tags: [] - coffee/roasting - coffee/roasting/thermodynamics aliases: - Coffee bean thermal conductivity - Heat conduction in coffee

Thermal Conductivity¶

Tags: #coffee/roasting #coffee/roasting/thermodynamics Aliases: Coffee bean thermal conductivity, Heat conduction in coffee Related: Roasting MOC | Conduction | Heat Transfer Coefficient | Heat Capacity | Roast Density Status: ✅ Complete

Overview¶

Thermal conductivity (λ) is a measure of how readily a material transfers heat by conduction — that is, through direct molecular contact without fluid movement. Expressed in watts per metre per kelvin (W/m·K), it quantifies how quickly a temperature gradient across a material results in heat flow. In coffee roasting, the thermal conductivity of the green and roasting bean is a key parameter because it governs how rapidly heat applied to the bean surface reaches the bean core. Low thermal conductivity means the core heats slowly and lags significantly behind the surface — a phenomenon that influences roast profile design, development time requirements, and the occurrence of heat soak.

Thermal Conductivity of Coffee Beans¶

Coffee beans are poor thermal conductors relative to metals and many food materials:

Material	Thermal conductivity (W/m·K)
Copper	~400
Stainless steel	~15
Water	~0.6
Green coffee bean (approximate)	0.10–0.15
Roasted coffee bean (approximate)	0.07–0.12
Air	~0.025

Green coffee beans have thermal conductivity of approximately 0.10–0.15 W/m·K — significantly lower than water and most dense food materials. Roasted coffee is lower still (0.07–0.12 W/m·K) due to the increased void space and porous structure created by CO₂ release, cell wall rupture, and density loss during roasting. This porous structure traps air, which is an even poorer conductor (0.025 W/m·K).

Why Low Thermal Conductivity Matters in Roasting¶

Surface-to-core temperature gradient: Because the bean's thermal conductivity is low, heat applied to the bean surface by conduction or convection takes significant time to reach the core. At any given moment during roasting, the core is cooler than the surface — the magnitude of this gradient depends on how fast heat is being applied and how long it has been applied.

Development time requirements: The Development Time Ratio (DTR) and total roast time are partly determined by the bean's thermal conductivity. More time in the development phase allows heat to soak from the surface to the core more completely (see Heat Soak). A very short development phase may leave an adequately coloured surface with an underdeveloped core.

Bean size effect: Larger beans (higher screen size) have more mass per surface area; the centre of a large bean is further from the surface. With the same thermal conductivity, a larger bean takes longer to heat from surface to core. This is one reason high-screen lots (e.g., Kenyan AA, screen 18) require slightly longer development phases than smaller-bean lots roasted to the same colour.

Density and thermal conductivity: Higher-density beans (from high altitude, compact structure) have less void space and slightly higher thermal conductivity than lower-density beans. However, as roasting progresses and voids develop, the effective thermal conductivity decreases. The transition from dense green coffee (relatively better conductor) to porous roasted bean (poorer conductor) occurs during the roast itself.

Thermal Conductivity of the Drum and Drum Wall¶

The roaster drum's thermal conductivity affects how heat moves from the gas flame to the drum interior and then to the beans: - Cast iron (traditional drum material): ~50 W/m·K — good conductor; holds and transfers heat effectively - Stainless steel: ~15 W/m·K — lower than cast iron but adequate; common in modern drum roasters - Drum wall thickness affects the total thermal resistance (thicker = more resistance to heat flow despite similar conductivity)

Key Facts¶

Thermal conductivity of green coffee: ~0.10–0.15 W/m·K; roasted coffee: ~0.07–0.12 W/m·K — both are poor conductors
Porous roasted bean structure (voids filled with air) further reduces conductivity relative to green coffee
Low thermal conductivity means the bean core lags behind surface temperature; DTR and total roast time govern how completely the core develops
Larger beans (higher screen) require more time for core-to-surface heat penetration due to greater mass per surface area
Cast iron drum (~50 W/m·K) conducts heat more readily to beans than stainless steel (~15 W/m·K) drums

References¶

Changelog¶

Date	Change
2026-04-27	Note created

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