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Extraction · Fundamental

CO₂ and the bloom

What happens to the carbon dioxide left in ground coffee when hot water reaches it – the bloom in filter brewing and the crema on espresso – how much gas remains at brewing time, and what is known about its effect on extraction.

Across the chain: Science

Source-checkedby Coffearium verification pass· Last source-checked 20 Oct 2026· Prepared with AI assistance· 13 sources (8 tier A)· 1 contested· Updated 20 Oct 2026
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Contents
  • Freshly roasted beans hold 1–2% of their weight as trapped gas, mostly carbon dioxide; in one study, up to 75% escaped during and just after grinding.1
  • Part of the carbon dioxide leaves ground coffee only when water reaches it. Its fast release is the bloom in filter brewing and the gas of espresso crema.1,3,4
  • In one study, coffee particles grew by about 15% in size in water, regardless of roast degree; the swelling was complete within 4 min.5
  • The Specialty Coffee Association (SCA) freshness handbook states that the released gas raises resistance to water flow, yet in its own espresso test fresh coffee ran faster.3
  • No study opened for this entry measured how a bloom changes extraction yield or flavour; one published recipe bloomed 15 g of coffee with 60 mL for 15 s.11

What the bloom is#

Roasting forms gas inside the coffee bean, mostly carbon dioxide (CO₂).1,3 Freshly roasted beans hold 1–2% of their weight as trapped gas.1 How the gas forms is covered in the roast gases entry; how it leaves the beans in storage is covered in the degassing entry.

Much of the gas is still in the coffee when it is ground and brewed. According to the Specialty Coffee Association (SCA) freshness handbook, it escapes fast when hot water meets the grounds.3 In filter brewing this release is called the bloom.3 On espresso, the same gas forms the crema, the foam on top of the shot.3,4 The handbook links fresher coffee to stronger blooms and more crema.3

In brewing practice, the bloom is also the name of a step. A small first pour wets the grounds, followed by a pause before the main pours.11,12 The pour-over technique entry describes that step. This entry looks at the physics behind it and at what has been measured.

How much gas is left at brewing time#

Grinding destroys or damages the large pores of the bean, and much of the gas escapes at once.1,3 In a 2018 weighing study, up to 75% of the trapped gas was lost during and just after grinding freshly roasted coffee for espresso.1 In a 2014 study, the finer the grind, the more carbon dioxide was lost.2

That 2014 study also measured what stayed in the coffee. Just-ground coffee held 4.46–11.82 mg/g of residual carbon dioxide, depending on roast and grind.2 Whole beans of the same roasts held 11.04–15.62 mg/g.2 Darker roasts held more in both forms.2

0246810121416residual CO₂, mg per g of roasted coffeeMedium roast, whole beans11.04–11.512 Medium roast, coarse grind(654 µm)7.75–8.542 Medium roast, medium grind(467 µm)6.80–7.662 Medium roast, fine grind(299 µm)4.46–5.492 Dark roast, whole beans15.36–15.622 Dark roast, coarse grind(654 µm)11.63–11.822 Dark roast, medium grind(467 µm)9.56–10.262 Dark roast, fine grind(299 µm)6.62–8.542
Data table
Pointresidual CO₂, mg per g of roasted coffeeSource
Medium roast, whole beans11.04–11.5111.04–11.512
Medium roast, coarse grind (654 µm)7.75–8.547.75–8.542
Medium roast, medium grind (467 µm)6.80–7.666.8–7.662
Medium roast, fine grind (299 µm)4.46–5.494.46–5.492
Dark roast, whole beans15.36–15.6215.36–15.622
Dark roast, coarse grind (654 µm)11.63–11.8211.63–11.822
Dark roast, medium grind (467 µm)9.56–10.269.56–10.262
Dark roast, fine grind (299 µm)6.62–8.546.62–8.542
Carbon dioxide still held by whole beans and by freshly ground coffee, by roast degree and grind size (mean particle radius). Brazilian arabica, two roast temperatures per span; chemical method.2

After grinding, most of the rest leaves within hours to days, not weeks.2 In the same study, at 25 °C, a fine grind stopped releasing measurable gas within 50 h.2 Whole beans kept degassing for weeks.1,2 The freshness and storage entry covers what this means for how long ground coffee keeps.

What happens when water meets the grounds#

Several things happen at once when the first water reaches the bed.

