Cut open
A real roast takes 10–15 minutes: 1× is real time, so you could roast along with it. 8× shows a whole roast in under two minutes.
Seed to Swan
Roast green coffee one step at a time: what the heat does to the beans, how the roast curve should run, when the two cracks come, and how light, medium and dark roasts end up tasting.
A real roast takes 10–15 minutes: 1× is real time, so you could roast along with it. 8× shows a whole roast in under two minutes.
Everything behind the animation: the machine, what happens inside a bean, the two cracks, the roast curve, the roast levels and how they taste, the common faults, why every coffee roasts differently, and how to choose, rest and brew roasted beans.
Most specialty coffee is roasted in a drum roaster: a gas burner under a turning steel drum, with a fan pulling hot air through the tumbling beans. Here it is cut open in the middle of the medium roast.
Green coffee is a hard, wet seed that smells of grass and plants, nothing like coffee. Heat first drives the water out, then turns sugars and proteins into colour, aroma and gas. Here is one bean through the whole medium-to-dark journey.
Green coffee is about 10–12 % water. Until roughly 150 °C much of the heat goes into turning it to steam: the beans go pale, then yellow, and smell of grass and hay. Browning can’t really get going while the bean is still wet.
From about 140–150 °C sugars react with amino acids (bits of protein), as on bread crust or a seared steak: brown colour (melanoidins) and hundreds of new aromas, from toast to nuts to caramel.
A side branch of the Maillard reaction. It makes much of the smell we call “coffee”, and nearly all of the CO₂ that stays trapped in roasted beans.
From about 160–170 °C the sugar (sucrose) breaks down, like making caramel: sweeter and more complex at first, more bitter the further it goes. It also uses up the bright fruit acids.
Steam and CO₂ build up inside the cells. At about 196 °C they split the bean with a pop (first crack). At about 224 °C the cell walls break again (second crack) and oil is pushed out to the surface.
Very hot for long enough, the bean starts to char. A little gives the plain “roasty” taste of dark coffee; a lot turns it into charcoal, and in the end it can catch fire.
The two sounds every roaster waits for. Turn on the sound to hear them on the stage, or play them here.
Roasters log both probes every second. The bean-temperature line and how fast it climbs (the rate of rise, °C per minute) are the roast profile: the same green coffee on a different curve tastes different.
Shorter: clearer, brighter, lighter body, but more pressure and more risk of defects. Longer: more body and complexity, but it can turn dull.
Shorter: simpler, lighter body, cleaner. Longer: more complex, heavier body, less clarity. Under about 2 minutes risks an underdeveloped cup.
Shorter: brighter acidity but it can taste vegetal. Longer: rounder acidity and more complexity, until it turns flat.
Less: more sugar left, simpler, possibly green. More: less sugar left, more complex, then roasty, burnt tones.
There is no official scale: one roaster’s light is another’s medium. What really changes is how far past first crack the beans go, and so the colour, the oil and the taste. Click a level to roast it on the stage.
As a roast goes on, bitterness climbs all the way, acidity rises early and then fades, and the coffee’s own origin flavours appear and then get covered by a generic roast taste. Sweetness peaks in between. Dark roasts from different countries end up tasting alike.
Bright, juicy acidity; fruit, flowers, citrus, tea-like and light in body. Shows the farm and the process best. Brew finer and hotter.
Balanced: caramel, nuts, milk chocolate, a little fruit; medium body. The most forgiving roast and the most drunk.
Dark chocolate, toasted sugar, roasted almonds, a hint of spice; low acidity, fuller body.
Bittersweet, smoky, heavy; almost no acidity; little of the origin left. Stands up to milk and sugar.
Roasting hardly touches caffeine: by weight light ≈ 1.2–1.4 %, dark ≈ 1.1–1.3 %. Light beans are denser, so a scoop of them holds about 5–10 % more. “Bold” is a taste, not a caffeine level: for less caffeine, use less coffee or decaf.
Light roasts are more acidic (bright, sour-ish), not more bitter. Bitterness comes from darker roasting, and from over-extracting when you brew.
