Drum vs. air is the coffee roasters' version of a partisan political debate as most engaged already have their minds made up, and no amount of debate will dissuade them from their strongly held views. But, occasionally someone will approach the question with an open mind, cut through the rhetoric, and make an informed decision based on the exigencies present in their own business and relative strengths and weaknesses of both systems. This attempt at defending the much aligned drum is for those intrepid few.
Drum = full conduction
This misconception is perhaps the most damning of all fallacies associated with drum roasters, and it is often used as the basis for many other arguments against the drum. Most modern, well-manufactured drum roasters transfer most of their heat through convection. It is true that conduction exists in drum roasters--both drum to bean and bean to bean (which also occurs in air roasters and in both cases later in the roast)--but it is neither the only form of heat transfer nor the most dominate. Convective heat transfer ratios on drum roasters generally fall in the 4/5 range, convection to conduction, depending of course on drum material and airflow dynamics (which can be very negatively affected by either poor installation or poor maintenance). But, for clean and well installed drum roasters, these numbers are solid. This means, in short, that most of the heat transferred to the coffee in a drum roaster is decidedly not transferred through the drum wall (conduction). Instead, it is transferred through airflow (convection). So, if drum roasters are not 100 percent conductive, then that allows us to debunk two further arguments advanced by the air-only crowd:
- Weekly drum cleaning to eliminate tipping, and
- Carcinogenic chemicals attached to coffee through the carbonization of chaff.
Since most of the heat is indeed transferred through airflow then there must be a significant amount of air that moves through the drum --and there is. Even table-top drum roasters can approach a CFM (cubic foot per minute) rate of air movement of 150 or so, depending on size. For drum roasters, this would be negative pressure. That means that the air through the drum is sucked not blown. In nearly all brands of drum roasters, the air moves through the drum only once, not twice, taking smoke and chaff with it toward the blower. If drum roasters did not use a significant amount of air then those of us that use drum roasters would likely have to wear gas masks as our spaces would be full of smoke, not just from the coffee roasting process but from the burning chaff as well. Additionally, if drum roasters burnt all the chaff off in the roasting chamber then we would have no need for chaff collection systems. This is just not so. So, if the smoke produced in a drum roaster is pulled away from the coffee as the coffee roasts and the chaff is pulled away as well, then the claim by air roasters that drum roasters are cancer-causing is false. Not only is the cancer-causing claim false, it is untested and therefore unverified and dangerous to our industry. As for tipping, if the majority of heat transferred through a drum roaster were indeed conductive then perhaps this would be a problem. It isn't and it isn't. I know of no drum roaster that cleans his drum once a week. In fact, I can think of no drum roaster operator that includes drum cleaning in any of his periodic maintenance--not weekly, monthly, nor yearly--because a dirty drum is just not an issue. Tipping is not an indicator of an unclean drum, it is in fact an indicator of a dirty blower causing the ratio of convection to conduction to change. And, it is true that a drum roaster blower must be cleaned periodically, as all blowers should, so as not to degrade performance. I truly believe that many air roaster advocates have confused drum roasters with ball roasters. I say confused because surely they would not intentionally malign the mighty, durable and versatile drum roaster, which has effectively made this industry, right?
Drum roaster offense
Drum roasters, while tracing their lineage back about 120 years, are hardly tools of the past. In fact, a case can easily be made that many modern drum roasters are actually "younger" than air roasters. This is especially true in the areas of profiling and control. In its basic design, the drum roaster has the flexibility that allows for an almost infinite number of roast profiles, producing many different taste permutations from the same coffee. This flexibility (read variability) is pushing roaster manufacturers to try and harness the full range of these profiles with more and more sophisticated roast control systems. The variability of the drum and the control of PC and PLC based systems is making profile roasting absolutely repeatable and consistent across a much wider range. Air roasters--being inherently more consistent in their roast patterns--do not have this flexibility. Furthermore, the competition between the 20 or so drum roaster manufacturers worldwide has and is leading to many positives for the buyer and operator of drum roaster, such as:
- Separate cooling blowers
- Standardization of bean probes and digital controllers
- Easier maintenance
- More and better training opportunities for drum roasters
- Advancements in control systems and pollution control
The six-minute roast
While it is true that it is difficult for most drum roasters to do a consistent six-minute roast, the bigger issue is, "Why would you want to do a six-minute roast?" Roasters in the specialty end of the coffee industry have known that roasting too quickly does not allow the roasted coffee to fully develop. For coffee, the sweetest spot appears to be in the 15 to 20-minute range, depending on equipment and what flavors a roaster wishes to coax from a given green coffee. Six-minutes, eight-minutes, 10-minutes are all too fast for proper bean development. Just because something can be done quickly doesn't mean it should be done quickly. The question that should be asked is, "What is the best time and profile for the coffee I wish to produce?" Coffee quality is the issue here, not roast time. There are many good companies roasting great coffee on air roasters, just as there are many companies producing great coffees on drums. Both of these roasting systems are capable, productive and proven. They both help to ensure the continuing success of our industry as a whole. While it is impossible to tell what will happen in the future or what new roasting technology will arise to challenge both air and drum roasters, one thing is for certain: drum roasters and drum roaster manufacturers will continue to improve through competition and innovation.
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