  • Gas is pushed out. Water spreads into the voids of the particles while gas inside and between them is pushed out of the bed.4,9 This was described for the first seconds of an espresso, but the same displacement shows as bubbles in a filter bloom.3,4
  • Some gas is released only now. The 2018 weighing study found that part of the carbon dioxide stays bound in dry coffee. It is released only when the coffee is extracted with water.1
  • The water adds gas of its own. Coffee acids react with the bicarbonate (hydrogen carbonate) in the water and form more carbon dioxide.1,10
  • Particles swell. In a 2020 study, roasted coffee particles grew by about 15% in size in water.5 The swelling was fast at first and complete within 4 min, inside normal brewing times.5

The visible bloom is the sum of these effects: bubbles, foam and a bed that stands higher than before. No study opened for this entry measured how much of the rise of the bed comes from gas and how much from swelling.

A · Dry groundsB · During the bloom123465

A · Dry grounds

  1. Gas left in the grounds. Much of the trapped gas escapes on grinding, more the finer the grind.1,2

B · During the bloom

  1. First pour. A small amount of water wets all the grounds; one recipe used 60 mL on 15 g of coffee and paused 15 s.11
  2. Water in, gas out. Water fills the voids of the particles and pushes the gas out.4,9 Part of the carbon dioxide is released only on wetting.1
  3. Bubbles and foam. The gas rises and escapes; fresher coffee gives a stronger bloom.3
  4. Bed rises. Gas and swelling both play a part; particles grow about 15% in size in water.3,5 Their shares have not been measured.
  5. Gas from the water. Coffee acids neutralise bicarbonate in the water and form more carbon dioxide.1,10
A bed of ground coffee before water (A) and during the bloom (B), as described in the sources cited. Rings in the accent colour are gas; hairlines are water; the dashed line in B marks the dry bed level. Point at a number or a line of the legend to highlight the part. Schematic, not to scale.

Gas and extraction: what is measured#

The 2018 weighing study states that carbon dioxide affects the extraction process.1 According to the SCA freshness handbook, the gas released as water reaches the grounds raises the resistance to water flow.3 It also affects contact between water and coffee.3 The handbook gives this as the reason espresso settings are adjusted to the age of the coffee.3 Which way fresh coffee shifts the flow is not settled (see Contested).

A 2011 review describes a related effect for espresso. In one interpretation it reports, reactions with bicarbonate in the hot bed release carbon dioxide and compact the bed.4 At high alkalinity, the SCA water handbook adds, the carbon dioxide formed from bicarbonate can lengthen extraction time and lead to over-extraction.10

These are explanations, not tests of extraction yield. No study opened for this entry compared extraction yield or flavour with and without a bloom. None compared coffees that differed only in their gas content, either. The SCA handbook states that the gas affects contact between water and coffee.3 A practitioner article goes further and reports a researcher’s suggestion that the gas may hinder wetting and cause uneven flow (channelling).13 Neither idea has been tested in the studies opened.

The bloom in practice#

In a pour-over, the bloom is a small first pour followed by a pause. One published research recipe poured 60 mL of water on 15 g of coffee and waited 15 s before the next pour.11 A second study by the same group used a 15 s bloom in an AeroPress protocol. It described the pause as time for the grounds to saturate fully and release carbon dioxide.12 No study opened for this entry tested a bloom amount or time.

The pour-over technique entry gives the full recipe and how pouring changes the brew. Water temperature, ratio and grind are covered under extraction variables, brew temperature and brew ratio and strength.

Crema and pre-infusion#

Espresso crema is a foam whose gas phase is mainly carbon dioxide formed during roasting.4 According to a coffee science textbook, pressure is decisive: brew methods without it form no crema.8 Fresher coffee gives more crema, according to the SCA freshness handbook.3 Even long-stored coffee still forms a little, from carbon dioxide formed by the water’s bicarbonate or already present in the water.3 The espresso entry covers crema in full.

Pre-infusion wets the espresso bed at low pressure before full pressure is applied. The espresso machine entry describes it and reports no controlled study of its effect. No study opened for this entry measured how pre-infusion changes gas release.

Free and bound carbon dioxide#

The 2018 gravimetric study distinguishes two pools.1 “Free” carbon dioxide leaves dry whole or ground coffee in storage. “Bound” carbon dioxide is released only when the coffee is wetted, or else very slowly.1 The authors base this on a gap between methods. Amounts released by weight were lower than residual amounts from chemical (wet) methods, in which carbon dioxide is extracted with water and trapped on a column.1 For fine-ground dark roasts, the 2014 study had found 6.62 mg/g (fast roast) and 8.54 mg/g (slow roast) of residual carbon dioxide by the wet method.2 The 2018 study cites these as 6.6 and 8.4 mg/g. Its own release measured by weight was 5.5 mg/g for both roasts.1

