Espresso Crema: What It Is and What It Tells You
What espresso crema is made of, what changes its volume and staying power, and why it is a clue about freshness and extraction, not a taste test.
Crema is the layer of foam on top of an espresso. It forms from carbon dioxide that was trapped in the beans during roasting, dissolved into the water under pressure and released as the shot drops to normal air pressure, then held together by emulsified coffee oil and other compounds. A thick crema mostly tells you the beans were fairly fresh and the shot was pulled under pressure. It does not tell you the espresso will taste good.
What crema is made of
A review of the espresso foam literature describes the foam as bubbles of roasting-derived carbon dioxide in a liquid that is itself an oil-in-water emulsion, and notes that it may also carry tiny fragments of coffee cell wall. In a normal 25 to 30 mL cup, the crema makes up at least 10% of the volume. Another review found that protein-like fractions extracted from roasted coffee help the foam form, while polysaccharides help it last. Wikipedia's espresso entry makes the point that water under very high pressure dissolves more carbon dioxide, which is why the foam is specific to espresso.
Crema is not only decoration. The same review reports that a consolidated crema retains volatile aroma compounds and slows the drink's cooling. A study using a capsule system found that after an initial burst of aroma in the first 2.5 minutes, a more stable crema acted as a stronger barrier to aroma over the next several minutes. Water with a different mineral content gave liquids with similar volatiles but foams of different structure, which is a reminder that water shapes crema as well as flavour.
The standard shot, for reference
| Parameter | Value the Italian Espresso National Institute is reported to specify |
|---|---|
| Water pressure at entry | 9 ± 1 bar |
| Water temperature leaving the unit | 88 ± 2°C (about 190°F) |
| Temperature in the cup | 67 ± 3°C (about 153°F) |
| Percolation time | 25 ± 5 seconds |
| Volume in the cup, including crema | 25 ± 2.5 mL |
These come from Wikipedia's summary of the institute's specification, and they are a reference point rather than a rule for every café. One roaster's blog gives the working pressure as about 7 bar, so quoted figures vary.
What changes how much crema you get
| Factor | What the sources say | How firm it is |
|---|---|---|
| Freshness | A machine maker says coffee loses 30 to 40 percent of its carbon dioxide within a few days of roasting and most of the rest within weeks; ground coffee loses it far faster. A roaster treats crema as a rough freshness indicator | Consistent across sources, though both are sellers |
| Grinding long before the shot | The same maker compared shots pulled 30 seconds, 30 minutes and hours after grinding beans under 48 hours old; the freshest grind gave the most crema, possibly more than ideal | One informal test |
| Robusta | A roaster states robusta can hold up to twice as much carbon dioxide and about half the oil of arabica, giving more foam that is drier and less glossy | One seller's claim |
| Roast level | The same roaster says longer roasting yields more foaming compounds and so more crema. No controlled study was found to settle this | Contested |
| Water minerals | Changes foam structure in a controlled capsule study | One study |
Two practical points follow. The equipment maker says pre-ground or stale coffee has lost nearly all of its crema-forming gas and produces little or no crema. And "too fresh" coffee can give an excessive, unstable foam, which fits one roaster's advice that espresso tastes best about two to three weeks after roasting.
Does a thick crema mean a good shot?
No, and a thin one does not mean a bad shot. The machine maker says crema alone says little about quality and mostly reflects bean freshness and extraction. A roaster adds that tiger stripes in the foam come from fine coffee particles that detach during extraction and that they appear more with darker roasts and conical grinders, so a striped crema is not proof of an even extraction. Because crema follows freshness, species and roast, a modest one can come with an excellent shot. Colour charts are common online, but the sources used here give no standard that ties crema colour to taste.
Use crema as a diagnostic:
- If crema is almost absent, check the roast date and whether the coffee was pre-ground.
- If it is excessive and bubbly on very fresh coffee, rest the beans a few days.
- If it vanishes in seconds, suspect old coffee before blaming technique.
- Whatever the crema looks like, make the call on taste: sweetness, balance and finish. The dial-in advice in the espresso ratio dial-in guide shows how to adjust for taste.
Who should care about crema
If you drink straight espresso and buy whole beans, crema is a useful quick check on freshness and a prompt to look at your dose and grind. If you mostly make milk drinks, crema matters little because the milk covers it. Skip any machine or bean marketed mainly on crema volume, because it is a weak proxy for flavour. Home users with pressure-less brewers should not expect a true crema: it is described as specific to espresso.
Questions
Is espresso crema bitter?
On its own it tastes bitter and acidic according to one roaster, but it also carries carbon dioxide and aromatics that contribute to the shot's character, so removing it changes the drink. Stirring it in before tasting gives a fairer impression.
How long should espresso crema last?
There is no standard in the sources used here. The capsule study followed aroma for several minutes; one roaster says crema lasts up to 40 minutes while turning drier and more porous. Fast collapse usually points to old coffee.
Can a moka pot make crema?
Probably not. Crema is described as specific to espresso, and Wikipedia's moka pot article puts moka extraction at 1 to 2 bar against the 9 bar espresso specification, so do not expect to reproduce it.
Is espresso crema just foam?
It is a foam, but an unusual one: its liquid phase is an oil-in-water emulsion. Steamed milk foam is a different structure altogether.
Related
- Espresso ratio and yield
- Coffee storage and freshness
- Home espresso machines
- Pressure and espresso physics
- Coffee freshness chemistry
Sources
- Neglected food bubbles: the espresso coffee foam. Food Biophysics 2011. doi:10.1007/s11483-011-9220-5
- Unraveling the science of coffee foam, a comprehensive review. Crit Rev Food Sci Nutr 2021. doi:10.1080/10408398.2020.1765136
- Influence of foam structure on the release kinetics of volatiles from espresso coffee prior to consumption. J Agric Food Chem 2011. doi:10.1021/jf201758h
- Wikipedia, Espresso, for the institute specification and the high-pressure carbon dioxide description; Moka pot, for its 1 to 2 bar pressure.
- Flair Espresso (equipment maker), CO2, the spark plug, 2018, cited for what the maker claims.
- Kaffeemacher (roaster and retailer), Espresso crema, cited for what the seller claims.