Coffee roasting is the process of heating green coffee beans until they turn brown and develop the aroma and flavor of coffee. It also makes the beans soluble enough to brew. Heat drives out moisture and triggers chemical reactions inside the bean. The Maillard reaction and caramelization are the big ones. A raw green bean smells grassy and won't brew into anything you'd want to drink. A few minutes in a roaster changes all of that.
Roasting sits in the middle of coffee's journey. Farmers grow, harvest and process the cherries. Roasters turn the dried green seeds into roasted coffee. Baristas and home brewers do the rest. Every roast decision, from charge temperature to the moment the beans drop into the cooling tray, shows up in the cup.
Below, you'll see how a roast works stage by stage. Then come seven facts that explain why two bags from the same farm can taste so different.
Roasters describe a roast in three overlapping stages. The temperatures below are typical bean temperatures. Your machine, batch size and green coffee will shift them a little.
| Stage | Approximate bean temperature | What you see, hear and smell | What is happening inside the bean |
|---|---|---|---|
| Drying | Room temperature to about 150°C (300°F) | Beans fade from green to pale yellow. Grassy, hay-like smell. | Water evaporates and the bean stores heat for later reactions. |
| Browning | About 150°C to first crack, around 196°C (385°F) | Color moves to tan and light brown. Smells like toast and baking bread. | The Maillard reaction and caramelization build sweetness and aroma. |
| Development | First crack to drop, with second crack near 224°C (435°F) | Audible popping, then darker color. Oils appear on dark roasts. | Flavor is set. The roaster decides the final roast level and drops the beans to cool. |
Development usually takes 15 to 25% of the total roast time. Many roasters track it as a development time ratio. Cut it short and the coffee can taste grassy or sour. Stretch it too far and the cup turns flat and baked.
The browning stage is where green coffee becomes recognizably coffee. Want the chemistry in more detail? Our guide on why green coffee beans turn brown walks through the Maillard reaction step by step.
Want to see a roast play out? Pick a machine and a roast level below, enter your green batch weight, and follow each stage from charge to drop.
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Pick a machine and a roast level, enter your green batch weight, and see each stage from charge to drop, plus how much roasted coffee you'll get back.
How we calculate: total roast time of 12 to 15 minutes on a drum and 6 to 8 on an air roaster, longer for darker roasts. Drying takes about 45 to 50% of the roast, browning 30 to 35%, and development 15 to 25%, longest for dark roasts. Weight loss runs about 14 to 15% (light), 15 to 17% (medium), 17 to 19% (medium-dark) and 19 to 20% or more (dark). These are planning rules of thumb. Your machine, batch size and green coffee will shift them.
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Coffee roasting started in the 15th century Ottoman Empire, where beans were roasted over fire in shallow pans that looked like giant metal spoons. The craft has changed a lot since then. These seven facts cover the parts that matter most today.
A drum roaster tumbles green coffee inside a rotating metal drum. A burner or electric element heats the drum. The beans pick up heat from the hot metal (conduction) and from hot air moving through the drum (convection). Probat, Diedrich, Giesen and US Roaster Corp all build drum roasters.
Air roasting, also called fluid-bed roasting, works differently. A strong stream of hot air lifts the beans and keeps them moving, so heat arrives almost entirely by convection. Michael Sivetz introduced the first commercial air roaster in the 1970s. Coffee Crafters and Sonofresco build air roasters today.
Air roasters are faster. A batch can finish in about 6 to 8 minutes, where a drum roaster usually takes 12 to 15. Neither method is better on paper. Drum roasts often lean toward body and layered sweetness. Air roasts often taste bright and clean. Most roasters pick the machine that suits their coffee and their space.
Roasting is loud if you know what to listen for. As beans heat up, water inside them turns to steam and pressure builds. Eventually the bean structure splits open with an audible pop. That's first crack, and it sounds a lot like popcorn.
Keep going and you'll hear second crack, a quieter, faster crackle like rice cereal in milk. Light and medium roasts are dropped after first crack and before second crack. Dark roasts go into or past second crack.
Sound is only one cue. Roasters also watch color, smell the trier and read bean temperature on the control panel or in roast software. Put those signals together and you can repeat the same roast batch after batch.
Roast level is named by color, and each color group comes with a fairly predictable flavor.
| Roast level | Color and surface | Typical flavor | When it is dropped |
|---|---|---|---|
| Light | Light brown, dry surface | Highest acidity. Fruity, floral and origin notes stand out. | Shortly after first crack |
| Medium | Medium brown, dry surface | Balanced acidity and sweetness | Between first and second crack |
| Medium-dark | Rich brown with a light sheen | Low acidity, fuller body, bittersweet finish | At the start of second crack |
| Dark | Very dark, oily surface | Roast flavors lead. Smoky, bitter, heavy body. | Into or past second crack |
Color names aren't standardized between roasters. A "medium" bag can mean different things on different shelves. Professional roasters measure color with an Agtron-style reader to keep it consistent. Our guide to the Agtron scale explains the numbers. For how roast level and flavoring choices work together, see our guide to coffee roasting and different flavors.
