Coffee Roasting Explained
What this covers
Coffee roasting is fascinating, and there's so much more to talk about than we cover here. We'll definitely dive deeper in the future - so do please share any ideas or suggestions for things you'd like to see. Thank you to Allpress Espresso and Climpson & Sons for letting us film and interrupt your busy working days: https://uk.shop.allpressespresso.com/ https://climpsonandsons.com/ Thank you also to the roasting team at Square Mile Coffee Roasters for the assistance in making this.
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The speaker explains that coffee roasting is a complex chemical process where roasters manage heat profiles to balance bitterness, acidity, and origin characteristics, with lighter roasts preferred in specialty coffee to preserve unique flavor qualities despite higher acidity.
- Roasting creates flavor through Maillard reactions and caramelization, with longer roasting increasing bitterness while origin characteristics fade into generic roast taste
- Specialty coffee favors lighter roasts despite acidity because darker roasts homogenize flavor and obscure the unique characteristics imparted by growing conditions
- Roasters use roast profiles (varying heat input over time) rather than constant temperature to achieve consistency and desired flavor outcomes
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Specialty coffee roasters primarily focus on lighter roasts that preserve origin characteristics (unique flavors from coffee's growing region), while darker roasts are rare in specialty coffee because they destroy these distinct characteristics and produce a generic roast taste that masks regional differences.
“However, it's rare to find dark roasted coffees in the specialty coffee world. And that's because of the third characteristic we need to talk about, which is difficult to talk about, without sounding a little bit pretentious. And that is origin characteristics.”
Modern consumer expectations for coffee freshness (roasted, bagged, and shipped the same day) create a practical challenge for quality control because roasters cannot adequately taste coffee before it ships, forcing them to rely on other quality assurance checks.
“Modern expectations say that the consumer wants their coffee roasted, bagged and shipped the same day, so that it is as fresh as possible. And this means really functional tasting is difficult to do before the coffee goes out the door. So they're looking for other ways, other checks that the the coffee can pass or fail to hit, to make sure that what they bag up and ship is tasting the way that they want it to.”
Roasters do not aim for one perfect roast profile but instead aim for consistency—ensuring each batch follows the desired profile path within acceptable tolerances to maintain reproducible flavor quality.
“Roasters aren't aiming for one perfect profile, but they are aiming for consistency in their roasts.”
During roasting, water that has not escaped turns to steam and builds up pressure inside the coffee bean; simultaneously, other roasting chemistry creates gases like CO2, which continue to pressurize the bean until 'first crack' occurs.
“Firstly, all of that water that hasn't escaped is turning to steam and is building up pressure. And then other roasting chemistry is causing gases to form, gases like CO2... And you build up pressure inside the coffee bean, and a really key moment in the roasting process is called first crack, because at some point relatively early on, that pressure becomes too much and the gas escapes from the coffee bean causing it to crack open”
Quenching (spraying a small amount of water on hot coffee beans) cools coffee very rapidly through evaporative cooling and has no downsides to coffee quality, though quenched coffee requires slightly less rest time before degassing and will stale slightly faster.
“here, a very, very small amount of water is sprayed onto the coffee beans. That water instantly evaporates because the beans are so hot and the evaporative cooling really very quickly cools the beans down. Now, there's no real downsides to quenching, except that that coffee will need a little bit less rest before cooling and it will stale slightly quicker. But quenching does not impact the quality of the coffee whatsoever.”
After first crack, browning reactions become complex and rapid, and the color changes happening are relatively minor compared to the large flavor changes occurring—big changes in acidity, sweetness, and bitterness happen during this phase even though visual differences are subtle.
“And from this point onwards, the browning reactions are really underway and those are so complex and so rapid that this is a point from now on where you are really paying a lot of attention. Those color changes are relatively minor, but the flavor changes happening now are actually surprisingly large.”
The roasting process increases bitterness in coffee beans; the longer coffee is roasted, the darker its color becomes and the more bitter it tastes, similar to how darker caramel is more bitter than lighter caramel.
“And the first one is bitterness. The longer we roast the coffee, the darker the color of it will be. And a little bit like caramel, a darker caramel will also taste much more bitter. So there's an increase in bitterness during the roasting process.”
