{ "id": "degassing", "url": "https://coffearium.org/entries/degassing/", "license": "CC-BY-4.0", "title": "Degassing", "summary": "The gas that roasting traps inside coffee – mostly carbon dioxide, with some carbon monoxide – where it is held, how it escapes by flow and diffusion, what speeds it up, how it is measured, and what it means for packing, brewing and safety.", "domain": "roasting", "kind": "Process", "stage": "roast", "topics": [ "science" ], "topicSections": {}, "lang": "en", "status": "verified", "reviewer": "Coffearium verification pass", "lastVerified": "2026-10-12T00:00:00.000Z", "updated": "2026-10-12T00:00:00.000Z", "added": "2026-10-12T00:00:00.000Z", "brief": [ "Roasting forms gas inside the bean, mostly carbon dioxide with some carbon monoxide and nitrogen. Freshly roasted beans hold 1–2% of their weight as trapped gas; green coffee holds none.1,2,8,10", "Gas sits in large voids left by roasting and in porous cell walls. It leaves by pressure-driven flow from large pores and, much more slowly, by diffusion through fine ones.2,5", "Darker and faster roasts release more gas, faster. Whole beans degas over weeks, ground coffee over hours to days; grinding releases a large share at once, more when finer.1,4", "The gas shapes packaging (valves, nitrogen, resting before sealing), the bloom in filter coffee and the crema on espresso.2,5,12", "Roasted coffee also releases carbon monoxide. Occupational health studies found peaks above 200 ppm near grinders and open hoppers, and advise monitoring and, where needed, ventilating storage spaces.13,14,15" ], "quickFacts": [ { "label": "Main gas", "value": "Carbon dioxide (87% of the gas formed, in a classic textbook estimate); also CO and N₂" }, { "label": "Trapped gas in fresh beans", "value": "1–2% of weight" }, { "label": "Where it sits", "value": "Gas-filled cell voids (macropores) and porous cell walls (micropores); some adsorbed or dissolved in oil" }, { "label": "How it leaves", "value": "Pressure-driven flow and diffusion" }, { "label": "Whole beans", "value": "Weeks; a dark roast took more than 800 h at 25 °C to release about 90% of its CO₂" }, { "label": "Lost on grinding", "value": "26–30% (coarse) to 45–59% (fine) in one study; up to 75% at espresso fineness in another" }, { "label": "Effective diffusivity of CO₂", "value": "0.5–10 × 10⁻¹³ m²/s (roast and ground coffee)" }, { "label": "Carbon monoxide", "value": "NIOSH ceiling 200 ppm; peaks above it measured near grinders and open hoppers" }, { "label": "Not to be confused with", "value": "Resting of parchment before hulling" } ], "aka": [ "Off-gassing", "Degassing of roasted coffee" ], "linkTerms": [ "degassing", "degas", "off-gassing", "one-way valve", "degassing valve" ], "related": [ "freshness-and-storage", "roasting", "packaging", "roast-degree-and-colour", "roast-profile", "espresso-machine", "brew-methods", "extraction-yield-and-tds", "cupping", "moisture-and-water-activity", "roast-gases-and-co2", "porosity-and-cell-structure", "co2-and-the-bloom" ], "contested": [ "Share of carbon monoxide in the released gas: the Specialty Coffee Association (SCA) freshness handbook gives one figure, citing a 1987 textbook2, and a grinding chapter in a 2017 coffee science book gives a clearly higher range.9 Neither is a primary measurement opened for this entry, so no figure is given here.", "Gas lost during grinding: a grinding chapter says a large part of the trapped gas is released instantly when the larger pores are opened9; the gravimetric study measured up to 75% for freshly roasted coffee at espresso fineness1; and a study using a chemical method found 26–59%, rising from coarse to fine grinds.4 Grind size, roast and method differ between them.", "Total gas formed by fast and slow roasts: a roasting chapter states that, at the same roast degree, fast-roasted beans generate larger gas quantities than slow-roasted beans.8 In a fluid-bed study, the fast roast released less carbon dioxide during roasting but more during 63 days of storage; its measured total came out slightly higher, but the authors, assuming that release had levelled off by day 63, read the totals as almost equal and total formation as depending on roast degree only, not on roasting temperature.3 In a third study, residual CO₂ at the same lightness differed little between fast and slow roasts.4", "Amount of gas released by whole beans: published totals are spread over a large range, which the gravimetric study attributes to different methods, roasting profiles and degassing conditions.1 Values measured as weight loss, as pressure rise and as