Agtron Differences and Roast Technique
A practical guide to flavour clarity versus layering
https://www.rubasseroasters.com/article_d.php?lang=en&tb=7&id=1152
Every roaster learns to hit a target colour. Fewer take the next step, which is understanding why two batches at the same Agtron can taste completely different. The answer is how heat reaches the bean core, how fast it gets there, and how evenly the surface and centre develop. This guide explains the physics, shows how different roaster designs shape flavour, and finishes with a user experiment you can replicate, complete with Agtron results.
Hero image: Article header or hero visual. Caption, “Roast colour is a starting point. Development balance does the heavy lifting.” Rubasse NIR Digital Coffee Roasters
Roaster design and what it does to flavour
Quick take: Hot air, hot metal, or near-infrared, each heats beans differently. That difference shows up as clarity or as layers in the cup.
- Hot-air dominated systems such as well-known German or US convection designs rely on even air contact. Done well, this yields high clarity, the sort of definition many roasters chase.
- Direct fire or half hot-air drums lean on contact conduction, the drum heats the surface first, the core catches up later. Expect larger surface-to-core gradients and more layered flavour.
- Short-wave near-infrared (NIR) radiates and penetrates across the bean mass. This can raise uniformity and clarity when radiation leads the heat mix.
Image: Three-panel graphic, convection, conduction, NIR. Short labels only. Rubasse NIR Digital Coffee Roasters
Two dimensions to watch, uniformity versus layers
How to read the cut bean: Small core-to-surface colour difference usually means purity and tidy flavour. Big difference means layers and complexity, sometimes at the cost of uniformity.
- High clarity and high uniformity: Minimal colour gap between surface and core. You taste a purer expression of the roast degree, fruity lift at lighter roasts or clean bittersweet at darker roasts.
- Multilayered: Noticeable colour contrast from outside to inside. You may taste light-roast fruit notes with medium-roast mellowness. Push too far at light colour and you risk grassy hints from an under-developed core, plus faster flavour drift because the wetter core keeps reacting after roast. Consistency suffers.
Image: Cross-sections showing a uniform bean versus a layered bean. Caption, “Same Agtron, different internal balance.” Rubasse NIR Digital Coffee Roasters
The physics in plain English, why speed changes the outcome
Think heat paths, then think time: Radiation and hot air tend to heat more evenly. Contact conduction cooks the surface first. Roast quickly and the core lags, roast slowly and the core catches up.
- Fast roasting: Shorter exposure means the surface finishes ahead of the centre. Expect more layering.
- Slow roasting: Extra time lets heat conduct toward the core. Uniformity and perceived purity rise.
Image: Simple heat-transfer sketch with a note on “fast equals more layers” and “slow equals more uniform”.
Applying the principles across roaster types
Use the machine you have, and steer the heat mix on purpose.
- Hot-air roaster: You can still build layers by pushing a fast roast. Some systems can fully cook in as little as about three minutes, which naturally raises surface-to-core contrast.
- Traditional half hot-air or direct fire: Lower heat and a more closed damper increase hot-air conduction time. Uniformity goes up, purity increases. Conversely, higher heat and stronger airflow raise layering.
- Infrared roaster: Designed to achieve high purity and high uniformity through NIR radiation. You can still dial in more or less layering by changing airflow, drum speed, and total time.
Image: Short table with “Machine type” and “How to emphasise uniformity” and “How to emphasise layering.” Rubasse NIR Digital Coffee Roasters
How Rubasse mixes the heat, three contributors that you can control
Why this matters: NIR never works alone. You tune the balance with airflow and contact time.
- Direct NIR irradiation and penetration at short wavelength for even energy delivery.
- Hot-air convection created by cooling the centre NIR lamp with incoming air inside a glass tube, which then exits as heated process air into the drum. Heat is re-used efficiently and lamp life improves. Fire and damper settings let you change the proportion of hot air in the total energy.
- Contact conduction from two places, the heated glass tube itself and the drum walls, which warm as radiation is absorbed and re-emitted inside the enclosed pot. Drum speed and time change how much contact heat the beans pick up.
Images:
a) NIR path and centre glass tube.
b) Convection schematic showing air through the tube then into the drum.
c) Drum surface temperature trace with the note that the drum rises by roughly fifty degrees Celsius during roast. Rubasse NIR Digital Coffee Roasters
Technique, how to aim for multilayered versus high-uniformity profiles
Dial like a pro: Adjust drum speed, exhaust, total time, and damper position to bias one side or the other.
To emphasise multilayered flavour
- Lower drum speed to increase contact time.
- Lower exhaust speed, then use a faster overall roast to deepen contrast.
- Turn the damper down to reduce convection.
To emphasise high uniformity and purity
- Raise drum speed to reduce contact-heat dominance.
- Raise exhaust speed to support a purity-first approach.
- Roast more gently to encourage fragrance, sweetness, and even development.
- Turn the damper up to increase convection.
Image: Two neat checklists side by side, “Multilayered” and “High uniformity”. Rubasse NIR Digital Coffee Roasters
A user experiment you can copy, layered profile on a digital infrared roaster
No mystery, just clear steps: Here is how one high-volume roaster built layers on purpose, complete with parameters and Agtron results.
- Coffee and batch: Ethiopia Yirgacheffe, 500 g, on a 3 kg digital infrared roaster.
- Charge and time: Charge around 150 °C, total roast about 9 minutes 43 seconds, drop near 214.5 °C, cool in about 2 minutes.
- Method: Hold exhaust, fire and drum speed through dehydration and Maillard. As first crack begins in earnest, raise the damper to vent steam, reduce fire to slow RoR quickly, and drop drum speed to about 30 RPM to increase contact time and lift the surface roast slightly.
- Result: Whole bean Agtron around 65.5 on a CM-100, ground Agtron around 82.9. The difference, roughly 17.4 points, indicates a deliberate surface-to-core contrast.
Images:
a) The recorded curve with annotations at first crack and the control changes.
b) Agtron measurements, bean and ground, shown side by side. Rubasse NIR Digital Coffee Roasters
Takeaway: Electric or NIR-based roasters are sometimes accused of lacking layers. The data here shows that when you control exhaust, fire and drum speed with intent, you can create diverse profiles, from pure and uniform to distinctly layered, on the same machine. Practice makes the difference.
Summary, use Agtron as a waypoint, not a finish line
Colour is essential, but it is not the whole story. Aim for the development balance you want, then use the tools on hand, NIR proportion, airflow, contact time and total roast time, to get there. If you want clarity, reduce the surface-to-core gap. If you want layers, widen it on purpose. Same colour, smarter flavour.
Considered as one of the region’s pioneers in specialty coffee education and Dubai cafe culture development, Ryan Godinho is an Australian entrepreneur who is accredited as the country's first SCAA AST and National Coffee Championships Coordinator. He is a frequent contributor to Forbes and Entrepreneur Magazines and also holds a postgraduate Certificate of Advanced Studies in Coffee Excellence from Zurich University (ZHAW).
