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s5", "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 profiles show that the roast degree has a stronger impact than roast speed on the total amount and dynamics of degassing of whole roasted beans.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s6", "title": "Effect of roasting conditions on carbon dioxide degassing behavior in coffee", "authors": "Wang, X.; Lim, L.-T.", "year": 2014, "publisher": "Food Research International 61, 144–151", "url": "https://www.sciencedirect.com/science/article/abs/pii/S0963996914000337", "doi": "10.1016/j.foodres.2014.01.027", "tier": "A", "quote": "Table 2 also shows that residual CO2 content for coffee beans processed to the same degree of roast, using either HTST or LTLT processes, were not significantly different (P>0.05).", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s7", "title": "A Kinetics and Modeling Study of Coffee Roasting Under Isothermal Conditions", "authors": "Wang, X.; Lim, L.-T.", "year": 2014, "publisher": "Food and Bioprocess Technology 7, 621–632", "url": "https://link.springer.com/doi/10.1007/s11947-013-1159-8", "doi": "10.1007/s11947-013-1159-8", "tier": "A", "quote": "However, a reversed correlation was observed above dark roast degree.", "abstractOnly": true, "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s8", "title": "Pore structure of coffee beans affected by roasting conditions", "authors": "Schenker, S.; Handschin, S.; Frey, B.; Perren, R.; Escher, F.", "year": 2000, "publisher": "Journal of Food Science 65(3), 452–457", "url": "https://agris.fao.org/search/en/records/65e012417c7033e84bf0075e", "doi": "10.1111/j.1365-2621.2000.tb16026.x", "tier": "A", "quote": "The roasting conditions were found to have a major impact on microstructure.", "abstractOnly": true, "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s9", "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": "The amount of gas development is dependent on the degree of roast.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s10", "title": "Coffee roasting and quenching technology – formation and stability of aroma compounds (Diss. ETH No. 17696)", "authors": "Baggenstoss, J.", "year": 2008, "publisher": "ETH Zürich (doctoral thesis)", "url": "https://doi.org/10.3929/ethz-a-005666902", "doi": "10.3929/ethz-a-005666902", "tier": "B", "quote": "However neither the mechanisms of CO2 entrapping and sorption nor the driving forces and mechanisms of degassing within coffee beans are fully understood.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s11", "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": "issues of CO2 production are neglected as there is currently insufficient experimental data to fit parameters.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s12", "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, 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": "Until 500 s, the intensities of the VOCs are low and hardly any of the typical coffee aroma compounds are observed.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s13", "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": "Because roasters are typically natural gas-fired, carbon monoxide (CO) and carbon dioxide (CO2) emissions are expected as a result of fuel combustion.", "accessed": "2026-10-20T00:00:00.000Z" }, { "id": "s14", "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": "Storage spaces may need to be ventilated to control CO concentrations to safe levels.", "accessed": "2026-10-20T00:00:00.000Z" } ], "markdown": "## What roast gases are\n\nGreen coffee contains no more carbon dioxide than normal air, according to the Specialty Coffee Association (SCA) freshness handbook.2 Roasting changes that. Heat drives chemical reactions that turn part of the bean into gas.3 A roasting textbook chapter names **carbon dioxide** (CO₂) as the main gas formed, with carbon monoxide and nitrogen as other important ones.3 Volatile organic compounds, the aroma molecules, leave the roaster too.12\n\n**Water vapour** also fills the bean. Part of it is the green coffee's own moisture, part is water formed by the same reactions.1 By weight, water is by far the largest thing leaving the roaster. In a 2005 laboratory study, 10.2–11.4 g of water left per 100 g of green coffee, against 0.4–0.5 g of carbon dioxide.1 Water in roasting has its own entry. This entry is about the other gases, above all carbon dioxide: how it forms, and how much of it the bean keeps.\n\n## Where the carbon dioxide comes from\n\nAccording to the SCA handbook, two reactions contribute to the formation of carbon dioxide.2\n\n- **Strecker degradation.** This reaction runs alongside the Maillard reaction. It turns amino acids into aldehydes, which are aroma compounds, and gives off carbon dioxide.2 A roasting chemistry chapter describes it as a deamination and oxidative decarboxylation of the amino acid.4 The amino acid loses its amino group and its acid group.4\n- **Pyrolysis.