residual CO₂ are not directly comparable (see Reference values)." ], "corrections": [], "sources": [ { "id": "s1", "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": "Compared to the amount of degassing from whole beans, up to 75% of the trapped gas was released during and immediately after grinding (within 90 s, until the first measurement), for freshly roasted coffee.", "accessed": "2026-10-12T00:00:00.000Z" }, { "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": "There are two fundamental mechanisms that can explain how carbon dioxide is released from roasted coffee; one is release via hydrodynamic flow, and the other by diffusion – as represented in Figure 10.", "accessed": "2026-10-12T00:00:00.000Z" }, { "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": "Total carbon dioxide formation and evolution seem to be dependent on only the degree of roast and not on roasting temperature.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s4", "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": "As shown, substantial amounts of CO2 were lost during grinding, and the finer the grinds, the greater the amount of CO2 loss.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s5", "title": "The diffusion kinetics of carbon dioxide in fresh roasted and ground coffee", "authors": "Anderson, B. A.; Shimoni, E.; Liardon, R.; Labuza, T. P.", "year": 2003, "publisher": "Journal of Food Engineering 59(1), 71–78", "url": "https://experts.umn.edu/en/publications/the-diffusion-kinetics-of-carbon-dioxide-in-fresh-roasted-and-gro", "doi": "10.1016/S0260-8774(02)00432-6", "tier": "A", "quote": "It appears that pressure flow and Knudsen diffusion take place and are important during tempering.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s6", "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 resulting pore structure controls mass transfer phenomena during roasting and storage.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s7", "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 pore structure controls mass transfer phenomena during storage and may determine the high gas adsorption capacity and the gasdesorption properties", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s8", "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": "2026-10-12T00:00:00.000Z" }, { "id": "s9", "title": "The Grind – Particles and Particularities", "authors": "von Blittersdorff, M.; Klatt, C.", "year": 2017, "publisher": "In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 13, pp. 311–328", "url": "https://eld.bl.uk/catalog/018161895", "tier": "B", "quote": "For safety reasons it is recommended to ventilate the environment of a grinder or, in case of industrial grinding, to exhaust the gas from a grinder and from R&G coffee storage containers in a controlled way.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s10", "title": "Protecting the Flavors – Freshness as a Key to Quality (chapter 14)", "authors": "Yeretzian, C.; Blank, I.; Wyser, Y.", "year": 2017, "publisher": "The Craft and Science of Coffee (ed. B. Folmer), Academic Press, 329–353", "url": "https://doi.org/10.1016/B978-0-12-803520-7.00014-1", "doi": "10.1016/B978-0-12-803520-7.00014-1", "tier": "B", "quote": "Coffee that has been stored for some period of time will have less entrapped gases and consequently a lower rate of gas release.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s11", "title": "Influence of Water Quench Cooling on Degassing and Aroma Stability of Roasted Coffee", "authors": "Baggenstoss, J.; Poisson, L.; Luethi, R.; Perren, R.; Escher, F.", "year": 2007, "publisher": "Journal of Agricultural and Food Chemistry 55(16), 6685–6691", "url": "https://agris.fao.org/search/fr/records/65e011e00f3e94b9e5deef67", "doi": "10.1021/jf070338d", "tier": "A", "quote": "Degassing is faster in water-quenched coffees with higher moisture content, but pore size distribution in the different coffee samples did not correlate with degassing behavior.