If a roaster expects a specific roast loss percentage (e.g., 14.5%) and obtains a different result (e.g., 15%), this deviation should serve as a red flag that something went wrong in the roasting process.
“But if you're expecting 14.5% roast loss and you get 15%, that should be a red flag that things haven't gone as expected.”
Home roasting coffee on a sheet tray in a 200-degree-Celsius oven produces poor results because beans touching the metal tray experience excessive conduction heat and burn in those contact areas, and because constant temperature is suboptimal compared to varied heat input that professional roasters use.
“If you look at the roast profile, you'll see that the temperature of the coffee would increase and it would begin to slow, the closer it gets to the target temperature. When you take it out though, that's not going to be the best tasting coffee, for a couple of different reasons. Firstly, where the coffee beans have been touching the metal the whole time, well they'll have had a lot more conduction heat transfer and they're probably gonna be a little bit burnt around those spots.”
Filter coffee roasting requires less solubility optimization because the brew method provides ample time and water for extraction, making it easier to achieve proper extraction compared to espresso.
“It it's typically a more soluble roast than it would be for a filter coffee, 'cause it's easier to properly brew coffee when you brew it as a filter coffee. [13:33] You've got a lot more time and you've actually got a lot more water to do the extraction work with.”
Raw coffee seeds have little to no flavor and smell similar to generic plant seeds; all the flavor complexity consumers experience comes from the roasting process transformation.
“And right now it smells and tastes, well, pretty terrible. It doesn't smell of much at all. It smells kind of like, well plant seeds a little bit, 'cause that's what it is. All of the flavor happens in that process of transformation inside the roasting machine and it is an astonishing process of transformation.”
Green coffee (raw coffee before roasting) contains significant moisture that must be dried out before browning reactions can occur efficiently, as moisture presence limits the temperatures that can be achieved.
“Green coffee has quite a lot of moisture in it and what the roasting machine needs to do first, is essentially dry it out. It's really hard to have those browning reactions take place with moisture present. It kind of limits your temperatures.”
At the point after first crack, coffee has become drinkable and of good quality, and roasters may choose to stop roasting at this point depending on their roasting vision, or continue roasting slightly longer.
“At this point, you could probably take the coffee out and drink it and enjoy it, depending on your vision as a roaster, on how you like coffee to taste. The coffee may roast a little bit longer. This point, everything is drinkable and good.”
Modern specialty coffee roasters use roasting machines equipped with multiple temperature probes positioned around the roaster, feeding data into computer software that tracks the bean probe temperature (the closest measurement to actual bean temperature), which becomes the basis for roast profile management.
“Regardless, they're all using multiple temperature probes positioned around the roaster to collect data and feed it into some sort of computer software. What that's ultimately gonna track, is the temperature of the bean probe. That's our closest guess to the temperature of the beans themselves during the roast.”
While Agtron color measurements are useful internal quality control tools, publishing Agtron numbers to consumers is of limited value because color alone doesn't tell the full story about roast quality, though significant deviations from expected color ranges indicate problems.
“I'm not sure that's very useful. It's really a good solid internal measurement. As we talked about with roast profiling, the color alone doesn't tell you all that much, but it's definitely gonna be a problem if that color is too dark or too light.”
The drum in drum roasters is typically made of cast iron and is heated directly by a gas burner underneath, which also heats the air being pulled through the machine by a large exhaust fan.
“Now the way that these work is that you have underneath here a heat source, typically it's a gas burner and that's gonna heat two things. By proximity, it's actually gonna heat the big metal drum typically made of cast iron, but primarily it's gonna heat air.”
Drum roasters produce potentially different flavor profiles compared to other roaster types, based on conventional wisdom, though the speaker expresses uncertainty about how true this distinction actually is given the variety of ways drums can be operated.
“It does produce a different kind of flavor, conventional wisdom says, to other kinds of roasters but I don't know how true that really, really is. There's a lot of different ways to use a drum roaster and you can get a lot of very different results from it.”
The speaker intends to make videos about home coffee roasting in the future as part of the master plan, but does not yet know when.
“Now, I will say, some of you are gonna ask a question and that question is, when are you gonna make a bunch of videos about roasting coffee at home? I don't know. But I will tell you that I definitely, definitely want to make those videos. They are in the master plan, the big what we will do in the future. I just don't know when.”