There are several aspects to be considered before a process can be implemented to improve a design. Often it is possible to identify, let's say for the sake of discussion, 10 variables that will affect the process. What is usually impossible to predict is the rank or the level of dominance of each variable unless you are really really really a gifted analyzer or clairvoyant. Personally I resort to testing a few variables at a time to better understand their effect. There will undoubtedly be surprises and inconsistencies when various variables are tested together but it is simpler to test in parts than in whole.
The testing results are unlikely to produce a clear graph such are joys of solving problems with multiple unknowns. The results will give a better idea about the trade-offs that are required before a process can be adopted. We maintained that coffee beans should be uniformly heated so that the chemical reactions that occur within the beans progress evenly. It was also deemed important that the beans not be thermally overloaded. To clarify; the bean has a certain capacity to transmit heat to its interior, if the heat applied is greater than what can be evenly distributed the exterior surface becomes much hotter and creates a risk of scorching. It is also for this reason that we roast at a relatively lower temperatures of 240°C to 250°C instead of ... 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. Yet this fluidized bed roaster has a cycle time of less than 12 minutes versus up to 20 minutes in a drum roaster. That is Better Efficiency!
The even distribution of heat is critical to preparing the beans for the roasting process. We force heated air between the beans so that they are all heated, not just the outside layer. There are small domestic roasters that also use air to create a fountain of beans. It is quite dramatic but if the air flows through a narrow passageway then the remaining beans are not being heated until they gradually make it to the fountain. Consequently, if you are looking for a home roaster look for one that pushes the air through all the beans. Typically, the batch will move upwards as the beans are levitated by the flowing air then the air will break through and mix the beans. The release of air pressure allows them to drop within the chamber but are then suspended again. The air cushion has the added advantage of keeping the beans off the hot metal which often causes tipping which appear as little pieces of the exterior wall that leave behind a shallow cavity.
It is a simple concept that is regulated by many variables which turn it into a complex process.
There is more to the efficiency quest than even thermal transfer and the recycling of air mentioned in the previous blog which leaves something to write about later. Now it is time for an espresso.