Your roasting-machine salesperson will likely claim that you can roast 15 kg per batch in a 15kg machine. The job of a salesperson is to sell machines, not to help you roast the best-possible coffee, so take any claims lightly. The salesperson may be technically correct, because the drum can surely fit a 15-kg batch, but a full batch may take 15:00–20:00 to roast, which is longer than ideal. If quality roasting is your goal, it’s usually safe to assume you will roast 3–3.5 lf batches per hour at 50%–70% of a machine’s stated capacity. Then deduct the 14%–20% weight lost per batch to calculate how much roasted coffee per hour a machine can produce.
For example, if one were to roast three and a half batches of 7 kg green coffee per hour in a Diedrich IR-12, with an average weight loss of 15%, the machine would produce just under 21 kg of roasted coffee per hour (3.5 * 7 kg * .85 = 20.8 kg). That’s more realistic than assuming the machine will roast 48 kg per hour.
Some machines are more reliable than others. Machines with fewer parts, fewer high-tech features, and heavier builds tend to be more durable and reliable. Older, simpler roasters, such as the fabled UG-series Probats are examples of rugged, low-tech machines built to last. Of course, all design decisions entail tradeoffs. Some modern technology may lack reliability but make quality roasting easier and more repeatable. Again, ask other users about reliability before buying— I’m sure most roasters would be happy to share their experiences, especially if they have complaints! While roasters are not always objective about their own roast quality, they tend to be somewhat objective about the reliability of their machines.
Many brands may not offer service or support in your country. Further, some companies offer poor support once you have paid for your roaster. I won’t publicly discuss which companies, in my experience, neglect their customers, but I implore you to ask other users of a brand about service quality before you put a deposit on a machine. Even if you have a pleasant initial sales experience with a company, that does not guarantee future service quality.
This may seem like a trivial consideration, but if you’re going to spend 20–40 hours per week using a roaster, a well-designed user interface is important. The interface isn’t just about convenience and comfort, it can also affect roast quality and repeatability. For example, machines that require you to repeatedly tap an up- or down-button to change the gas setting can be tedious and slow to respond. In comparison, a machine with an analog gas dial or a smart touchscreen is more responsive, makes it easier to replicate curves, and can be a pleasure to operate. Other ease-of-use considerations involve having large, well-positioned digital manometers, timers, and temperature readouts.
You may want to consider aesthetics if you are installing a machine for use in a retail cafe or other public space. A beautifully refurbished vintage machine may make a nicer impression than a budget, modern machine.
I discussed this topic in detail in The Coffee Roaster’s Companion, but will repeat the basics here. Common architectures include classic-drum roasters, indirectly fired roasters, recirculating roasters, and fluid-bed roasters. Each design has pros and cons.
Classic drum roasters: In these machines a drum rotates above a gas flame, and a fan pulls hot air from the burner through the drum and out of the roaster. Most smaller machines are classic drum roasters. Classic drum roasters get the job done, though many models provide too much conductive heat transfer, due to having a thin single-walled drum or an improper distance between the burner and drum. If too much heat is transferred to the beans via direct contact with the drum, coffee will taste harsher and less delicate. If you choose a classic drum roaster, I suggest you seek a machine with a double-walled drum and a burner with sufficient btu/hr (or kj/hr) for your needs. Compared to other designs, classic drum roasters offer good thermal stability but slower responses to gas changes.
Indirectly heated drum roasters: In these machines the burner chamber is separated from the drum and hot air passes from the burner chamber through the drum. The design allows the drum’s surface to remain cooler because the flame is not in contact with the drum. Indirectly heated roasters are more difficult to control than classic drum roasters, because they require skillful management of airflow, while classic drum roasters rarely require much, if any, airflow adjustment.
Recirculation roasters: These machines recirculate a portion of the roasting exhaust air back through the burner and roasting chamber. Such machines are energy-efficient but often run the risk of imparting smokey or polluted flavors on coffee. To avoid smoke taint, it’s important to heat the recirculated air to a sufficiently high (afterburner-level) temperature before passing it through the drum.
