Drum roasting requires well developed skills because there is an extended lag between the moment the RoastMaster views the beans in the Bean Sampler and the termination of the exothermic reaction. Several factors affect the degree of roast in a drum roaster; the air temperature is considerably higher than what is required for roasting ("Espresso Coffee" R. Eggers writes the typical gas temperature of a drum roaster is 400° - 550°C. ), using a Bean Sampler is a relatively slow procedure, and the Cooling fans are relatively inefficient at moving air through several layers of beans. Sucking the air downwards through the beans is also bad design because it packs beans together which greatly hinders air flow. Many drum roasters spray water on the beans to cool them more quickly. This is not good nor is it a measured cooling method.
The high temperature cooking analogy for beans hit by the super hot air is cooking Scrambled Eggs while using the stoves highest heat setting. Even if they are continually mixed the heat is much higher than the egg can transmit to the interior. The higher heat of the drum roaster still yields a relatively slow roast which highlights the inefficient transfer of energy. For more details see "NOT Sustainable Roasting" and "Heating Coffee Beans to Roasting Temperature"
It is said that every second counts when "calling the roast" yet it takes many seconds to determine the degree of roast using a Bean Sampler especially if multiple readings have to be taken to follow the progression.
Many roasters are stressed by "calling the roast" even after they have been roasting for nine months. The beans continue to roast during the dumping process and at the beginning of the air cooling cycle. It is a common drum roasting technique that water is sprayed unto the beans to soak up heat. It is impossible to accurately meter water coming from a garden hose. Moisture is the #1 enemy of roasted coffee. That lingering roast period means the drum RoastMaster must anticipate the roast quite a bit in advance of the required roast level. An appropriate analogy is shooting ducks; the expert hunter aims for the position where the duck will be when the buckshot arrives. The hunter must compensate for altitude, shell powder load, temperature, duck speed, and probably many other things that only a good hunter would know. For various reasons a lot of people have a problem with duck hunting (no ducks were harmed for this analogy). The drum RoastMaster does not deal with less variables.
To this end the NXT aka Roastaire uses a fluidized bed in which 240°C (adjustable as required) air is pushed, under pressure, through the beans. This significantly increases the heat transferred to the entire batch of beans. The beans have a limited ability to absorb the heat therefore the temperature gradient is more uniform throughout the batch with the forced air stream. The added benefit is that the air flow continually churns the beans and carries away the chaff.
The beans are cooled within the Roasting Chamber therefore "calling the roast" is done in real time. The blast of cooling air also churns the beans so that there is even and rapid cooling. The Roast Monitor communicates to the RoastMaster the roasting progress so that the Cooling Cycle is started at the proper moment. It only takes a few seconds to cool the beans ( below 180°C according to R.Eggers and below 200°C according to Clarke & Macrae) to stop the roasting process. After this 30 second Cool Mode the beans are transported to the Packaging Hopper while the NXT aka Roastaire loads nother batch freeing the RoastMaster to appreciate a freshly roasted coffee.