The authors suggest this bound pool explains why even old coffee can still form some crema.1 They also suggest that dark roasts may hold carbon dioxide more strongly by sorption.1 The study called for further work to separate the two pools.1

Release on the brewing time scale#

In a 2014 study, release from whole beans and ground coffee at 25 °C was fitted with a Weibull model.2 The time-scale parameter was 190–335 h for whole beans and 2–25 h for ground coffee.2 In the 2018 gravimetric study at 35 °C, release reached up to 40 mg/h from 12 g of freshly ground coffee. From 15 g of whole beans it reached up to 2 mg/h.1

Neither study measured release during the seconds or minutes of a brew. The time scales above describe dry coffee in air. They show how fast the gas available for a bloom falls after grinding, not how fast it leaves under water. No study opened for this entry measured carbon dioxide release from grounds during a filter brew.

Gas, swelling and bed rise#

A 2020 study measured the swelling of roasted coffee particles in water by laser diffraction and microscopy.5 The total increase in particle size was about 15%.5 It did not depend on the initial particle size or on roast degree, and particle shape did not change.5 Swelling was complete after 4 min.5 The study also found particle erosion, with more fines after wetting at higher temperatures.5

According to a coffee science textbook, particles in an espresso bed swell during wetting because of water-insoluble polysaccharides.8 With flow and pressure they rearrange, and the bed loses porosity (consolidation).8 In a filter bloom, bed rise therefore has at least two causes: gas held in the wet bed, and swollen particles.3,5 No study opened for this entry separated the two.

Carbon dioxide from the water#

The water is a second source of carbon dioxide during brewing. Coffee acids neutralise its bicarbonate, and carbon dioxide forms.1,10 A 2011 review names two routes: thermal decomposition of bicarbonate and its neutralisation by coffee acids.4 A water treatment called decarbonisation turns bicarbonate into carbonic acid. In an in-line system that keeps the gas in, the SCA water handbook reports excessive crema with large bubbles that collapse quickly.10 The hardness and alkalinity and water treatment entries cover the water chemistry.

Acidity. According to the SCA freshness handbook, carbon dioxide dissolved in the brew forms carbonic acid, which could make fresh coffee taste more acidic.3 The handbook notes that this had not been studied systematically in coffee.3

Gas and the extraction measurement#

Carbon dioxide does not count as dissolved solids. The SCA water handbook notes that, as a gas, it adds nothing to total dissolved solids (TDS) and almost nothing to conductivity.10 Gas still in the grounds is weighed with the dose, however.

A 2021 immersion study discussed this as a possible source of deviation. Carbon dioxide and other volatiles can cause mass losses of about 10 mg/g of roasted coffee, about 1% of its mass.6 The authors found slightly higher extraction yields in cupping brews of same-day coffee than in 1-litre beaker brews of coffee rested two days.6 They argued that gas leaving during brewing should lower the measured yield, not raise it, and left the difference unexplained.6 The extraction yield and TDS entry explains the calculation.

Espresso foam and the gas#

A 2011 review reports a study in which foam volume rose with the carbon dioxide content of the coffee. It went from about 2.5 mL at 0.25 mg/g to about 12 mL at 4.5 mg/g.4 In another study described in the review, holding the coffee for 24 h under ambient conditions reduced foam volume from 10 to 6.5 mL.4 The review states that the bubble formation mechanism had not been investigated in detail.4

A 2020 study pressurised the sealed extraction chamber of a Caffè Firenze espresso brewer with different gases.7 With carbon dioxide as the working gas, the espresso held 4.21% TDS, against 3.36% with air, a significant difference.7 Its foam collapsed within 2 min, which the authors relate to the low solubility of carbon dioxide in hot water.7 The study used one coffee and one device. In that set-up, carbon dioxide in the brewing system went with more extracted solids, not fewer.7

