What Is Espresso Crema? The Science Behind the Golden Foam
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Espresso crema is the layer of fine, hazelnut-colored foam that sits on top of a freshly pulled shot. Scientifically, it is a foam: tiny bubbles of carbon dioxide, released from freshly roasted coffee, trapped in a thick liquid of dissolved coffee solids, emulsified oils and microscopic bits of the bean. The espresso machine’s high pressure is what forces that gas into the liquid in the first place.
Crema looks like the badge of a good espresso, and baristas have judged shots by it for decades. But the science says it is more interesting, and less reliable, than that. Here is what it is made of, what changes it, and how much attention it deserves.
Key takeaways
- Crema is mostly carbon dioxide bubbles held in a liquid of coffee solids, oils and fine particles.
- The CO2 comes from roasting, which is why very stale coffee makes thin crema and very fresh coffee can make too much.
- Robusta tends to give darker, longer-lasting crema, but research does not support the idea that Robusta is essential for good crema.
- Crema is a useful clue about freshness and extraction, not a guarantee of flavor; the taste of the shot is the real test.
In this article
- What crema is made of
- How crema forms in the cup
- What changes crema
- Arabica vs Robusta crema
- Crema myths
- How to judge a shot (beyond the crema)
- FAQ
What crema is made of
In a detailed 2011 review in the journal Food Biophysics, Ernesto Illy and Luciano Navarini described espresso foam as a gas dispersed in a liquid, where the liquid is itself a complex mixture. Breaking it down:
| Component | Where it comes from | What it does in crema |
|---|---|---|
| Carbon dioxide (CO2) | Produced inside the bean during roasting | Forms the bubbles |
| Melanoidins and protein-like material | Brown compounds formed during roasting | Surface-active: help bubbles form |
| Polysaccharides | Large sugar chains from the bean’s cell walls | Thicken the liquid and help bubbles last |
| Coffee oils (lipids) | Natural fats in the bean, emulsified under pressure | Add body and aroma, but tend to destabilize foam |
| Fine solid particles | Tiny fragments of ground coffee cell walls | Create the speckled “tiger” pattern |
The review notes that crema is a metastable foam: it is not permanent, but under good conditions it can persist for many minutes. A common rule of thumb in the research is that crema should make up at least about 10% of the volume of a single espresso.
How crema forms in the cup
Step 1: roasting loads the bean with gas
During roasting, chemical reactions inside the bean create gases, and carbon dioxide is by far the biggest share. Much of that CO2 stays trapped in the bean’s porous structure after roasting and then slowly escapes over the following days and weeks. This is the degassing you hear about with fresh coffee.
Step 2: grinding releases some of it
Grinding opens up the cells, and gas starts leaving fast. Illy and Navarini cite data showing that more than 70% of the CO2 can escape once beans are ground to around 0.5 mm, which is one reason they recommend brewing within half an hour of grinding.
Step 3: pressure dissolves gas into the coffee
An espresso machine pushes water at around 92 to 94 °C (198 to 201 °F) through the coffee at about 9 bar of pressure. At that pressure, more CO2 dissolves into the liquid, the same way a soda bottle holds gas under pressure. Oils get broken into tiny droplets and emulsified at the same time.
Step 4: the pressure drops and bubbles appear
When the coffee leaves the basket and hits normal air pressure, the liquid suddenly holds more gas than it can keep dissolved. Bubbles form everywhere, like opening a soda. Surface-active compounds coat the bubbles and the thick, sugar-rich liquid slows how fast they drain and pop. The result floats to the top as crema.
That is also why a moka pot, which brews at much lower pressure, gives only a light froth, and why drip coffee has none.
What changes crema
| Factor | Effect on crema |
|---|---|
| Roast date | Very fresh beans (a day or two) can give big, bubbly, unstable crema; stale beans give thin, pale or no crema |
| Time since grinding | The longer ground coffee sits, the less CO2 is left for foam |
| Roast degree | Darker roasts produce more CO2 and more foam volume; in one study, persistence peaked around a medium roast |
| Pressure | Higher pressure in the normal range gave more foam volume in research cited by Illy and Navarini |
| Coffee species | Robusta foam tends to be darker and longer-lasting; Arabica foam is lighter and often tiger-striped |
| Grind and tamp | Too coarse lets water rush through (pale, thin crema); too fine chokes the shot (dark, patchy crema) |
| Water temperature | Within the normal range (about 88 to 98 °C), studies found little effect on foam volume |
Why freshness matters most
Of all these factors, freshness is the one most people at home can control. A bag roasted months ago has already lost most of its gas, and no machine can create crema from CO2 that is not there. That is a big reason supermarket espresso often looks flat, as we explain in fresh roasted coffee vs supermarket coffee. On the other hand, beans roasted yesterday can be so gassy that the shot sputters and the crema is huge but coarse. Many baristas rest espresso beans for several days after roasting so the gas settles.
