tags: [] - coffee/roasting - coffee/roasting/heat-transfer aliases: - Conductive heat transfer - Drum conduction

Conduction¶

Overview¶

Conduction is one of the three mechanisms of heat transfer in coffee roasting, alongside convection and radiation. It involves the direct transfer of thermal energy through physical contact — between the hot drum walls and the coffee beans, and between individual beans as they tumble in contact with each other. In a drum roaster, conduction is responsible for a significant portion of early-roast heat transfer, particularly during the drying phase when beans are in frequent contact with the drum surface. The relative contribution of conduction, convection, and radiation to total heat transfer varies by roaster design, batch size, drum rotation speed, and airflow, and roasters control this balance through drum speed and energy input adjustments to shape the roast profile.

Conduction in Drum Roasting¶

In a conventional drum roaster, green coffee beans tumble in the rotating drum and contact the hot drum walls and drum flights (the paddles or ribs inside the drum that lift and cascade the beans). Each contact event transfers heat from the hot metal surface directly into the bean. The rate of this heat transfer is governed by:

Temperature differential: The greater the temperature difference between the drum surface and the bean surface, the faster conductive heat transfer occurs — which is why charge temperature selection strongly influences initial heat transfer rate
Contact duration and frequency: Determined by drum rotation speed; faster rotation means more frequent contact but shorter duration per contact; slower rotation means longer contact per event but lower frequency
Bean surface area: Denser, smaller beans have proportionally more surface contact per unit mass

Conduction is inherently surface-biased — it heats the bean exterior more directly than the interior. If conductive heat transfer is too rapid relative to the bean's internal thermal conductivity (its ability to move heat from outside to inside), the result is scorching: the surface overheats while the interior remains relatively cool. This is one mechanism behind tipping and scorching defects.

Conduction Versus Convection¶

The balance between conductive and convective heat transfer is one of the key design variables distinguishing drum roaster models:

Transfer Mode	Primary source in drum roasting	Control variable
Conduction	Drum wall and flight contact	Charge temp, drum speed
Convection	Hot air moving through the drum	Gas burner output, airflow/damper
Radiation	Radiant heat from drum walls and burner	Burner type, drum temperature

Some roaster designs emphasise convection (fluid-bed roasters use almost entirely convective heat transfer; the Loring roaster's recirculating air system shifts the balance strongly toward convection); traditional drum roasters combine all three, with conduction typically more prominent in the early drying phase and convection becoming relatively more important as airflow and gas input are modulated through the roast.

Conduction and Roast Defects¶

Excessive conduction — usually associated with too-high charge temperatures or inadequate drum rotation speed — is the primary cause of scorching and tipping defects. When the bean surface contacts drum metal that is significantly hotter than the bean interior can accommodate through its internal thermal conductivity, localised overheating darkens and damages the surface before the interior has developed. Conversely, insufficient conductive heat transfer in the early phase (from too-low charge temperature) delays heat absorption and can contribute to a stalled drying phase.

Key Facts¶

Conduction: heat transfer through direct physical contact between hot drum surfaces and coffee beans
One of three heat transfer mechanisms in roasting; also includes convection and radiation
Rate determined by: temperature differential between drum and bean, drum rotation speed, bean surface area
Surface-biased: heats bean exterior first; risk of scorching if differential is too large
Drum rotation speed is the primary control variable for conduction intensity
Fluid-bed roasters eliminate drum contact; rely on convection; no significant conduction
Excessive conduction → scorching and tipping; insufficient conduction → stalled drying phase

References¶

Changelog¶

Date	Change
2026-04-27	Note created

This article is part of All-About-Coffee.com - The comprehensive coffee knowledgebase.