Heat Transfer – The Basics

2010 January 8
by Joe Johnston
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Roasting coffee is quite simply adding thermal energy to green coffee been over time until they have dried and then undergone a series of chemical reactions. There are three primary ways of transferring heat: conduction, convection and radiation.

Heat Transfer Modes

Conduction is the physical contact of a cooler object with a hotter object. In cooking, this would be using a pan to sear meat or scramble eggs. Thermal energy is transferred to the food via contact with the hot pan. In the coffee world, pure conduction is pan roasted coffee, still used in many parts of the world.

Convection is heat transfer using a hot gas or liquid flowing around a cooler object. In cooking, this would include a convection oven (hot air), boiling (hot water), and frying (hot oil). The greater the capacity of the fluid or gas to carry thermal energy, the faster the process (frying is much faster than convection air baking). In coffee roasting, pure convection is utilized in a hot air roaster such as a popcorn popper at home, a Sivetz or Neuhaus Neotec roaster commercially.

Radiation is heat transfer by radiant heat from a hot object to a cooler object through space. In cooking, the broiler is largely radiant heat transfer. There is no coffee roaster that I know of that works primarily using radiant heat transfer, however radiant heat transfer is present to some degree in most roasters.

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2 Responses
  1. June 6, 2010

    There are industrial conveyor belt roasters, similar to the commercial toasters where you stick a slice of bread in one side and toast comes out the other, that use only radiant heat to roast coffee. They are efficient in terms of quantity, but not so effective in terms of quality. Artisan roasters (the people not the machines) tend to believe that better results are achieved with a blend of convection and conduction heat.

    • June 9, 2010

      I agree with your premise. I have added a radiant element to the roaster as something to be able to experiment with. Since standard drum roasters have no way to vary the radiant heat transfer in a way decoupled from the drum itself, I wanted to introduce the ability to control/add a radiant component. I may find that it is not useful, but one never knows what can be discovered in experimentation.

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