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s12", "title": "Neglected Food Bubbles: The Espresso Coffee Foam", "authors": "Illy, E.; Navarini, L.", "year": 2011, "publisher": "Food Biophysics 6(3), 335–348", "url": "https://link.springer.com/article/10.1007/s11483-011-9220-5", "doi": "10.1007/s11483-011-9220-5", "tier": "A", "quote": "In the case of espresso coffee, the gas phase is mainly the carbon dioxide generated during coffee roasting and entrapped within the cell structure", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s13", "title": "Carbon monoxide emission rates from roasted whole bean and ground coffee", "authors": "LeBouf, R. F.; Aldridge, M.", "year": 2019, "publisher": "Journal of the Air & Waste Management Association 69(1), 89–96", "url": "https://stacks.cdc.gov/view/cdc/76991", "doi": "10.1080/10962247.2018.1515125", "tier": "A", "quote": "Storage spaces may need to be ventilated to control CO concentrations to safe levels.", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s14", "title": "Evaluation of exposures and respiratory health at a coffee roasting and packaging facility (Health Hazard Evaluation Report 2016-0144-3305)", "authors": "Harvey, R. R.; Hawley, B.; Martin, S. B.", "year": 2018, "publisher": "National Institute for Occupational Safety and Health (NIOSH), CDC, U.S. Department of Health and Human Services", "url": "https://www.cdc.gov/niosh/hhe/reports/pdfs/2016-0144-3305.pdf", "tier": "A", "quote": "TWA personal CO measurements ranged from <0.1 ppm to 12.3 ppm and were below the NIOSH REL of 35 ppm for CO exposure", "accessed": "2026-10-12T00:00:00.000Z" }, { "id": "s15", "title": "Carbon Monoxide Exposure in Workplaces, Including Coffee Processing Facilities", "authors": "Cox-Ganser, J. M.; Cummings, K. J.; Hawley, B.", "year": 2017, "publisher": "American Journal of Respiratory and Critical Care Medicine 196(8), 1080–1081 (letter)", "url": "https://stacks.cdc.gov/view/cdc/210564", "tier": "A", "quote": "High CO levels were observed where coffee beans were stored or ground.", "accessed": "2026-10-12T00:00:00.000Z" } ], "markdown": "## What degassing is\n\nAccording to the Specialty Coffee Association (SCA) freshness handbook and a coffee science textbook, green coffee holds no trapped gas.2,10 A roasting chapter in the same textbook states that roasting forms large amounts of gas inside the bean.8 The chapter describes this gas, together with water vapour, as what makes the bean swell.8 In its account, part of the gas escapes in the roaster; a large part stays trapped, leaving slowly in storage and quickly on grinding.8 The SCA handbook calls that slow release degassing, or off-gassing.2\n\nA weighing study and the textbook both put the trapped gas in freshly roasted whole beans at 1–2% of their weight.1,10 The freshness entry covers what this means for staling. This entry looks at the gas itself: what it is, where it sits and how it gets out. It also covers what the gas means at the roastery, at the bar and for the air people breathe.\n\n## Which gases form\n\nThe SCA handbook and the roasting chapter name **carbon dioxide** as the main gas from roasting, and carbon monoxide and nitrogen as other important ones.2,8 A classic coffee technology textbook, as cited in a peer-reviewed roasting study, puts the share of carbon dioxide at 87% of the gases formed.3 The SCA handbook adds that the composition may vary with variety, roast degree and roast profile.2 The share of **carbon monoxide** is not settled (see Contested); its presence, and its importance for safety, are (see Carbon monoxide and safety).13,14\n\nAccording to the SCA handbook, carbon dioxide forms in two groups of roasting reactions.2 One breaks down amino acids; the other, late in the roast, breaks down carbohydrates by heat.2 The Deep dive gives the chemistry and the amounts measured.\n\n## Where the gas is held\n\nRoasting turns the dense green tissue into a porous one. According to the roasting chapter, cell contents are pushed against the intact walls, leaving a large gas-filled void in the centre of each cell.8 These inflated cells are called **macropores**. The SCA handbook describes the cell walls as porous too, with **micropores** far too small to image.2 One study and a related thesis describe the pore structure as controlling how substances move in and out of the bean in roasting and storage.6,7\n\nNot all of the gas is free: some is held in the solid material or dissolved in oil (see Pore structure and bound gas).3 The roasting chapter describes darker roasts as creating more volume and porosity, although swelling levels off at some point.8\n\n## How it gets out: flow and diffusion\n\nThe SCA handbook describes two mechanisms that release the gas.2\n\n- **Pressure-driven flow.** In the handbook's account, gas held at elevated pressure in the macropores flows out through small openings, from high to low pressure.2 This flow depends only on the total pressure, not on which gas surrounds the coffee.2\n- **Diffusion.** Diffusion is the slow spread of molecules from where they are many to where they are few. In the handbook's account, carbon dioxide held in the cell walls or dissolved in oil diffuses through the fine pores to the surface.2 Diffusion is much slower than flow.2 It depends on the carbon dioxide outside the coffee, not on the total pressure.2 So a nitrogen atmosphere, even at high pressure, does not stop it.2\n\nRelease measured in individual studies is discussed under Release kinetics. The SCA handbook noted that it was not fully understood which mechanisms were responsible.2\n\n
