Having a firm grasp on the concepts of heat energy and its transfer requires an understanding of the various methods through which this energy is passed from the roasting machine to the coffee beans.
Heat energy is transferred through the processes of convection, conduction, and radiation. Convection is the heat transfer through the mass motion of a fluid, such as air, as this heated fluid moves away from the source of heat and carries heat energy along with it.
Conduction is the transfer of heat arising from the direct contact between particles, and can be thought of as the transfer of heat energy through the touching of matter without a bulk motion of that matter.
Radiation is the transfer of energy in empty spaces, through the emission of photons or electromagnetic waves, mainly in the frequency range of infrared radiation.
In coffee, most of the measurable energy transfer from roasting machines is done through conduction and convection, with the energy generated from radiation too complex and difficult to measure for practical consideration. Often, a combination of airflow and speed of drum rotations are the two aspects one would adjust to manipulate the convection and conduction energy transferred from the roasting machine to the coffee bean.