** Pyrolysis is breakdown by heat alone. Late in the roast, carbohydrates break down this way and release carbon dioxide.2\n\nOther sources are named as well. A 2005 study, citing a coffee textbook, lists the breakdown of chlorogenic acids and of sucrose and the decarboxylation of amino acids.1 A roasting chemistry chapter states that almost all free sugars are lost in roasting, giving water, carbon dioxide, colour, aroma and taste.4 How much each source contributes was not measured in any study opened for this entry.\n\n## When it forms\n\nGas formation is slow at first. According to the roasting chapter, in isothermal roasting it accelerates strongly in the second half of the roast.3 In the 2005 study, a fine probe sat in the core of a single bean. Carbon dioxide rose in relevant amounts only once the core reached 180–200 °C.1 First crack is reported in the same temperature range (see first crack).\n\nAccording to the SCA handbook, the Maillard route gives most carbon dioxide up to and around first crack. Pyrolysis adds carbon dioxide only in darker roasts.2 In the 2005 study, carbon dioxide in the exhaust rose sharply at the end of the fast roast. The authors suggested the roast was moving towards pyrolysis.1\n\n## Released or retained\n\nNot all the gas leaves in the roaster. According to the roasting chapter, part escapes during roasting, and a major part stays trapped in the beans.3 The 2005 study measured both parts. Carbon dioxide in the exhaust came to 0.4–0.5 g per 100 g of green coffee.1 Cooling added at most 0.1 g, as estimated.1 Sealed whole beans then released 0.83–0.99 g in 63 days of storage.1\n\nSo about a third left in the roaster and cooler and about two thirds afterwards. This is our calculation from the study's values, and it treats the 63-day total as all the gas formed.1 The trapped gas sits in pores, is bound to the bean's large molecules, or is dissolved in the oil and the remaining water.1,2 Its slow release after roasting is degassing, which has its own entry.\n\n
\n
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Where the carbon dioxide in a roast comes from and where it goes. Amounts per 100 g of green coffee from one laboratory fluid-bed study, fast and slow roast to the same colour.1,2,3,4,5,9 Schematic, not to scale.
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\n\n## Pressure, swelling and cracks\n\nTrapped gas pushes outwards. According to the roasting chapter, gas pressure is the main force that swells the bean, while the thick cell walls hold against it.3 Water vapour adds to the push.3 The walls stretch rather than break, so the bean swells steadily, not in one burst.3\n\nIn places the structure does give way. The roasting chapter describes small cracks that release a little gas in a sudden burst, heard as cracking and popping.3 The SCA handbook explains first crack as pressure from water vapour and carbon dioxide.2 It explains second crack as a renewed build-up once the bean has become brittle.2 No source opened for this entry measured the pressure directly; only calculated values are cited (see Contested).3,10\n\nThe same pressure moves oil. According to the roasting chapter, it pushes coffee oil through tiny channels in the cell walls to the surface.3 First crack, porosity and cell structure, and the glass–rubber transition each have their own entry.\n\n## Roast degree and roast speed\n\n**Darker roasts form more gas.** A doctoral thesis and the roasting chapter describe gas formation as rising with roast degree.3,9 In one study, carbon dioxide left in whole beans more than doubled from light to dark roast. It rose no further in very dark roasts.6 In a second study by the same group, the trend reversed above dark roast.7\n\n**Roast speed matters less.** In a 2018 weighing study, roast degree had a stronger effect than roast speed on the gas that whole beans released after roasting (see figure).5 Fast roasts released more in medium and dark roasts, but not in light roasts.5 Whether fast roasts also form more gas in total is disputed (see Contested).1,3,6\n\n
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Gas released by whole arabica beans after roasting, for three roast degrees at three roast speeds. Measured by weight loss over at least 400 h at 35 °C; lower Colorette values are darker.5 Measured points only; lines join them.
