Features to Consider when Selecting a Roaster

Every roasting company has its unique list of needs and preferences when choosing a roaster, such as aesthetics, machine footprint, cost, and so on. While I can’t comment on those company-specific requirements, I offer the following technical recommendations to help you choose a roaster.

First, decide how much roasting capacity you need. Second, use a manufacturer’s stated capacity as a starting point and look up a machine’s BTU rating to estimate what its realistic capacity might be. Finally, given that every machine will have different heat-transfer efficiency, I recommend that you contact a few users of a given machine to ask about their typical batch sizes and roast times. Using those three pieces of information, you should have a good sense of the machine’s realistic capacity.

A roasting machine’s configuration probably has the greatest effect on the quality of coffee that it can produce. As I’m sure you’ve gathered by now, I recommend single-pass roasters over recirculation roasters, despite the latter’s energy efficiency. I also recommend an indirectly heated drum, or a double drum, over a standard flame-on-drum design. A single-pass roaster with a double drum or indirectly heated drum will maximize your chances of producing great coffee and minimize potential flavor taints due to bean-surface burning or a smoky roasting environment.

If you buy a classic drum roaster with a fiame-on-drum configuration, I recommend choosing a machine with a carbon-steel drum. Contrary to popular belief, most old, German “cast-iron roasters” have carbon-steel drums, not cast-iron drums. Those machines and many others often have cast-iron faceplates, drum spokes, and drum paddles, but steel drums. I have seen one machine with a cast-iron drum (a small, newer roaster manufactured in Taiwan) and one machine with a sheet-iron drum, but every other machine I’ve ever seen has had a steel drum.

Most roasting drums are made of carbon steel, but some manufacturers have recently begun building machines with stainless-steel drums; this seems reasonable, but I don’t have enough experience with them to have an opinion about their performance. Stainless steel drums may develop hot spots more easily than mild carbon steel ones, but that’s probably not a serious concern, given the drum’s rotation and an adequate thickness.

I’ve come across few roasters with inadequate airflow but several machines with poor airflow adjustment mechanisms. Ideally, your exhaust fan’s RPM should be adjustable in minute, stepless increments. Subtler airflow adjustments will produce smoother roast profiles. Machines with two or three discreet airflow settings, usually controlled manually by a damper, are acceptable but limiting. Not only are the settings usually too far apart, forcing the machine operator to compromise and choose a suboptimal setting, but the large shifts in airflow when adjusting settings may cause undesirable jumps in rates of convective heat transfer.

Some machines use one fan to draw air through the roasting drum and cooling bin. I do not recommend most of these machines; they inhibit management of the drum temperature while beans cool between batches, and they limit airflow options early in a batch if the operator is roasting and cooling simultaneously. Most of these roasters also tend to cool beans too slowly, as the single fan is rarely as powerful as other machines’ dedicated cooling fans.

Full batches should cool to near room temperature in 4 minutes or less. I recommend testing a roaster’s cooling efficiency before committing to it. Rapid cooling prevents baked flavors and loss of sweetness and allows more precise termination of the roasting process.

Beyond having adequate gas power, a roasting machine should offer steplessly adjustable gas settings. Virtually every larger machine of 30 kg capacity or greater offers stepless gas adjustment, but many smaller machines offer either stepped gas adjustment or a mere two or three settings. Stepless adjustment offers much more flexibility when an operator is trying to replicate desired roast profiles across a variety of batch sizes. I’ve challenged several manufacturers about why they offer limited gas control on smaller machines while they offered stepless adjustment on their larger machines. They have usually replied with vague references to smaller machines having “different physics,” whatever that means. So far, none have made a compelling argument for the benefit of limited gas control. I suspect that the real reason they offer limited burner control on their smaller machines is that such burners are substantially cheaper to produce, and the manufacturers want to remain price competitive in the small-machine market.

Adjustable drum speed is probably the least important of the various roasting controls, but it can help fine-tune roasts. As a roast progresses, the beans expand, which changes the way they rotate in the drum. Small, incremental increases in drum RPM will maintain ideal rotation for uniform roasting as the beans expand. Adjustable drum speed is also useful when one is roasting a variety of batch sizes.

Successful roasters today use everything from fully manual machines to fully automated machines. Regardless of how you feel about roasting technology, I recommend that you use, at the very least, a digital bean-temperature probe, a digital environmental probe, and a manometer or other indicator of gas pressure. If you are not using automated profiling software, I recommend using a data-logging aid such as Cropster^® software to track and log roast profiles. Data-logging software offers real-time graphical feedback about roast progress, profile tracking, and, in the case of Cropster, the rate-of-rise curve, which is indispensable. These programs do not control the roaster, but they provide today’s best option for real-time roasting feedback and record keeping.

This brand of software usually tracks profiles better than most, but in this example, the software abruptly changed the exhaust fan RPM in a desperate attempt to stay on profile. Most profiling software seems to make similar, extreme adjustments at unpredictable times.

Be aware that upgrading a vintage roaster to work with modern technology will be an expensive headache. The chore of replacing older gear with digital probes, solenoid valves, variable-speed motors, and so on somehow always takes longer and costs much more than anyone expected. It’s not unusual for the modifications to cost more than the roaster itself. If that sort of project doesn’t appeal to you, consider buying a more modern machine.

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