Reference values#

Parameter Value reported Conditions Source
Trapped gas, fresh whole beans 1–2% of weight Freshly roasted Smrke et al.1
Gas lost during and just after grinding Up to 75% Freshly roasted, espresso fineness, within 90 s Smrke et al.1
Residual CO₂, whole beans 11.04–11.51 mg/g (medium); 15.36–15.62 mg/g (dark) Brazilian arabica, fluid bed, 230 and 250 °C, chemical method Wang & Lim2
Residual CO₂, just ground, medium roast 7.75–8.54 (coarse); 6.80–7.66 (medium); 4.46–5.49 mg/g (fine) Mean particle radius 654, 467 and 299 µm Wang & Lim2
Residual CO₂, just ground, dark roast 11.63–11.82 (coarse); 9.56–10.26 (medium); 6.62–8.54 mg/g (fine) Same Wang & Lim2
Weibull time scale of degassing 190–335 h (whole beans); 2–25 h (ground) 25 °C Wang & Lim2
Fine grind, release plateau Within 50 h 25 °C Wang & Lim2
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
Bound CO₂, fine-ground dark roast Wet method 6.62–8.54 mg/g against 5.5 mg/g released by weight Gap read as CO₂ released only on wetting Wang & Lim2, Smrke et al.1
Particle swelling in water About 15% in size; complete after 4 min Laser diffraction; independent of roast degree Hargarten et al.5
Mass of CO₂ and volatiles lost About 10 mg/g (≈1%) Discussed as a source of deviation in yield Liang et al.6
Bloom in a published recipe 60 mL on 15 g; 15 s pause V60 cone, paper filter Santanatoglia et al.11
Espresso foam vs CO₂ content ≈2.5 mL at 0.25 mg/g; ≈12 mL at 4.5 mg/g Study reported in a review Illy & Navarini4
Ground coffee held 24 h in air Foam 10 → 6.5 mL Study reported in a review Illy & Navarini4
CO₂ as working gas 4.21% TDS (air 3.36%); foam gone within 2 min Gas-pressurised espresso chamber, one coffee Angeloni et al.7

Open questions#

  • Bloom and extraction. No study opened for this entry measured how a bloom, or its water amount and time, changes extraction yield or flavour.
  • Gas and flow. Whether gas released in the bed slows or speeds espresso flow, and whether it causes channelling, is not settled (see Contested).3,13
  • Bed rise. How much of the rise of a blooming bed comes from gas and how much from particle swelling has not been separated.3,5
  • Free and bound gas. The 2018 gravimetric study called for further work on carbon dioxide released only on wetting.1
  • Carbonic acid and acidity. Whether carbon dioxide from fresh coffee makes the brew more acidic had not been studied systematically, according to the SCA freshness handbook.3
  • Release under water. No study opened for this entry measured how fast carbon dioxide leaves grounds during a brew.

Key studies#

Smrke et al., 2018 – Gravimetric degassing. Weighed whole and ground coffee continuously in containers with a capillary outlet at 35 °C. Grinding released up to three quarters of the trapped gas at once. The authors compare their results with wet chemical methods and propose that part of the carbon dioxide is released only when coffee meets water.

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

Wang & Lim, 2014 – Roast conditions and grind. Measured residual carbon dioxide in whole and ground arabica coffee from two roast temperatures and four roast degrees, and followed its release at 25 °C. Grinding left 4.46–11.82 mg/g in the coffee, and ground coffee degassed on a time scale of hours rather than weeks.

Food Research International 61, 144–151 · ScienceDirect

Hargarten et al., 2020 – Swelling of coffee particles. Followed roasted coffee particles in water by laser diffraction and microscopy. They grew by about 15% in size within 4 min, regardless of initial size and roast degree, and shed fines at higher temperatures.

Journal of the Science of Food and Agriculture 100(10), 3960–3970 · doi:10.1002/jsfa.10440

Illy & Navarini, 2011 – The espresso foam. Reviewed crema as a foam whose gas is mainly carbon dioxide from roasting. It reports foam volume rising with the carbon dioxide content of the coffee, and falling when ground coffee was held in air before brewing.

Food Biophysics 6(3), 335–348 · doi:10.1007/s11483-011-9220-5

Angeloni et al., 2020 – Working gases in a gas-driven brewer. Brewed one coffee with six pressurising gases in a Caffè Firenze machine. Carbon dioxide gave a stronger extract than air but a foam that collapsed within 2 min.

Foods 9(12), 1825 · doi:10.3390/foods9121825

Contested
  • Fresh coffee and espresso flow: the SCA freshness handbook states that gas released as water reaches the grounds raises the resistance to water flow.3 In the espresso demonstration reported in the same handbook, freshly roasted coffee needed a finer grind setting than coffee rested for 10 days to reach the same brew ratio, and at the rested coffee’s setting it gave more beverage in the same time; older coffees also ran faster.3 The SCA water handbook states that large amounts of carbon dioxide formed from bicarbonate at high alkalinity can increase extraction time.10 A practitioner article discusses the faster shots of very fresh coffee and reports a researcher’s untested suggestion that the gas may hinder wetting of the bed and favour channelling.13 No peer-reviewed study opened for this entry resolves the direction or the cause.
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Change log