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\n\n**Fast roasts build a more open bean.** In a 2000 study, beans roasted at high temperature had greater volume, more pore volume and larger pores in the cell walls than beans roasted at low temperature to the same degree.8\n\n\n\n## Reference values\n\nValues reported in the sources. They describe particular coffees, roasters and methods; compare within a study.\n\n| Parameter | Value reported | Conditions | Source |\n|---|---|---|---|\n| Roast conditions, 2005 study | 100 g green coffee, 8.3 g/100 g water (wet basis); air 260 °C for 170 s (fast) or 228 °C for 720 s (slow); both to L* 22–23 | Electrically heated laboratory fluid bed | Geiger et al.1 |\n| Start of relevant CO₂ and reaction-water formation | Bean core 180–200 °C | Same study | Geiger et al.1 |\n| CO₂ in the exhaust during roasting | 0.4 g (fast); 0.50 g (slow) per 100 g green coffee | Same study, infrared gas analyser | Geiger et al.1 |\n| CO₂ during cooling | 0.1 g (fast); 0.0 g (slow) per 100 g green coffee | Estimated by extending the curves 20 s | Geiger et al.1 |\n| CO₂ released in storage | 0.99 g (fast); 0.83 g (slow) per 100 g green coffee | Whole beans, closed flasks, 63 days at 23 °C | Geiger et al.1 |\n| Share of measured CO₂ leaving in roaster and cooler | ≈34% (fast); ≈38% (slow) | Our calculation from the three rows above | Geiger et al.1 |\n| Water evaporated during roasting | 10.2 g (fast); 11.4 g (slow) per 100 g green coffee | Same study | Geiger et al.1 |\n| Roast loss explained by water, CO₂ and chaff | ≈96% (fast); ≈93% (slow) | Same study, allowing for the spread of the data | Geiger et al.1 |\n| Residual CO₂, whole beans | 6.29–6.70 mg/g (light); 11.04–11.51 (medium); 15.36–15.62 (dark); 15.11–15.97 (very dark) | Brazilian arabica, 230 °C and 250 °C; no significant difference between speeds | Wang & Lim6 |\n| Gas released by whole beans after roasting | Light 2.5–2.8 mg/g; medium 4.7–6.6; dark 8.5–11.9 | El Salvador arabica, 1 kg batch roaster, roasts of 4–5, 9–10 and 11–12 min; ≥400 h at 35 °C | Smrke et al.5 |\n| Volatile organic compounds in roaster off-gas | Low until 500 s; strong rise to the end of roasting (894 s); a change at about 730 s with first crack | 15.6 kg Guatemalan arabica, gas-heated drum | Wieland et al.12 |\n| Pressure in the bean | No direct measurement; calculated values only | See Contested | Schenker & Rothgeb3; Baggenstoss10 |\n\n## Formation chemistry\n\n**Strecker degradation.** A roasting chemistry chapter describes it as part of the Maillard reaction network. An amino acid loses its amino group and is oxidatively decarboxylated, giving a Strecker aldehyde.4 Carbon dioxide is a by-product.2 The aldehydes include malty 3-methylbutanal, potato-like methional and honey-like phenylacetaldehyde.4 Strecker degradation has its own entry.\n\n**Pyrolysis.** According to the SCA handbook, carbohydrates decompose by heat late in the roast, during caramelisation. This forms carbon dioxide alongside a large number of other compounds.2 In the handbook's account, this route matters only in darker roasts.2\n\n**Chlorogenic acids.** A doctoral thesis on hot-air roasting reviews an earlier model of bean expansion in which chlorogenic acids are key components.9 In that model, the acids decompose sharply near the glass transition temperature of the bean, with substantial carbon dioxide release.9 No study opened for this entry measured how much carbon dioxide comes from chlorogenic acids in a roasting bean. Chlorogenic acids have their own entry.\n\n**Reaction conditions.** A second doctoral thesis describes the roasting bean as a set of tiny closed reactors: thick cell walls and no spaces between cells.10 High temperature comes with considerable pressure, and reactions may run in the gas phase or on cell-wall surfaces.10 The thesis warns that results from model reactions in water must therefore be read with care.10\n\n**Timing.** In the 2005 study, carbon dioxide and reaction water were expressed per degree of bean core temperature, to compare the two roasts on one scale. Both rose only above a core of 180–200 °C.1 Over the whole roast, the slow roast released more carbon dioxide in the exhaust, because it lasted much longer.1\n\n## Retention mechanisms\n\nThe 2005 study lists the forms in which carbon dioxide stays in the bean. Part is bound to polar sites of the bean's polymers, part dissolved in the oil or in the remaining water.1 Another part may be trapped in collapsed amorphous zones.1\n\nA doctoral thesis summarises older work on where the gas sits. One earlier author concluded that most carbon dioxide is held in the fine pores of the tissue.10 In that view, gas in large cavities is probably lost during roasting through relatively large fissures.10 Other authors suggested that most carbon dioxide in ground coffee is held by sorption, and only a minor part in collapsed structure.10 The thesis states that neither the mechanisms of trapping and sorption nor the driving forces of degassing were fully understood.10\n\nSome gas may not leave dry coffee at all. The 2018 weighing study measured less gas than a method that extracts carbon dioxide with water.5 The authors suggest that part of the carbon dioxide is adsorbed in the coffee and released only when the coffee meets water.5 They call for studies of this “free” versus “bound” gas.5\n\nStructure decides how much can escape during the roast. One study found that roasting conditions have a major impact on the bean's microstructure.8 At the same roast degree, high-temperature roasting gave more pore volume and larger micropores.8 A thesis reviewing the literature reports that, despite the pressure, scanning electron microscopy showed no evidence of cell walls breaking.10 It attributes this, probably, to the walls turning from glassy to rubbery at high temperature.10\n\n## Gas in the roaster exhaust\n\n**Measuring it.