The longer beans stay in the roaster, the more their natural sugars break down. Light roasts keep more of the green coffee's own character. Dark roasts trade that for roast-driven flavors like dark chocolate, smoke and bitterness.
Caffeine follows a similar pattern, just on a much smaller scale. Caffeine is fairly heat-stable, so the difference between roast levels is slight. By weight, light and dark roasts carry almost the same caffeine. Dark beans are less dense. So a scoop of dark roast holds slightly less coffee, and slightly less caffeine, than a scoop of light. Brew method and dose matter far more than roast color. Roast level changes brewing, too. Darker beans are more soluble and extract faster, so many baristas grind a little coarser or shorten the brew for a dark roast.
The same logic explains a common mix-up. "Espresso beans" aren't a different species. They're usually a darker roast or blend built for espresso. Our breakdown of espresso beans vs coffee beans covers it in detail.
Roasting builds carbon dioxide inside every bean. That gas keeps escaping for days after the roast. It's why coffee bags have one-way valves, and why a sealed bag of fresh beans can puff up like a balloon.
Brew too soon and the escaping gas gets between the grounds and the water. Extraction turns uneven and the cup can taste sharp. Wait too long and the coffee loses the aromatics that made it taste fresh. Many roasters rest filter coffee for a few days and espresso a little longer. Our guide on how long to use coffee beans after roasting lays out resting windows by brew method.
Green coffee still holds water when it reaches the roaster, up to 12.5% under International Coffee Organization export standards. Roasting drives most of that out, along with some dry matter that burns off as gas.
The result: roasted coffee weighs roughly 14 to 20% less than the green coffee you put in. Light roasts sit at the low end. Dark roasts sit at the high end or beyond. A medium roast often lands near 16%.
That loss matters when you plan production. To sell 100 kg of roasted coffee, you need to roast about 118 kg of green. Moisture also affects how green coffee keeps before it ever reaches the drum. Our guide on how to store green coffee beans covers humidity, temperature and packaging.
Every roaster depends on people thousands of miles away. Coffee grows in more than 70 countries. Farmers pick and process the cherries. Importers ship the green coffee. Roasters turn it into something people drink every morning.
The equipment travels too. A sample roaster that cupped a farm's first lots might later sit in a café's back room. A 12 kg drum that built one roastery often starts the next one after its owner moves up a size. Roasting is a craft passed from person to person, and so are the machines.
The chemistry is the same at any scale. What changes is control and consistency.
At home, you can roast in a popcorn popper, a skillet, an oven or a small electric drum like an Aillio Bullet. It's a good way to learn first crack by ear and taste how roast level changes a coffee. Start with our guide on how to roast coffee beans at home. Then try a single origin with our walkthrough to home roast Colombian beans.
Commercial roasting adds precise burner and airflow control and roast logging software like Cropster or Artisan. You also need a cooling tray, a chaff collector and, in many areas, an afterburner. Café roasters often start on a 1 to 3 kg shop roaster such as a Diedrich IR-2.5 or a San Franciscan SF-6. Production roasteries move up to 12 kg drums and beyond. Browse current small-batch coffee roasters (1 to 4 kg) to see what a first commercial setup includes.
Roast logging is now standard on the used market, too. Of the 206 roasters listed on CoffeeTec from November 2024 to October 2026, 127 were listed as ready to connect to Cropster or Artisan. Most ran on natural gas (108) or propane (52), and 40 were all electric.
CoffeeTec has served the coffee industry since 1972. We work with roasters at every stage. That includes café owners roasting their first batch and production roasteries running drum and fluid-bed machines side by side.
Ready to roast? Our marketplace carries new and used roasters from Probat, Diedrich, US Roaster Corp, Coffee Crafters and more. Moving up to a bigger machine? List your used roasting equipment on CoffeeTec and reach buyers who are already searching for it.
Green coffee contains the acids, proteins and caffeine of coffee but none of its familiar taste. Roasting creates the aroma and flavor compounds and makes the bean brittle and soluble enough to grind and brew.
You can steep green beans in hot water, but the drink tastes grassy and tea-like, not like coffee. Green beans are also very hard to grind. That's why green coffee is mostly sold for roasting or as an extract.
A drum roaster usually takes 12 to 15 minutes per batch. An air roaster can finish in about 6 to 8 minutes. Cooling adds a few more minutes, and darker roasts run a little longer than light ones.
Processing happens at origin. It removes the fruit from the coffee cherry and dries the seed into green coffee. Roasting comes later, usually in the consuming country, and turns that green coffee into roasted beans.
A roast profile is the plan for how temperature changes over time during a roast. Roasters record it as a curve, often in Cropster or Artisan. That lets them repeat a roast and adjust it batch to batch.
The basics are simple: apply heat, listen for first crack, cool the beans. Consistency is the hard part. Most roasters build skill through practice, cupping every batch and keeping roast logs. Many also take SCA roasting courses.
At minimum, you need a commercial roaster with a cooling tray and chaff collector. Add a vented exhaust that meets local air rules, a scale and a bag sealer. Most roasteries add a destoner and roast logging software early on.
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