Fluid-bed roasters: These machines rely on a bed of rising hot air to circulate the beans and keep the beans aloft. Fluid-bed roasters eliminate the risk of conductive-heat damage, and are usually capable of developing beans well in short amounts of time. While there is no theoretical downside to fluid-bed roasters, in practice their control systems are usually too simplistic to fulfill the machines’ potential. Given the current, rapid evolution in roast-control and data-logging software, I expect the utility and popularity of fluid-bed roasters to grow rapidly in the near future.
None of the features listed below are necessary to roast a good batch of coffee, but each may contribute to improved roast quality or repeatability.
Double drum (applies only to classic drum roasters) and powerful burner: The foundation of a good classic drum roaster is its burner and drum. As noted previously, burner output determines a machine’s true capacity. Double drums allow for faster and hotter roasting with less risk of tipping or scorching. Make drum quality and burner output your first two concerns when choosing a classic drum roaster. You can easily replace or upgrade fans, valves, ducts, etc, but you cannot easily replace a drum, and upgrading a burner can be expensive.
Variable-speed-drive (VSD) fan: As long as your roaster’s fan provides a reasonable amount of draw, you don’t need a variable-speed fan to produce good roasts. But without a VSD fan, it’s impossible to maintain consistent airflow levels day to day. The combination of a digital air-pressure manometer and a VSD fan is essential for expert-level roast repeatability.
Air manometer (aka drum-pressure manometer): A manometer in the duct between the roasting drum and exhaust fan is a relatively new, worthwhile addition to a roaster. The manometer reads pressure, not flow, but that pressure reading correlates with airflow. Using the same fan setting every day does not ensure consistent roasting because airflow may vary day to day with the weather and other factors. Having an air-pressure manometer helps one know how to adjust the fan to provide consistent airflow every batch. (Note: directly measuring airflow requires installing probes in the exhaust duct, but the probes get dirty too quickly during roasting to work effectively. Using an air-pressure manometer is the best current option to monitor and maintain consistent airflow batch to batch. However, the relationship between pressure and flow will shift slowly as the ducts get dirty, so frequent chimney cleaning is critical.)
High-resolution gas manometer: Most roasting machines come with small, cheap analog manometers that offer imprecise gas-pressure measurements. I recommend replacing your stock analog manometer with a high-resolution digital manometer. Analog manometers may be aesthetically pleasing, but they make discernment of precise readings too difficult.
Proper probes and probe locations: To be a great roaster by today’s standards, one needs better green, lighter roasts, quality data collection, precise controls, and software to track and analyze the data. To ensure adequate data collection, insist on having a bean probe and an environmental probe, each with diameters of 2.5 mm– 4 mm. An inlet-temperature probe is helpful but not critical.
The optimal bean probe location in most machines is as follows:
The probe’s tip should be 3–5 cm from the inside of the machine’s faceplate.
The probe’s tip should be 3–5 cm from the inner drum edge. (2 cm is ok for machines with capacity of 1 kg or less.)
The probe tip should be in the heart of the bean pile, even when roasting very small batches. If the probe is too high in the drum or too close to the center axle, it may not be immersed in the bean pile of very small batches. Proper probe location should provide quality data for batches as small as 20% capacity.
Paying for a machine: Manufacturers typically require the buyer to deposit 50% of the machine’s price upon ordering, with the balance due upon shipment of the machine. The problem with such arrangements is that once a manufacturer has your deposit, he or she may lose motivation to deliver your machine on time. Salespeople routinely promise a machine in three months, secure a deposit, and then ship the machine six to nine months later, claiming unavoidable delays. The buyer is helpless as he or she pays rent on an empty roastery and loses money waiting for the machine to arrive. I have seen such delays happen on fully half of my clients’ orders. I strongly suggest insisting on a sales-contract clause guaranteeing delivery by a certain date, with a penalty against the manufacturer for late delivery.
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