During roasting in coffee roasters, heat is transferred to the beans by touch (conduction), reflection (convection) and radiation (radiation). In boiler coffee roasters, 70% of the heat transfer takes place by convection and 30% by conduction

Heat transfer by touch (conduction)

In direct transfer boiler coffee roasters that transfer heat directly to the coffee, the green coffee beans are first heated by conduction. This means that the heat reaches the coffee beans through the hot boiler walls. The burner flame heats the outer layer of the roasting vessel. At the same time, hot air vents at the back of the vessel allow hot air to come into contact with the green coffee. Before the roasting process begins, the coffee roaster must be brought to a certain temperature. This temperature should preferably be above 180 °C. In boiler coffee roasters, the heat from the hot boiler is transferred to the beans by conduction (by touch) during the first minutes of roasting, immediately after the beans are placed in the boiler. The temperature inside the boiler drops after the green coffee is placed in the boiler. Most of the heat transfer in this first moment is by convection

Heat transfer by reflection (convection)

The transfer of heat from the heated coffee grain to other grains is called heat conduction by reflection. 1 In boiler roasters heated by hot air, the heat source does not directly contact the boiler. Hot air is produced in a separate section, in the heat boiler. The hot air passing through and between the green grains in the boiler heats the grains. The boiler temperature is lower during the roasting process. Most of the heat transfer in coffee roasters with hot air systems takes place by convection

Heat transfer and temperature

At least the first two-thirds of the roasting process is endothermic, meaning that the beans absorb energy. The heat they contain also moves from the outside of the core to the inside. The heat exchange (∆T) highly determines the rate of heat transfer. The higher the ∆T, the faster the temperature of the inner core changes. The ∆T at the start of roasting is about 50 °C. This value either remains constant or increases slightly and decreases as the roasting process continues. In other words, the core’s internal temperature increases during the first few minutes of roasting until it equals the core’s external temperature. In general, “∆T” is highest in fast roasting processes and lower in slow ones. 1

Heat and mass transfer in the nucleus

During roasting, the moisture in the coffee bean evaporates starting from the outermost layer and as the roasting continues, the evaporation spreads to the inner layers. The cellulose structure of the inner bean, which is cooler than the outer layer, remains intact and traps the moisture in the center of the bean. This trapped water heats up, turns into steam, expands and increases the pressure inside the core, forcing the core to expand. This pressure, measured by experts and found to vary between 5.4 atm. and 25 atm., increases enough to disrupt the cellulose structure of the core and finally the first explosion (pop) occurs. As pressurized water vapor and CO2 gas escape through the crack formed in the explosion, the core’s core temperature increases. 2

Heat transfer and humidity

During roasting, both moisture in the roasting kettle and moisture in the kernel affect heat transfer. After the initial delay, moisture in the roasting kettle increases the effect of heat transfer, accelerating moisture loss from the kernel. Moisture in the kernel has more complex effects during roasting.High moisture content in the kernel has three main effects on heat transfer:1. It acceleratesheat transfer because moisture increases thermal conductivity, 2. It increases the heat uptake capacity of the core, which means that more energy is needed to raise the temperature of the core,3. It inhibits heat transfer to the inner core, allowing more vaporized moisture to be ejected out of the core. Moist cores definitely increase in temperature more slowly than dry ones. Therefore, machine operators should apply heat more intensely when roasting moist kernels and use more discretion when roasting dry kernels