In cooking the heating process is usually prescribed by the recipe. Yet, there are reasons why certain chemical reactions are promoted by heating method and temperature. Common sense tells us that a Blow Torch is not a good heat source for warming the body. The high temperature differential means that the outside layer will burn before the inside is warm. Perhaps the Blow Torch salesman would suggest that the body whirl like a Dervish near the flame to distribute the heat.
In "Espresso Coffee" R. Eggers writes the typical gas temperature of a drum roaster is 400° - 550°C. The high temperature cooking analogy for beans hit by the super hot air is frying eggs while using the stoves highest heat setting. To this end the Roastaire uses a fluidized bed in which 240°C (adjustable as required) air is pushed 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.
Considerable research has been done to understand the dynamic and continually evolving variables in the coffee roasting process. In the design phase, of the Roaster, the limitations of the bean and the requirements to start the chemical reaction, referred to as roasting, determine the roasting precepts. We do not believe in using extreme 400°C plus air temperature that mostly moves around the batch of beans. The beans will burn on the outside before it is hot inside. Agreed the beans are in motion and only subjected to intermittent burning temperatures. The average may be arithmetically acceptable but then so is keeping half your body in ice while the other half is in boiling water. Perhaps some manufacturers slavishly follow this century old trend because it is easy and obvious.
The starting point of the exothermic reaction ( above 180°C according to R.Eggers and at 200°C according to Clarke & Macrae). We commonly use an air temperature of 240°C delivered under pressure so that the air is forced through the batch without initially losing more than a few tens of degrees. The heat transfer to the beans, is slightly higher at the beginning of the roast when the beans are denser and able to transmit more heat internally. Initially the heat absorbed serves to evaporate the moisture which determines the heat slope (how quickly the beans increase in temperature). The decrease in bean density reduces the heat absorption by the beans hence the air temperature is rather uniform for the first and last layers. Actually when the exothermic reaction begins the heat generated in the first layers raises the temperature of the beans that are down stream. Delivering the air with sufficient pressure at 240°C is a challenge that will be addressed in a later post. Sufficient to say that no off the shelf hot air blowers were found, resulting in another product development.
The Blower's pressurized air heats the beans and mixes them with a "bubble action" which helps reduce the contact of beans and the hot metal sides of the Roasting Chamber. Some home roasters tout the advantage of their fountain mixing of the beans which is not optimal for roasting. A fountain is created when the air flow is concentrated in one area which means that the air is not flowing through the rest of the beans i.e. not heating them evenly.
Time for a freshly roasted coffee. Agree?