\n
\n

A · Whole bean

\n
    \n
  1. Macropores. Roasting leaves a large gas-filled void in each cell; the cell walls stay intact.8
    2. \n
  2. Porous cell walls. Micropores too small to image; carbon dioxide is also held on the solid material (adsorbed) and dissolved in the oil.2,3
    3. \n
  3. Pressure-driven flow. Gas at elevated pressure flows out of the macropores through small openings; only total pressure matters.2
    4. \n
  4. Diffusion. Molecules migrate through the fine pores to the surface – much more slowly, and driven only by the carbon dioxide outside.2,5
    5. \n
\n

B · Ground coffee

\n
    \n
  1. Opened macropores. Grinding destroys or damages large pores, and their gas escapes at once.1,2
    2. \n
  2. Short paths. Small particles have a larger surface and shorter diffusion paths, so the rest leaves faster.2,9
    3. \n
\n
Where the gas sits in a roasted bean and how it leaves, before and after grinding, as described in the sources cited. Schematic, not to scale.
\n
\n\n## What speeds it up or slows it down\n\n- **Roast degree.** Darker roasts hold and release more gas, and release it faster.1,4 In one study, whole arabica beans held about 6 mg/g of residual carbon dioxide in light roasts and about 15 mg/g in dark roasts.4 Very dark roasts held no more than dark ones.4 In the weighing study, ground arabica and robusta coffee released more gas from light to medium roast, but not from medium to dark.1\n- **Roast speed.** In the weighing study, faster roasts released gas faster; in medium and dark roasts they also released more.1 A study using a fluid-bed roaster, which roasts beans in a stream of hot air, found significantly faster degassing after high-temperature, short-time roasting.4 Whether fast roasts also form more gas in total is disputed (see Contested).3,8\n- **Grind size.** In one study, the finer the grind, the faster the degassing.4 According to the SCA handbook, grinding destroys or damages the large pores and shortens the paths for diffusion.2\n- **Temperature and moisture.** According to a grinding chapter in the same textbook, both make molecules diffuse faster and so speed up degassing, along with aroma loss.9 The SCA handbook states that freezing strongly slows degassing but does not stop it.2 In one study, water-quenched coffees with higher moisture content degassed faster than air-cooled ones.11 Their pore size distribution did not explain the difference.11\n- **Arabica and robusta.** In the weighing study, ground robusta released up to 3.9 mg/g after grinding at espresso fineness, and ground arabica up to 3.0 mg/g.1 No study opened for this entry compared whole beans of the two species.\n- **Origin and processing.** The SCA handbook reports whole-bean coffees of different origins and processes degassing at clearly different rates at the same roast degree.2 It suggests lower green coffee density as one possible reason for the fastest one.2\n\n## Whole beans versus ground\n\nAccording to the SCA handbook, whole beans release their gas over weeks.2 In the weighing study at 35 °C, whole beans were still releasing gas after 400 hours.1 A light roast measured for 800 hours had lost 3.4 mg/g and was still releasing.1 In another study at 25 °C, a dark, fast-roasted coffee took more than 800 hours to release about 14 mg/g.4 That was about 90% of its residual carbon dioxide.4\n\nGrinding changes the timescale. Part of the gas escapes during grinding itself, and more of it the finer the grind (see figure).1,4 In the weighing study, darker roasts lost a larger share on grinding than medium roasts.1 The authors suggest that more of their gas sits in large pores that grinding destroys.1 The rest then leaves within hours to days.4 In that second study at 25 °C, release from a fine grind levelled off within 50 hours.4 Coarse and medium grinds were still degassing slowly at the end of the test.4\n\n
\n
\n
Share of the trapped carbon dioxide lost during grinding. Brazilian arabica, medium and dark roasts, by grind size (mean particle radius), calculated from residual CO₂ in ground versus whole coffee.4 Freshly roasted coffee at espresso fineness, measured by weight loss.1 Methods differ; compare within a study.
