Updated August 12, 2016
There are three main forms of heat transfer that we’re concerned with in drum roasting; convection, conduction, and radiation.
Convection is the predominant form throughout the roast and is the key driver for coffee body and flavor formation. Convection heat is heat moved by a liquid or a gas and in the case of a drum roaster, we’re most concerned with heat moved by gasses present in the roaster, i.e. the air in the roaster. We have direct control over convection through heat application at the burners as well as air movement through the drum itself via fan speed, air dampers, etc, depending on the design of the particular roaster.
Because convection is the dominant form of heat transfer throughout the roast but air is also key to body and flavor development, mastery of air control is a must. Some things to keep in mind with air control:
- Air flow has a dehydrating effect.
- Water is necessary for forming better sugars later in the roast
As with everything in roasting, balance is key and experimentation is the best way to find what roast profile your particular beans need. Remember when experimenting; change only one variable at a time so you know what change had a particular effect. In the case of playing with airflow and sweetness, maintain time and temperature as normal for a given profile, but adjust airflow during one phase only and see what the outcome is. For example, if you normally have a 5:4:3 roast (5 minutes drying phase, 4 minutes maillard phase, 3 minutes roast-development phase) then maintain a 5:4:3 profile, with the same temperature curve as normal, but during the maillard phase, test using less air, then do another roast exactly the same except using more air in the maillard phase. Then cup and compare the two roasts and see which turns out better.
Conduction heat is transferred through direct physical contact. Heat travels from higher gradients to lower gradients and can occur with bean-to-bean and drum-to-bean contact. Controlling conduction is all about pre-heat. Think of a hotter drop temperature as more stored energy in the metal of the drum. As soon as beans come into contact with the drum, they begin to absorb heat directly from the drum.
Symptoms of too much conductive heat include tipping, uneven roasting, and mottled, scorched beans. Unfortunately, in a drum roaster, conductive heat is an inconsistent and unreliable method of heat transfer.
Some things to keep in mind with conduction heat:
- Conductive heat tends to produce a sweater roast
- Too much conductive heat can damage the surface of the bean and reduce cup quality
Radiated heat is the most complicated of the three forms of heat transfer and the most difficult to measure or control. Radiated heat travels at the speed of light and unlike convection or conduction, radiation requires no medium to carry it. Instead, it is emitted as electromagnetic waves. All surfaces within the roaster produce thermal radiation as do the beans themselves. Because of the difficulty in controlling or even measuring radiated heat, it’s best to simply be aware of its presence and it’s ability to affect total heat.
None of the heat transfer forms are independent of one-another. The drum will radiate heat to the air and the air will transfer heat to the bean and so on. While it’s difficult to control any single form, it is very useful to know which method is dominate at which stage of the roast.
According to Terry Davis, the following shows which heat transfer method is typically dominant during the three roast phases1:
- Drum, Air, Bean
- Air, Drum, Bean
- Bean, Drum, Air
At a minimum, you want to measure the environment temp and the temperature of the bean mass. To get the air temperature, we want a temperature probe extended into the airspace of the drum itself. We don’t want it too close to any metal surface and we don’t want it too close to the bean mass itself. For the bean temperature reading, we want a temperature probe placed so that the probe is always in contact with beans as they roll around the drum. Having temperature measured from these two locations give us good insight into what is happening inside the drum right now and the potential we have remaining, i.e. are we currently increasing environment temperature, or are we on course to maintain current environment temperature or are we losing environment temperature? Knowing this allows us to precisely reach a desired time and temperature target consistently and that get’s us half way to mastering the roasting process!
In a drum roaster, the bean temperature is the result of the environment temperature, which is the result of the burners, airflow, and heat radiated from hot surfaces inside the roaster and at later stages of the roast, the beans themselves. Roasting based solely on bean temperature is reactive. The optimal system controls the roast based on the rate of change in both environment and bean temperatures as well as the difference between the two.
In the roast development stage, just after first crack, the beans become exothermic meaning ongoing chemical reactions are producing heat as a byproduct and that heat is of course added to the total heat energy of the system. Therefore it is important to be aware of and accommodate for this new source of heat. There is infinitely more heat available at the end of the roast, which makes it very easy to lose control and overshoot your target roast. Remember your roast benchmarks as you go!
Some things to remember with heat transfer:
- Level of moisture present in green beans influences speed of roasting
- Higher airflow requires higher heat energy
Knowing the three methods of heat transfer in a drum roaster and having accurate and consistent temperature readings of the environment and the bean mass give us finite control of the roast and allow us to very accurately hit time and temperature targets. And knowing is half the battle.
Updated August 12, 2016: Added section headers.
1. Terry Davis, "The Heat is On, A Roaster's Guide To Heat Transfer," Roast Magazine May/June 2009↩