Drum Roasting

Almost all the coffee roasting today is carried out in a drum roaster of some type with the exception of a small number of roasters using the fluid bed roaster.  The large commercial roasters use a continuous design where the green coffee enters one end of the roaster and the roasted coffee exit the opposite end and the flow of coffee is continuous.  These roasters are designed for large volume roasting of the same coffee.  The artisan roaster of today uses the batch roaster almost exclusively because they are roasting many different types of coffee each day requiring different roast profiles and a different degree of roast.  The batches may be as small as 10 pounds and the variety may be dozens of different coffees each day.

The basic design of the batch drum roaster consists of a horizontal metal drum that rotates inside an oven where the temperature and the flow of air can be controlled around and through the drum.  At one end of the roaster is the cooling bin located below the drum where the beans are dumped and cooled after roasting.  Almost all the drum roaster ovens are gas heated but a few do operate on electric heat.  There are even some older wood fired roasters in use today.

Inside the drum of the roaster are paddles or flights that agitate the beans as the drum rotates to ensure even roasting.  The temperature inside the roaster is controlled within the desired temperature range of the roasting process and the flow of air through the drum is controlled using a damper.  The temperature needed to roast coffee is in the 425 to 500 degree F temperature range.  The lightest roasted coffee would be finished at a bean temperature of around 415 degree F and the darkest at around 470 degree F.  The roasting process takes from 8 to 20 minutes depending on the degree of roast and the desired result.

When the beans have reached the desired roast level the door is opened and the beans are dumped into the cooling bin.  The cooling bin contains a rotating set of paddles to agitate the beans while air is drawn over them to evenly cool them to room temperature.  This cooling is important to stop the roasting process.  After the roasting process the beans have increased in size by as much as twice their original size and lost 14% to 20% of their weight depending on the degree of roast.  The beans are now ready for packaging and shipping unless other processes are intended such as grinding or flavoring before packaging.

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