Coffee roasting
Roasting turns a hard, grassy seed into coffee in a matter of minutes, and nearly all of what coffee tastes of is made in that time. It is also the part of coffee with the most folklore. Much of what roasters teach one another about curves and timings has never been tested, and the research that exists is more cautious than the advice.
These pages keep the two apart. Where somebody measured something, they say who and on what. Where a figure would be a guess, they leave it out, which is why you will find no target temperature or ideal curve here.
A roast, in order
The stages of one roast
CoffeeHQ diagram
- Green coffee goes inA dense seed with roughly a tenth of its weight as water. How wet it is changes how fast it heats.
- Drying, then the colour changeThe seed loses water and turns from green towards yellow. Little aroma has formed yet.
- BrowningSugars and amino acids react, colour deepens and aroma builds fastest once the seed has dried out.
- First crackAudible popping as the beans swell. In one study the coffee's measured acidity peaked here, however the roast was run.
- After first crackThe stretch in which the roaster chooses where to stop. Acidity falls and roast-made flavours grow.
- Second crack, if the roast goes that farA faster, quieter crackle. Dark roasts end at or beyond it.
- CoolingThe roast is stopped by cooling the beans quickly. It is part of the roast, not the tidying up.
The order of events in a roast, with two findings from published studies noted where they occur. No times or temperatures are shown: they differ between machines, batch sizes and coffees, and a temperature read by a probe in one roaster is not the temperature of a bean.
Each stage is explained, with its sources, in what happens when coffee roasts.
What decides how a roast turns out
Four things, none of which has a correct setting. The table gives what a source reports for each, and beside it the claim that goes further than the evidence.
Four inputs, and how far the evidence goes for each
CoffeeHQ explanatory model — not a measurement
| Input | What a source reports | What has not been established |
|---|---|---|
| How the heat is applied over time | A thesis found the temperature profile the most important factor for both flavour and the structure of the bean. | That any particular curve shape is correct. One research group roasted several on purpose and says it knows of no published data behind a common belief about them. |
| How much air moves through, relative to the coffee | In the same thesis, small amounts of air gave better coffee in its trials and very large amounts gave blander coffee. | That one kind of roaster is better. A comparison of three machines found no consistent winner. |
| The green coffee, especially its moisture | In the thesis's trials, wetter beans heated more slowly and lost weight faster. | A setting that suits every coffee. The same settings give a different roast with a different coffee. |
| Where the roast is stopped | Colour follows one path, and measured acidity falls after first crack, whatever the profile. | A shared definition of light, medium or dark. One roaster's light can be darker than another's dark. |
Basis: A doctoral thesis on roasting, two studies of roast profiles on one drum roaster and one comparison of three roasters. Limits: Each cell reports one source about one set-up. The inputs interact, so changing one alters what the others do.
Where to go from here
- What happens inside the bean
Weight, volume, the reactions, the cracks and cooling, with what is measured kept apart from what is usually said.
- Roasters and heat transfer
Contact, moving air and radiation; drum and air roasters; and the experiment with very fast roasting that the industry abandoned.
- Reading a roast profile
What rate of rise is, what the probe actually measures, and why a curve does not travel between machines.
- Roast defects
Baked, scorched and underdeveloped, in the one study that made them on purpose, and why the words are looser than they sound.
- Roasting basics
The short version, and what light, medium, dark, espresso roast and filter roast on a bag can and cannot tell you.
- Roasting at home
The steps of a roast, and what the manuals for home roasters instruct about smoke, chaff and fire.
The names on the bag
Light, medium and dark describe how far a roast was taken. Espresso roast, filter roast and omni roast describe what the roaster had in mind. Neither set is standardised, so the words compare coffees from one roaster better than they compare roasters.
- Light roast
Roasted only to shortly after first crack. Retains more of the bean's origin character and acidity, with less roast-derived flavour than darker roasts.
- Medium roast
Roasted between first and second crack. Balances origin character with roast-developed sweetness and body.
- Dark roast
Roasted into or past second crack. Roast-derived bittersweet, smoky or roasted flavours dominate over origin character; body is typically heavier.
- Espresso roast
A roasting approach (rather than a fixed colour) developed specifically for espresso brewing, typically medium to medium-dark with roast development aimed at balancing the concentrated extraction and higher pressure of espresso. Distinct from 'dark roast' as a general colour category, though the two overlap in practice.
- Filter roast
A roasting approach aimed at filter and pour-over brewing, typically lighter than an espresso roast, intended to preserve acidity, aromatics and origin character that a longer, lower-pressure brew can extract without becoming harsh.
- Omni-roast
A single roast profile intended to give reasonable results across multiple brewing methods, including both espresso and filter, rather than being optimised for one. Increasingly used by roasters who want to offer one product without a dedicated espresso and filter version.
Learn it as a course
Coffee Expert level 7 covers the same ground as a lesson, with exercises you can run on coffee you already own. If you want to taste what roast degree does, light roast versus dark roast sets the two side by side and the Tasting Lab gives you a worksheet for recording what you find.
Roasting is not the end of the story. What happens to roasted coffee in the weeks after is covered in why coffee goes stale, and what brewing takes out of it in coffee extraction and the brewing methods.