** The 2005 study analysed the exhaust of an electrically heated laboratory fluid bed with an infrared gas analyser, for carbon dioxide and water vapour.1 Readings were corrected for the baseline of the hot air.1 With chaff weighed, the measured gases explained about 93–96% of the roast loss. The authors attribute the rest to abrasion, other gases and measurement inaccuracies.1 Roast loss has its own entry.\n\n**Aroma compounds.** In a 15.6 kg gas-heated drum, the off-gas was analysed online by mass spectrometry.12 Volatile organic compounds stayed low for the first 500 s, then most rose strongly until the end of the roast.12 A change in the profile at about 730 s coincided with first crack.12\n\n**Fuel gases.** In production roasters, the beans are not the only source. The United States Environmental Protection Agency (EPA) expects carbon monoxide and carbon dioxide in roaster exhaust from burning the fuel, because roasters are typically natural-gas fired.13 For a batch roaster, its uncontrolled carbon dioxide factor is 180 lb/ton (≈90 kg/t) of green coffee, rated D (limited test data).13 The 2005 fluid bed, heated electrically, measured 4–5 kg/t from the beans during roasting (our conversion).1 The two values come from different machines and methods. Roaster emissions has its own entry.\n\n## Carbon monoxide\n\nAccording to the SCA handbook, carbon monoxide forms during roasting together with carbon dioxide, in smaller amounts.2 Like carbon dioxide, part stays in the bean. A 2019 emission study showed that roasted coffee can rapidly emit carbon monoxide.14 Its authors recommend monitoring storage spaces and state that “Storage spaces may need to be ventilated to control CO concentrations to safe levels.”14 The degassing entry covers measured exposures and occupational limits; roaster emissions covers the exhaust.\n\n## Measurement methods\n\n| Method | What it measures | Used for | Source |\n|---|---|---|---|\n| Infrared gas analyser in the exhaust | CO₂ and water vapour leaving the roaster, online | Gas released during roasting | Geiger et al.1 |\n| Pressure rise in a closed flask | Gas released by the beans after roasting | Storage release over 63 days | Geiger et al.1 |\n| Chemical (sorbent) method | CO₂ left in the beans after roasting | Residual CO₂ by roast degree | Wang & Lim6 |\n| Gravimetric | Weight of all gas released by beans or grounds | Gas released after roasting, by roast degree and speed | Smrke et al.5 |\n| Online mass spectrometry (PTR-ToF-MS) | Volatile organic compounds in the off-gas | Aroma formation over the roast | Wieland et al.12 |\n\nEach method counts a different part of the gas: what leaves the roaster, what stays in the bean, or what leaves afterwards.1,5,6 The gravimetric study notes that earlier methods were either specific to carbon dioxide or assumed the released gas was pure carbon dioxide.5 Bases differ too: per 100 g of green coffee1 or per gram of roasted coffee.5,6\n\n## Open questions\n\n- **Formation kinetics.** A 2017 heat and mass transfer model left carbon dioxide production out of its calculations, because there was not enough experimental data to fit the parameters.11\n- **Contribution of each precursor.** No study opened for this entry measured how much carbon dioxide comes from amino acids, sugars, chlorogenic acids or polysaccharides in a roasting bean.\n- **Pressure in the bean.** Only calculated values exist; none was measured directly in the sources opened.3,10\n- **Trapping and sorption.** The mechanisms that hold carbon dioxide in the bean were not fully understood as of 2008.10 The split between free and bound gas needs further study.5\n- **Other coffees and roasters.** The 2005 authors noted that their measurements need to be extended to other coffees, origins and roasting conditions.1\n\n## Key studies\n\n
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Geiger et al., 2005 – Carbon dioxide during roasting and storage. Measured carbon dioxide and water vapour online in the exhaust of an electrically heated laboratory fluid bed, for a fast and a slow roast to the same colour, then gas release from the beans for 63 days. Carbon dioxide formed from a bean core of 180–200 °C; the larger part left after roasting, and the authors read the totals of both roasts as almost equal.

