During roasting, heat is transferred to coffee beans in the roaster by conduction, convection and radiation.
In drum roasters, 70% of heat is transferred by convection and 30% by conduction.
Heat transfer by conduction (touching)
Green coffee beans are first heated by conduction (direct contact) in direct heating drum roasters which transfer heat directly to the beans, i.e., heat reaches the beans as they contact hot drum walls.
Outer layer of drum is heated by burner while hot air inlets at the back of the drum allow hot air to contact with green coffee beans.
Coffee roaster’s inner temperature must reach to a certain temperature level which is preferably over 180 °C before roasting.
Right after beans are loaded into the drum, heat is transferred to the beans by conduction (touching) in the first minutes.
After beans are loaded, interior temperature level starts to drop. Most of the primary heat transfer is by convection.
Heat transfer by convection (reflection)
Hot coffee beans transferring their own heat to other beans by reflection is the heat transfer by convection.
Heat source has no direct contact with drum in hot air drum roasters. Hot air is generated in a separate heater, and heats the beans as it flows through. Surface temperature of the drum is low during roasting.
In hot air roasters, most of the heat is transferred by convection.
Heat transfer and temperature
At least the first 2/3 of roasting process is endothermic, i.e., beans absorb energy and heat moves from outside to inside the beans.
Temperature change (∆T) determines the ratio of heat transfer. Higher “∆T” value changes the heat inside the bean faster.
“∆T” value is approximately 50 °C at the beginning of roasting. This value either stays fixed or increases very slightly, then decreases as the roast cycle proceeds. In other words, the inner bean temperature increases until
it is equalised to the outer bean temperature during the first few minutes of roasting. Generally, ∆T value is at top level in fast roasting, while lower in slow roasting.
Heat and mass transfer inside the bean
During roasting cycle, moisture inside the bean starts to evaporate from the outer layer, and evaporation expands to inner layers as roasting goes on.
Cellulose matrix of the inner bean, which is cooler than the outside, remains unchanged and traps the moisture at the center. The water trapped in the bean gets warm, turns into vapor, expands and forces the bean to swell by increasing the pressure inside.
Experts’ measurement results indicate that interior pressure, varying from 5,4 atm to 25 atm, increases enough to rupture bean’s cellulose matrix, and then first pop occurs. Central heat of the bean increases while pressurized water vapor and CO2 gas escape from the crack.
Heat transfer and moisture
During roasting, moisture inside the drum as well as the bean, affect the heat transfer.
After the first delay, moisture inside the drum increases the effect of heat transfer and accelerates moisture loss inside the bean.
However, bean’s inner moisture has more complicated effects during roasting.
Three fundamental effects of high moisture levels to heat transfer are:
1. High moisture accelerates heat transfer cause it increases heat conductivity,
2. High moisture increases bean’s heat intake (absorption) capacity, which causes bean to need more energy to be heated,
3. High moisture provides more vaporized moisture discharge from the bean by blocking heat transfer into the bean.
Moist beans should be exposed to higher heat while dry beans to lower, since moist beans certainly get heated slower than the dry ones.