  • – The Caffè Firenze study is now described as pressurising a sealed extraction chamber with different gases, not as using gas instead of a pump; the CO₂–air TDS difference is marked as significant. Reason: The paper describes a pump feeding water and a separate gas supply to the chamber; its Table 2 gives different significance letters for CO₂ and air.
  • – Wet-method residual CO₂ for fine-ground dark roasts is now given as 6.62 and 8.54 mg/g and attributed to the 2014 study; the 2018 study's rounded citation (6.6 and 8.4 mg/g) is noted. Reason: The values are from Wang & Lim's Table 3; the 2018 paper only cites them, and its 8.4 differs from the original 8.54 used in the chart.
  • – The brief and the Contested item now show that the SCA freshness handbook's own espresso test ran the other way from its statement on flow resistance; the practitioner article is cited only as a reported, untested suggestion. Reason: Balance of evidence and the rule that a Tier C source never stands alone.

Sources

  1. ASmrke, S.; Wellinger, M.; Suzuki, T.; Balsiger, F.; Opitz, S. E. W.; Yeretzian, C. (2018). Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee. Journal of Agricultural and Food Chemistry 66(21), 5293–5300. doi:10.1021/acs.jafc.7b03310 · other copies
  2. AWang, X.; Lim, L.-T. (2014). Effect of roasting conditions on carbon dioxide degassing behavior in coffee. Food Research International 61, 144–151. doi:10.1016/j.foodres.2014.01.027 · other copies
  3. BSmrke, S.; Sage, E.; Wellinger, M.; Yeretzian, C. (2018). The Coffee Freshness Handbook (ISBN 978-0-9995807-2-1). Specialty Coffee Association, Santa Ana.
  4. AIlly, E.; Navarini, L. (2011). Neglected Food Bubbles: The Espresso Coffee Foam. Food Biophysics 6(3), 335–348. doi:10.1007/s11483-011-9220-5 · other copies
  5. AHargarten, V. B.; Kuhn, M.; Briesen, H. (2020). Swelling properties of roasted coffee particles. Journal of the Science of Food and Agriculture 100(10), 3960–3970. doi:10.1002/jsfa.10440 · other copies
  6. ALiang, J.; Chan, K. C.; Ristenpart, W. D. (2021). An equilibrium desorption model for the strength and extraction yield of full immersion brewed coffee. Scientific Reports 11, 6904. doi:10.1038/s41598-021-85787-1 · other copies
  7. AAngeloni, G.; Masella, P.; Guerrini, L.; Spadi, A.; Bellumori, M.; Innocenti, M.; Parenti, A. (2020). Physical and Chemical Effects of Different Working Gases in Coffee Brewing: A Case Study of Caffè Firenze. Foods 9(12), 1825. doi:10.3390/foods9121825 · other copies
  8. BMestdagh, F.; Glabasnia, A.; Giuliano, P. (2017). The Brew – Extracting for Excellence. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 15, pp. 355–380. doi:10.1016/B978-0-12-803520-7.00015-3 · other copies
  9. BFolmer, B.; Blank, I.; Hofmann, T. (2017). Crema – Formation, Stabilization, and Sensation. In: Folmer, B. (ed.), The Craft and Science of Coffee, Academic Press, ch. 17, pp. 399–417. doi:10.1016/B978-0-12-803520-7.00017-7 · other copies
  10. A/BWellinger, M.; Smrke, S.; Yeretzian, C. (2018). The SCA Water Quality Handbook, edition 2 (ISBN 978-0-9995807-3-8). Specialty Coffee Association.
  11. ASantanatoglia, A.; Cespi, M.; Perinelli, D. R.; Fioretti, L.; Sagratini, G.; Vittori, S.; Caprioli, G. (2023). Impact of the human factor on the reproducibility of different coffee brewing methods. Journal of Food Composition and Analysis 124, 105698. doi:10.1016/j.jfca.2023.105698 · other copies
  12. ASantanatoglia, A.; Alessandroni, L.; Fioretti, L.; Sagratini, G.; Vittori, S.; Maggi, F.; Caprioli, G. (2023). Discrimination of Filter Coffee Extraction Methods of a Medium Roasted Specialty Coffee Based on Volatile Profiles and Sensorial Traits. Foods 12(17), 3199. doi:10.3390/foods12173199 · other copies
  13. CBarista Hustle (2023). Why do espresso shots run faster when I use super fresh coffee?. Barista Hustle (online coffee education company).

Tier A · authoritative · B · trade and expert · C · general. Supporting quotes for every source are in the data version of this entry. If a publisher’s site refuses access, try the doi link or “other copies”.

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