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

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Smrke et al., 2018 – Gas released by nine roast profiles. Weighed whole beans of one arabica coffee, roasted to three degrees at three speeds, for at least 400 h at 35 °C. Roast degree mattered more than roast speed; faster medium and dark roasts released more gas, and part of the carbon dioxide seemed to leave only with water.

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

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Wang & Lim, 2014 – Residual CO₂ by roast degree. Measured the carbon dioxide left in arabica beans roasted fast and slow in a fluid bed to four degrees. Residual carbon dioxide more than doubled from light to dark, did not rise further in very dark roasts, and did not differ significantly between the two speeds.

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Food Research International 61, 144–151 · ScienceDirect

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Schenker et al., 2000 – Pore structure and roast conditions. Compared beans roasted at high and low temperature to the same degree. High-temperature beans had greater volume, more pore volume and larger micropores in the cell walls, a pore structure that, the authors state, controls mass transfer during roasting and storage.

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Journal of Food Science 65(3), 452–457 · doi:10.1111/j.1365-2621.2000.tb16026.x

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Wieland et al., 2012 – Off-gas monitored online. Followed volatile organic compounds in the off-gas of a 15–20 kg drum roaster in real time by mass spectrometry. Aroma compounds stayed low for the first 500 s and then rose strongly, with a change in the profile at first crack.

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

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