\n
\n\nThe SCA handbook reports a storage result that does not fit a simple picture. Beans stored whole for several weeks and then ground released more gas than the whole beans could still release.2 The handbook offers two explanations: gas pockets that only grinding opens, or a very slow release mechanism not seen in whole beans.2\n\n## Packing and degassing in practice\n\n**Why bags have valves.** According to the SCA handbook, coffee sealed airtight without a valve visibly inflates and can burst.2 The handbook and the roasting chapter describe valve bags as letting the gas out instead.2,8 In the handbook's account, total pressure acts only on flow from the large pores; diffusion depends on the carbon dioxide outside the coffee.2 The handbook adds that the released gas pushes oxygen out of the bag, which slows oxidation.2\n\n**Packing before or after degassing.** One study lists three ways manufacturers handle the gas: vacuum packing so the pack does not billow, a special film that lets carbon dioxide escape, or tempering.5 Tempering means holding the coffee before packing to release as much gas as possible.5 According to the roasting chapter, industrial lines use tempering silos for beans and degassing silos and machinery for ground coffee.8 This avoids overpressure in later steps or in sealed packs such as capsules.8\n\nThe grinding chapter states that ground coffee for sealed containers, from which no gas can escape, must be fully degassed.9 In its account, a barista grinding small amounts does not need to rest the grounds.9 The SCA handbook uses capsules to show the trade-off: too little degassing builds pressure that can deform or burst them, too much costs aroma.2 The freshness entry summarises cupping trials on resting before bagging and on nitrogen flushing.\n\n**Nitrogen flushing** replaces the oxygen in the pack.2 According to the SCA handbook, it does not stop degassing, because carbon dioxide still diffuses out into a nitrogen atmosphere.2\n\n**Swollen bags** are a sign of gas still leaving the coffee. In the SCA handbook's reading, a swollen sealed bag or a deformed capsule means the coffee was packed before enough gas had left.2\n\n## In the cup\n\nAccording to the SCA handbook, the gas left in the grounds is released the moment water reaches them. This raises resistance to water flow and changes contact between water and coffee.2 Espresso settings are adjusted to the coffee's age for that reason.2 In filter brewing, the same release shows as the **bloom**.2 On espresso, the gas forms the **crema**, the foam on top of the shot.12 A review of espresso foam describes its gas phase as mainly carbon dioxide generated during roasting and trapped in the cell structure.12\n\nIn a demonstration reported in the SCA handbook, even fully degassed coffee, stored for a long time, still formed some crema.2 The handbook attributes this to carbon dioxide formed when coffee acids neutralise bicarbonate in the water, and to carbon dioxide already present in the water.2 The brew-methods and espresso machine entries cover brewing.\n\n\n\n## Formation chemistry and amounts\n\nThe SCA handbook names the reactions that form carbon dioxide: Strecker degradation, which runs alongside the Maillard reaction and turns amino acids into aldehydes and carbon dioxide, and pyrolysis, the thermal breakdown of carbohydrates late in the roast.2 According to the handbook, the Maillard route contributes mostly up to and around first crack; pyrolysis adds carbon dioxide only in darker roasts.2 In one laboratory fluid-bed study, reactions began to produce carbon dioxide and water in relevant amounts at 180–200 °C bean temperature.3 According to the roasting chapter, gas formation in isothermal roasting is slow at the start and accelerates strongly in the second half of the roast.8\n\n**How much forms.