Radiation vs Convection in Coffee Roasting, What Really Changes in the Cup
https://www.rubasseroasters.com/article_d.php?lang=en&tb=7&id=1026
If you care about flavour, clarity and repeatability, start at the heat path. Near-infrared radiation does one useful thing most systems cannot, it penetrates and heats the bean more evenly, surface and core together, which changes what is possible later in the roast. This article/guide explains the principle, then walks through a Yirgacheffe natural “runaway” profile that looks outrageous on paper yet cups outrageously delicious once rested.
Hero image: Page header from the source. Caption, “Direct radiation heating, what changes in the drum.” Rubasse NIR Digital Coffee Roasters
Radiation roasting, why it matters and how it works
Conduction sears the surface first, convection warms by air contact, radiation reaches deeper, so the centre does not lag as much.
Radiation is the only heat-transfer mode that is inherently penetrative. For coffee, that means you can pursue even development without “searing” the outside while the core plays catch-up. Rubasse builds around near-infrared as the radiant source, positions the emitter at the centre of the drum, and shapes drum and airflow for efficient capture. The intent is simple, more of the energy you supply is absorbed by the beans, less is wasted heating air and drum mass.
Place image: The internal NIR element view or NIR explainer visual. Short caption, “NIR selected for absorbance and responsiveness.” Rubasse NIR Digital Coffee Roasters
Design notes you should know:
- Near-infrared is chosen for strong grain absorbance and very fast response.
- Placing the source at the drum axis, then tuning airflow and drum geometry, improves energy efficiency versus conventional gas systems.
The experiment, a natural Yirgacheffe and a “runaway” profile
Set the scene: The profile breaks a few unwritten rules, then earns its place in the cup after rest.
Green coffee
Gedeb, Yirgacheffe, Ethiopia. Heirloom. Grade G1. Natural process.
Place image: Before and after roast photo of the Wote Konga lot. Rubasse NIR Digital Coffee Roasters
Roast overview
- Charge at roughly one quarter of drum capacity.
- First crack near 8:00.
- Development time about 30 seconds from crack to drop.
- Entering first crack with an RoR around 18 to 20 °C per minute.
- Strongly increase airflow toward the end, to roughly 60 on the scale, where similar loads might sit near 30 to 34.
- Colour results, Agtron whole bean about 77.2 and ground about 90.7.
Place image: The recorded roast curve and settings summary. Caption, “High late-stage airflow, short development, high RoR into crack.”
Why it looks wrong on paper: Conventional wisdom reads “ashy outside, raw inside” when development is that short and airflow spikes late. The surprise comes after rest.
Sensory results, day one and day seven
How it tastes matters more than how it looks: Gas will mask the picture on day one, so always re-cup.
- Immediately post-roast: White florals, blueberry, peach-like fruit, toffee, medium body with a velvety impression, some gassy artefacts, and a hint of caramel-popcorn note that suggests possible slight underdevelopment.
- After seven days’ rest: Gas clears, vibrancy increases, flavours sharpen, body stays full yet clean, tropical notes emerge, florals intensify.
What changed in the heat mix, and why it works
Read the controls with physics in mind: The late-stage strategy is to cut convective and conductive dominance while amplifying radiant input.
- Airflow raised dramatically near the end: This cools the bulk air and drum metal, so their contribution drops.
- NIR source driven near maximum: Radiant energy keeps the beans developing even as air and metal contribute less. RoR remains high through end-game.
- Drum rotation sped up: Beans spread and turn more, so exposure to the centred radiator is more uniform.
Place image: Schematic showing inner versus surface temperature convergence with radiation. Caption, “Radiation lets the core catch up faster.” Rubasse NIR Digital Coffee Roasters
The key mechanism: For similar surface temperatures at the same timestamps, inner-bean temperature rises faster when radiation leads, so the “done” point is achievable in less calendar time without the classic green or raw-nutty defect. This opens new ways to balance flavour, acidity, and body, especially on fruit-forward naturals.
Practical guidance if you want to try it
Approach this like a disciplined test, not a stunt.
- Log a stable reference first, same coffee, standard profile, unforced end-game. Taste and file.
- Replicate the “runaway” idea in small steps, lift airflow late while nudging radiant power up, and shorten development progressively. Taste at 24 hours and at 7 days.
- Watch RoR into crack, aim near 18 to 20 °C per minute if your machine permits, then control crash or flick with airflow rather than extra heat.
- Use colour as a waypoint, not the finish line. Note whole-bean and ground Agtron on both versions, then read the flavour differences alongside.
- Adjust drum speed for exposure, a little more RPM can spread the mass for better radiant contact on axis-lamp designs.
Takeaways you can bank
The short version: Radiant-led roasts can reach internal “done” faster, which lets you compress development without tasting raw.
Radiation’s penetrative nature reduces the surface-to-core gap at a given timestamp, so you have more latitude to design late-stage profiles and still cup clean. It does not replace convection or conduction, it shifts the balance to suit the flavour you want. For this Yirgacheffe, the bolder, late-air, high-radiation end-game produced a more vibrant and defined cup than the shop’s conventional approach, particularly after sufficient rest.
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).