Start from the medium roast, change one setting and watch the curve, the times and the cup move. The dashed line is the medium roast as planned.
The lab, the curve and the numbers on this page come from a small heat model of a 1 kg drum roaster, set to match the times and temperatures in the sources. They show which way each change pushes a roast, not what your machine will do.
All on the medium roast, one thing done wrong at a time. Press “Watch it” to see the whole roast go wrong on the stage, with the curve.
Green coffee is a farm product: no two lots behave the same in the drum, so a good profile for one coffee can be wrong for the next.
Dense beans (often grown high up) need more energy and can take a hotter start. Soft, low-density beans burn easily: be gentle.
Wetter beans take longer to dry before anything else can happen. Drier beans heat up and move through the roast faster.
Big beans take longer to heat through. Small round peaberries roll differently in the drum and roast faster and more evenly.
Naturals and honeys carry sugar on the surface: they roast faster and scorch more easily. Washed coffees are clean and roast evenly. How each process is made: the Green Coffee studio.
Different varieties have different structures and sugars, so they react to heat differently, and taste different at the same colour. Which variety gives what: the Coffee Tree studio.
Small batches heat fast and react quickly to changes; big ones have more momentum. A cold room or a cold machine slows a roast down.
The video test: 300 g of each, the same settings and changes all the way, and the same time after first crack. Our model of the same idea is in the right-hand columns.
Heat reaches a bean in three ways: hot air flowing past it (convection), touching hot metal (conduction) and glowing hot metal nearby (radiation). The machines mix them differently; none is “best”, they are tools for the taste a roaster wants.
Beans leave the drum above 200 °C and keep roasting until they cool. A stirred tray with a big fan gets them to room temperature in a few minutes. Big roasters sometimes add a fine water mist (“quenching”) that flashes off instantly.
Fresh beans keep breathing out CO₂ for days. Bags have a one-way valve: squeeze one and a hiss means the coffee is still fresh and still degassing.
Give beans at least 2–3 days. Too fresh, they bubble so much that water struggles to get in. Espresso usually wants a bit longer than filter. How to dial it in: the Espresso studio.
Whole beans stay near their best for about 2–4 weeks after roasting. Look for a roast date, not a best-before date, and grind just before brewing: ground coffee loses aroma in days.
Airtight, cool, dark, away from the stove. Oily dark roasts go stale faster: buy them in smaller bags.
Did it follow the planned curve? Is the weight loss as expected (14.5 % planned and 15 % measured is a warning)? Is the colour right on the meter? And they taste (cup) it.
A handy rule of thumb from one guide, the 15-15-15 rule: buy within 15 days of roasting, grind within 15 minutes of brewing, drink within 15 minutes of pouring.
Roast level is only a rough guide: the green coffee decides most of the taste. Roasting can turn blue into teal or navy, never into orange. Taste two very different coffees side by side and you will learn what you like faster than from any label.
| Species | Share of the world’s coffee | Caffeine | Taste |
|---|---|---|---|
| Arabica | ≈ 56–60 % | ≈ 1.2–1.5 % | Sweeter, more acidity and aroma; fruit, berries, chocolate. Grown high, costly to farm. Almost all specialty coffee. |
| Robusta | ≈ 40–44 % | ≈ 2.2–2.7 % | Bitter, earthy, strong body. Easy to grow low down, resists disease: instant coffee, cheap blends, some espresso blends. |
| Liberica | under 2 % | ≈ 1.2–1.5 % | Woody, smoky, bold, with floral and fruity notes. Big, long beans. |
| Excelsa | under 1 % | ≈ 0.8–1.0 % | Tart and fruity; adds complexity to blends. |
Bright and fruity: Ethiopia berries and jasmine, Kenya bold citrus. Often tea-like in body.
Lively acidity, crisp; citrus, apple, stone fruit with caramel or honey sweetness.
Smooth and mellow: chocolate, nuts, gentle acidity, medium body (Brazil, Colombia).
Full, earthy, spicy, herbal; Sumatra rich and syrupy with dark chocolate.