** The same fluid-bed study measured carbon dioxide online in the exhaust and then in closed flasks. Per 100 g of green coffee, 0.4–0.5 g of carbon dioxide left during roasting and a further 0.83–0.99 g during 63 days of storage at 23 °C.3 A further 0.1 g or less was estimated for cooling.3 The fast roast lost less carbon dioxide in the roaster and more in storage; assuming that release had levelled off by day 63, the authors read the totals as almost equal.3\n\n## Pore structure and bound gas\n\nThe SCA handbook notes that micropores in the cell walls can be measured only indirectly, by mercury porosimetry.2 Roast conditions change the pore structure. In one study, at the same roast degree, high-temperature roasted beans had greater bean volume, greater pore volume and larger micropores in the cell walls than low-temperature roasted beans.6\n\nNot all of the carbon dioxide is free gas. The roasting-and-storage study above describes part of it as bound to polar sites of the bean's polymers or dissolved in the coffee oil or in the residual moisture; part may be held in collapsed amorphous zones of the bean.3 According to the SCA handbook, carbon dioxide dissolved in oil can reach far higher concentrations than the volume of the coffee could hold as gas.2\n\n## Release kinetics\n\nThe SCA handbook lists several kinds of diffusion as involved, including Knudsen and surface diffusion.2 In one study, carbon dioxide release from roast and ground coffee followed Fick's law for unsteady diffusion with one or two effective diffusivities, and the authors concluded that pressure flow and Knudsen diffusion both take place.5 In whole beans, the gravimetric (weighing) study found the degassing of a fast-roasted dark coffee best described by two first-order processes: a fast one that dominates early but carries a small share of the gas, and a slow one that carries most of it.1 The authors attribute the difference between roasts probably to porosity, and suggest that surface diffusion and diffusion of carbon dioxide dissolved in coffee oils are likely the dominant processes.1 Release rates measured in the gravimetric study reached up to 40 mg/h for 12 g of freshly ground coffee, against up to 2 mg/h for 15 g of whole beans.1\n\n## Measuring degassing\n\nThe SCA handbook groups the methods as below.2 According to the gravimetric study, each measures something slightly different, which is one reason published amounts are spread over a large range.1\n\n| Method | What it measures | Limits | Used in |\n|---|---|---|---|\n| Chemical | Carbon dioxide driven out of the sample and captured on a sorbent; the sorbent's weight gain gives the residual CO₂ | Complex and slow; no kinetics unless repeated at different storage times2 | Residual CO₂ by roast and grind4 |\n| Volumetric | Volume change of a sealed container as gas is released (eudiometer) | Simple to build, but not accurate2 | – |\n| Pressure increase | Pressure and temperature in a closed rigid container; mimics a capsule | The most common method; treats the released gas as one gas1,2 | Storage release in closed flasks3 |\n| Infrared | CO₂ concentration in the headspace by its infrared absorbance; specific for CO₂ | Precise, but needs expensive custom-built equipment2 | Degassing kinetics, whole and ground4 |\n| Gravimetric | Weight loss of a sample in a container with a capillary outlet; mimics a valve bag | Measures all gas lost, not only CO₂; needs very stable balances, constant temperature and low humidity1,2 | Kinetics by roast degree, speed and grind1 |\n\nIn the gravimetric set-up, 15 g of whole beans or 12 g of ground coffee sat in a basket in a container with a stainless-steel capillary outlet, on a semi-micro balance in an oven at 35 °C, with silica gel to buffer the release or uptake of water and volatiles.1 The authors note that earlier methods were either specific to carbon dioxide or assumed that the released gas is pure carbon dioxide.1\n\nTwo cautions apply when comparing numbers. First, the basis: some studies report per gram of roasted coffee1,4, others per 100 g of green coffee.3 Second, curve fitting: the gravimetric study fitted its curves with a Weibull model, but for a light roast measured for 800 hours instead of 400 the fit gave quite different parameters, and the authors warn that the model does not allow correct extrapolation of the total amount.1\n\n## Residual CO₂ by roast degree\n\n
\n
\n
Residual carbon dioxide in whole arabica beans by roast lightness (L*; lower = darker), for a slower roast at 230 °C and a faster roast at 250 °C in a fluid-bed roaster, measured by the chemical method.4 Measured points only; lines join them.
