Today I received a Drum Roaster's Newsletter which was interesting since I would have thought that they know the topic of coffee roasting thoroughly. The first sentence informed me that drum roasters haven't really changed over the years. That does give the impression that there is more interest in marketing than innovation. As an outsider I think they should be devoting a lot of resources to curbing their energy waste. Using a 400,000 btu to 1 million btu Afterburner on a relatively small coffee roaster is wasteful. Perhaps it was innovative decades ago but the improvements since then have hardly been ground shaking. That leaves only marketing spin such as writing that the drum roasters are for those that enjoy their artistic side of roasting. Never quite thought of it that way, but I can understand that with one of those Afterburners the Roast Master would not want to dwell on the accounting side of roasting. So much heat is going up the chimney that a costly high temperature stainless steel chimney is required.
Apparently, so it says, "You can’t roast fast and dark at the same time without risking an exothermic reaction — you have to slow down the entire process to fully develop the flavor of the beans." The first half of the roasting process is endothermic,the bean is absorbing heat to evaporate the moisture. The steam pressure, I am told, swells the bean before escaping into the surrounding air. To recap; beans swell and the beans lose the weight of the evaporated moisture ( varies with humidity and darkness of the roast) which results in a significant decrease in density. The bean temperature then increases more rapidly because the moisture is essentially gone. Pyrolysis, it is written, occurs in a oxygen free atmosphere which describes the interior of the bean. It is during this time that there is a caramelization of the sugars. This reaction is exothermic therefore it has to be monitored closely in order to stop it at the proper moment with a lot of cool air. The high temperature of the drum roaster air is required to convey the required energy because the air only meanders through the beans. That results in a rapid decrease in air temperature as the first layers absorb most of the heat because of the large temperature differential. The danger here is if the bean is exposed to more heat than it can absorb there will be some burning of the outer layer especially if there is a large temperature differential and/or significant conductive heating. Mixing the beans attempts to average the amount of heat absorbed by the beans. One of the challenges is "calling the roast" when the circulating air is about 300°C hotter than the starting point of the exothermic reaction ( above 180°C according to R.Eggers and at 200°C according to Clarke & Macrae). When coupled with using the Bean Sampler to establish the level of the roast plus the time required to dump the beans into the cooling tray it is understandable why many roasters are still very stressed by the experience even after they have been roasting for nine months. An appropriate analogy is shooting ducks; the expert hunter aims for the position where the duck will be when the buckshot arrives. For various reasons a lot of people have a problem with duck hunting.
Like a typical chemical reaction the bean waits for the input conditions to be satisfied before it "goes exothermic". If voltages (or gas pressure) is lower, ambient temperatures are colder, humidity levels are higher it will take longer to reach the trigger point. A very light roast can be stopped but it will be classed as "not fully developed" and not considered optimal. If the air/vessel temperature is too low the bean will be baked rather than roasted. An interesting aside is an experiment done in the 90's (unintentionally) where the beans were baked then the oven was shut off without force cooling them, about 20 minutes later there was an exothermic reaction that created a mushroom cloud at the 18' ceiling. It was sort of induced or at least encouraged spontaneous combustion.
Perhaps one of the reasons why it is suggested that the roasting process has to be slowed down is that their roaster cannot transfer the heat to the beans any faster. Air has a low thermal density and if it only moves slowly through the beans by convection it is not very effective at heating beans. Increasing the temperature will scorch the outside before the inside is hot making the mixing action all important. 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.
Another marketing spin of the drum roaster claim that it is advantageous that 15% of the heat is transferred to the beans via conduction. This can be bean to bean contact but the text specifically says "The beans stay in constant contact as the drum turns..." which in my experience is the main cause of tipping. In" Coffee Technology Vol. 2" Clarke & Macrae write "Decreasing air temperatures and decreased proportion of conductive heat is a feature of recirculatory-type roasters ..."
Reading the newsletter is good training for critically reading the newspaper.
