When espresso foams, gas is the first explanation. Crema is a dispersion of carbon-dioxide bubbles. The gas forms during roasting and remains trapped within the porous bean. Hot water under pressure dissolves some of it; pressure drops at the outlet, CO2 forms microbubbles, and proteins, melanoidins, polysaccharides and very fine particles help stabilise their films.
Three practical consequences follow. Crema often indicates freshness, does not guarantee good flavour, and has different physics from milk foam.
Where the gas comes from #
Roasting above roughly 200°C causes Maillard reactions and thermal breakdown. Illy and Navarini's review reports carbon dioxide as about 87% of gases released by roasted coffee and carbon monoxide as about 7%. Some remains in cell pores and then escapes continuously.
Under the conditions studied by Wang and Lim, darker roasts retained more CO2: approximately 6.29 to 6.70 mg/g for light, 11.04 to 11.51 mg/g for medium and 15.4 to 15.6 mg/g for dark roasting. These are results from a defined experiment, not a guarantee for every bean.
Bicarbonate in water can also release some CO2 when heated and reacting with coffee acids. Most characteristic crema gas, however, comes from roasting.
Why espresso produces crema #
Classic espresso passes roughly 90 to 94°C water through a compact bed at around 9 bar. Pressure dissolves gas into the drink. At atmospheric pressure it becomes supersaturated and nucleates into bubbles on fine solid particles.
Filter, French press and Turkish coffee operate at atmospheric pressure. CO2 escapes during wetting rather than remaining to form a dense layer in the cup. Measurements by Navarini and colleagues show that a moka pot reaches only a fraction of espresso pressure and does not produce characteristic espresso foam. Very fresh coffee may make a light, short-lived froth, but it is not the same structure.
| Method | Pressure principle | Foam |
|---|---|---|
| espresso | high brewing pressure and rapid release | dense crema |
| capsule | system-specific pressure and outlet | a layer strongly shaped by the system |
| moka | around 1 to 2 bar | little or no true crema |
| filter and press | atmospheric pressure | bloom during brewing, no crema in the cup |
What keeps bubbles stable #
Creating bubbles and keeping them are different tasks. Protein- and melanoidin-rich fractions correlate with foaming in the studies summarised by Illy and Navarini. High-molecular-weight polysaccharides, especially galactomannans and arabinogalactans, improve the liquid film's stability.
Lipids generally work against foaming. Very small solid particles may coat bubble surfaces. Composition, roast, grind and extraction therefore act together; no single compound explains the entire layer.
Freshness is the practical lever #
CO2 diffuses from a bean after roasting. Grinding greatly expands surface area and speeds the loss. Research summarised in the review found a large share released when coffee was ground to around 500 micrometres. Finer grinding and open storage accelerate degassing further.
Grind shortly before brewing and protect beans from air, heat, light and moisture. A filter bloom shows gas escaping: hot water makes fresh grounds swell and bubble. It is the same CO2 as crema, released before the cup rather than trapped in it.
Crema fading over weeks after opening a bag suggests degassing more readily than sudden machine failure. An abrupt fast shot instead points towards grind, dose or puck distribution.
Is Robusta necessary? #
Robusta contains more CO2 on average and less lipid than Arabica. Some experiments found a greater crema volume, while others found no significant volume difference between comparably roasted pure Arabica and Robusta. Stability findings are not fully consistent either.
The fair conclusion is that Robusta may support volume but is not necessary. Freshness and extraction often matter more at home. Freshly ground Arabica can foam better than an old Robusta blend. The Arabica and Robusta guide covers other distinctions.
More is not necessarily better #
Darker roasting retained more CO2 in experiments and can increase volume, but very dark roasting may break down polysaccharides that stabilise bubbles. Volume and persistence therefore need not rise together. Some studies observed the longest persistence at medium roast.
Crema says little about enjoyment by itself. Tasted alone, it is bitter and astringent, and literature treats its sensory value as limited. Thick foam can cover an unbalanced espresso, while a thinner layer can accompany an excellent one.
Colour and texture provide hypotheses only. Pale crema may accompany fast, cool or under-extracted coffee. A dark central mark may appear with excessive flow restriction. Large pale bubbles can relate to heat or rapid destabilisation. Confirm every diagnosis by tasting and measuring the shot.
Other foams #
Milk foam contains mechanically incorporated air stabilised mainly by milk proteins. Dalgona contains whipped air, sugar and soluble coffee compounds. Nitro cold brew uses added nitrogen. Filter bloom releases roast CO2 before and during infusion.
Capsule geometry, outlet and extraction design strongly shape capsule-machine foam. A printed 15- or 19-bar figure normally describes maximum pump capacity, not guaranteed pressure through the coffee.
Takeaway #
Crema forms when roast CO2 dissolves under pressure and emerges as bubbles. Fresh coffee, grinding just before brewing and suitable extraction are the best practical levers. Robusta and dark roast can change volume but guarantee neither stability nor flavour. Judge with taste and recipe, not foam height alone.
Sources: Illy and Navarini 2011, Wang and Lim 2014, Wang et al. 2019, Lomolino et al. 2022 and Navarini et al. 2009.