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\n\nIn this study, residual carbon dioxide rose steeply from light to dark and then levelled off. At the same lightness, the two roast speeds differed little.4 The paper does not state how soon after roasting the beans were analysed.4\n\n| Roast (Brazilian arabica, fluid bed) | L* slower roast, 230 °C | Residual CO₂, mg/g | L* faster roast, 250 °C | Residual CO₂, mg/g |\n|---|---|---|---|---|\n| Light | 32.64 | 6.294 | 33.15 | 6.704 |\n| Medium | 24.07 | 11.514 | 24.44 | 11.044 |\n| Dark | 20.75 | 15.624 | 20.27 | 15.364 |\n| Very dark | 18.53 | 15.114 | 18.14 | 15.974 |\n\n## Reference values\n\n| Parameter | Value reported | Conditions | Source |\n|---|---|---|---|\n| Trapped gas in fresh whole beans | 1–2% of bean weight | Freshly roasted | Smrke et al.1, Yeretzian et al.10 |\n| Share of CO₂ in the gas formed | 87% | Textbook value cited in a peer-reviewed study | Geiger et al.3 |\n| CO₂ released during roasting | 0.4 g (fast); 0.50 g (slow) per 100 g green coffee | Laboratory fluid bed, 170 s and 720 s | Geiger et al.3 |\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, pressure method | Geiger et al.3 |\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, fluid bed, 230 °C and 250 °C, chemical method | Wang & Lim4 |\n| Gas released by whole beans | 2.5–2.8 mg/g (light); 4.7–6.6 (medium); 8.5–11.9 (dark) | Arabica coffee, ≥400 h at 35 °C, gravimetric | Smrke et al.1 |\n| Degassing rate still measured at 400 h | 3.1–4.5 µg/g per h (light); 5.1–5.6 (medium); 7.6–9.2 (dark) | Same study | Smrke et al.1 |\n| Two-process fit, dark fast roast | 2.18 mg/g with a time constant of 37.6 h; 10.34 mg/g with 182.5 h | Whole beans, 35 °C | Smrke et al.1 |\n| Whole-bean degassing, long run | 3.4 mg/g after 800 h, still releasing 2 µg/g per h | Light roast, medium speed, 35 °C | Smrke et al.1 |\n| Whole-bean degassing, dark roast | About 14 mg/g (≈90% of residual CO₂) in more than 800 h | Fast dark roast, 25 °C, infrared | Wang & Lim4 |\n| CO₂ lost during grinding | 26–30% (coarse, 654 µm); 33–38% (medium, 467 µm); 45–59% (fine, 299 µm) | Medium and dark roasts; mean particle radius | Wang & Lim4 |\n| Gas lost during and just after grinding | Up to 75% | Freshly roasted, espresso fineness, within 90 s | Smrke et al.1 |\n| Gas released by grounds after grinding | Up to 3.0 mg/g (arabica); up to 3.9 mg/g (robusta) | Espresso fineness, 35 °C | Smrke et al.1 |\n| Time to plateau, fine grind | Within 50 h | 25 °C | Wang & Lim4 |\n| Peak release rate | Up to 40 mg/h (12 g ground); up to 2 mg/h (15 g whole beans) | Freshly roasted, 35 °C | Smrke et al.1 |\n| Effective diffusivity of CO₂ | 0.5–10 × 10⁻¹³ m²/s | Fresh roast and ground coffee, during tempering | Anderson et al.5 |\n| CO emission, freshly ground coffee | 69.5 ± 33.5 mg per kg per h (mean ± SD) | Loose coffee, microchamber, about 24 °C | LeBouf & Aldridge13 |\n| CO, personal exposure (shift average) | <0.1–12.3 ppm; 7–30 ppm | National Institute for Occupational Safety and Health (NIOSH) evaluations of coffee roasting and packaging facilities | NIOSH14; Cox-Ganser et al.15 |\n| CO, short peaks | Above 200 ppm near grinders (up to 338 ppm); 584 ppm next to an open hopper of ground coffee | Same evaluations | NIOSH14; Cox-Ganser et al.15 |\n| CO exposure limits (US) | NIOSH recommended exposure limit (REL) 35 ppm (time-weighted average) and ceiling 200 ppm; Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) 50 ppm (time-weighted average) | Occupational limits | NIOSH14 |\n\n## Carbon monoxide and safety\n\nCarbon monoxide emitted from roasted coffee is described as a potential occupational respiratory exposure hazard for workers in the coffee industry.13 In NIOSH evaluations of coffee facilities, high levels were found where beans were stored or ground, and the biggest source was not the roaster, a combustion source, but the roasted beans themselves.15 Grinding gives a larger surface for the carbon monoxide to leave.15\n\n**What was measured.** Tested loose and unpackaged, as a worst case, commercially available whole and ground coffee was shown to emit carbon monoxide rapidly.13 Freshly ground coffee emitted significantly more than whole beans. Roast degree made no significant difference for whole beans, while ground medium-dark and dark roasts emitted more than medium and light ones.13 At workplaces, average exposures over a shift stayed below the NIOSH recommended exposure limit of 35 ppm in one plant (<0.1–12.3 ppm); a NIOSH letter on coffee facilities reports 7–30 ppm.14,15 Short peaks were much higher: above 200 ppm near grinders, two roaster operators with maxima above the NIOSH ceiling of 200 ppm, and 584 ppm for a worker standing next to an open hopper of ground coffee.14,15 High exposures to carbon monoxide and carbon dioxide can cause headache, dizziness, fatigue, nausea and altered mentation.14\n\n**What the authorities recommend.