Arabica vs Robusta crema
A common claim in espresso culture is that you need Robusta in the blend for a proper crema. The research is more nuanced.
- In the studies Illy and Navarini reviewed, foam volume rose with roast degree for both species, without a significant difference between Arabica and Robusta.
- Foam persistence was longer for Robusta: at a medium roast, one study measured around 49 minutes for Robusta versus about 34 minutes for Arabica.
- Arabica contains more oil, and coffee oils behave as anti-foaming agents, which helps explain why Arabica crema is often thinner.
- Robusta crema looks darker and greyer, while Arabica crema tends to show the reddish-brown “tiger skin” mottling.
The authors concluded that the idea of Robusta being indispensable for good crema is contradicted by the data. So Robusta can make crema denser and longer-lived, but a well-roasted, fresh, 100% Arabica espresso can have perfectly good crema too. Whether you want Robusta in your cup is really a question of taste and caffeine, which we compare in our guide to the best coffee beans in the world.
Crema myths
Myth 1: more crema means better espresso
Not necessarily. Very fresh beans, dark roasts and Robusta all boost crema, regardless of how the shot tastes. A sour, under-extracted shot from very fresh beans can still have a tall head of foam.
Myth 2: crema is the best part
Taste a spoonful of crema on its own and you will usually find it bitter and a little astringent; the review notes crema itself is of limited sensory interest. Many tasters stir it into the shot so the flavors blend, and some baristas even skim it off to taste the liquid underneath. Try all three ways and see which you prefer.
Myth 3: no crema means the coffee is bad
No crema usually means no gas: the beans are old, the coffee was ground long ago, or the brewer did not reach enough pressure. It tells you about freshness and method, not necessarily about bean quality.
Myth 4: crema color tells you everything
Baristas use rough rules: pale, thin crema hints at under-extraction; very dark crema with a pale spot hints at over-extraction or a grind that is too fine. Illy and Navarini point out that these rules come mostly from trial and error rather than measurement. Use them as hints, then confirm with your tongue.
How to judge a shot (beyond the crema)
- Look: crema should cover the surface, be fine-bubbled and hold together for at least a minute or two. Big soapy bubbles suggest very fresh or under-extracted coffee.
- Time and weigh: a classic starting point is about 18 g of coffee in, 36 g of espresso out, in 25 to 30 seconds. Our guide to making espresso at home walks through dialing in.
- Stir and smell: stir the crema in and breathe in the aroma.
- Taste: sour and thin means grind finer; harsh, bitter and dry means grind coarser. Sweetness and a lingering finish mean you are close.
Beans for a rich crema
Every Feel Espresso bag is roasted 1–5 days before delivery and arrives in 2–5 days, so the CO2 that makes crema is still in the beans. If you love a dense, long-lasting foam, our Italian Roast is our darkest roast, an Arabica and Robusta blend with a smoky, intense profile and thick crema. Our West Java Robusta brings cacao, Brazil nut and malt notes and plenty of body for blending your own shots. Prefer a brighter cup? African Espresso, a medium-dark blend of Kenyan, Tanzanian and Ethiopian coffees, gives red fruit and chocolate with a lively finish. Choose the Espresso grind, or better, whole beans ground just before brewing.
Espresso crema FAQ
Why does my espresso have no crema?
The most common causes are stale beans, coffee ground long before brewing, a grind that is too coarse, or a machine that does not reach real espresso pressure.
Should you stir the crema?
Many tasters do, because crema alone tastes bitter and stirring blends it with the sweeter liquid below. It is a matter of preference.
Does crema have more caffeine?
Crema is mostly gas bubbles and a thin film of liquid, so it holds only a small share of the shot. There is no good evidence that it is a caffeine-rich layer you need to keep or remove.
Why does crema have tiger stripes?
The mottled pattern comes from very fine solid coffee particles caught in the foam. It is common with Arabica and with well-extracted shots.
How long should crema last?
A good crema should hold for at least a couple of minutes. In laboratory conditions, espresso foams have lasted much longer, but you should drink the shot well before that.
Sources
- Illy E., Navarini L., “Neglected Food Bubbles: The Espresso Coffee Foam,” Food Biophysics, 2011. PMC3140933
- Nunes F.M. et al., “Foamability, foam stability, and chemical composition of espresso coffee as affected by the degree of roast,” Journal of Agricultural and Food Chemistry, 1997. FAO AGRIS record
Facts checked in October 2026.
Keep reading: How to make espresso at home · Fresh roasted vs supermarket coffee · Blends collection