** In the words of the peer-reviewed emission study: carbon monoxide concentrations \"should be monitored in storage spaces in service and manufacturing facilities as well as transport vessels to ensure exposures do not exceed occupational exposure limits\", and \"Storage spaces may need to be ventilated to control CO concentrations to safe levels.\"13 The NIOSH evaluation recommended carbon monoxide monitors near the roasters and grinders to alert employees if levels exceed the 200 ppm ceiling, and covering hoppers of whole and ground coffee to limit the release of carbon monoxide and alpha-diketones into the production space; it also recommended local exhaust ventilation at all grinders, there to reduce alpha-diketones.14 A grinding chapter recommends ventilating the area around a grinder and, in industrial grinding, exhausting gas from grinders and from storage containers of ground coffee in a controlled way.9\n\n**Silos and closed rooms.** Tempering and degassing silos exist to let coffee release its gas before later steps.8 No source opened for this entry gives silo measurements; the monitoring and ventilation advice above applies to storage spaces generally.13 Carbon dioxide itself is heavier than air and can displace oxygen at the bottom of a space; the SCA handbook considers releases large enough for suffocation unlikely with coffee.2\n\n## Open questions\n\n- Which mechanisms control release from each kind of pore was not fully understood when the SCA handbook was written.2\n- Whether some gas is released only on grinding, or by a very slow mechanism in whole beans, is not resolved in the SCA handbook.2\n- According to the SCA handbook, released carbon dioxide can form carbonic acid in the brew, which could make fresh coffee taste more acidic; the handbook notes that this had not been studied systematically in coffee.2\n- How roast degree and profile change the carbon monoxide share of the released gas was not known, according to the SCA handbook.2\n- Why coffees of different origin and processing degas at different rates at the same roast degree is left open in the SCA handbook; green coffee density is one suggestion.2\n\n## Key studies\n\n
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Smrke et al., 2018 – Gravimetric degassing. Weighed whole and ground coffee continuously in containers with a capillary outlet at 35 °C, for nine roast profiles of one arabica coffee and for ground arabica and robusta. Darker and faster roasts released more gas, faster; grinding released up to three quarters of the trapped gas at once; and a two-process model described dark, fast-roasted beans best.

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

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Geiger et al., 2005 – CO₂ during roasting and storage. Followed carbon dioxide and water vapour in the exhaust of a laboratory fluid bed during a fast and a slow roast, then gas release from the beans in closed flasks for 63 days. The fast roast lost less carbon dioxide in the roaster and more in storage; the totals were 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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Wang & Lim, 2014 – Roast conditions and grind. Measured residual carbon dioxide chemically and its release by infrared spectroscopy in arabica coffee roasted fast and slow to four degrees, whole and at three grind sizes. Residual CO₂ rose with roast degree up to dark, grinding released 26–59% of it, and finer grinds degassed faster.

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Food Research International 61, 144–151 · ScienceDirect

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Anderson et al., 2003 – Diffusion kinetics. Modelled carbon dioxide release from freshly roasted and ground coffee during tempering with Fick's law. Effective diffusivities came out at 0.5–10 × 10⁻¹³ m²/s, and the authors concluded that both pressure flow and Knudsen diffusion are at work.

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Journal of Food Engineering 59(1), 71–78 · doi:10.1016/S0260-8774(02)00432-6

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LeBouf & Aldridge, 2019 – Carbon monoxide from coffee. Measured carbon monoxide emission from 64 samples of 36 brands of commercially available coffee, loose, whole and ground. Ground coffee emitted significantly more than whole beans, and darker ground roasts more than lighter ones; the authors call for monitoring carbon monoxide in storage spaces and transport vessels and, where needed, ventilating storage spaces.

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Journal of the Air & Waste Management Association 69(1), 89–96 · doi:10.1080/10962247.2018.1515125

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