Pros and cons of various roast-control systems - Scott Rao

EXHAUST-TEMPERATURE CONTROL

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Managing roasts based on exhaust or environmental temperature is conceptually backwards. It is akin to driving by looking in the rear window of the car; it may work sometimes, but I wouldn’t trust it. This control system has been the standard on the Ikawa for a long time, it is an option on the Roest (that fewer and fewer people use), and it has been used in some third-party roast-automation software. 

PID CURVE MANAGEMENT

PID Curve Management has potential, but no one has fully realized that potential yet. Kaffelogic and Loring’s “profile roasting” automation use PID curve management, and Artisan software can manage curves using a PID. The idea is the software will make countless small adjustments to the power settings in an attempt to replicate a reference bean-temperature or ROR curve.

PID control has a few challenges. For one, if a roast falls behind the reference-curve target, it is critical that the machine has ample “extra” power to quickly get back on track. In the case of the Loring automation, for example, if you see the machine stuck at either the lowest or highest power setting for more than a few seconds, it means the machine is struggling to match the reference curve. A second challenge is that the optimal PID settings change throughout a roast: ideally, one would want a very “aggressive” PID during the first minute or so of a roast, when temperature changes are rapid and dynamic, and a “mellower” PID mid-roast, before again needing an aggressive PID to manage the rapid changes in temperature and moisture release during and after first crack. A third challenge is how to deal with the (fake) declining BT readings at the start of a roast, since the BT reading is not accurate until after the turning point. There is no easy answer to how to program an automated PID system to manage a curve while the BT data is both inaccurate and also changing rapidly.

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Fluid-bed roasters are efficient at transferring heat to the beans, and they allow for the fastest roasts of any type of roasting machine. The large volumes of hot air lift and mix the beans, making a lot of heat available to them. . Because the beans do not touch any surfaces for long, there is virtually no risk of bean-surface burning, allowing the use of high air temperatures. Furthermore, according to Goldfine (), high airflow blows away the layer of air immediately surrounding each bean, which would otherwise insulate the beans and reduce heat transfer.

Fast roasting reduces production time, but it also has distinct effects on the structure of the roasted coffee and the resulting flavour in the cup, which we will discuss fully in Part 2 of this course. While high air-to-bean ratios allow faster roasts, excessive airflow may strip away some of the aroma compounds or dry the beans too quickly, resulting in flat or dull coffee (Schenker ). With a fluid-bed design, the airflow is determined by the speed needed for fluidisation, so it’s not possible for a roaster to reduce the airflow to alter the roast profile.

A fluid-bed roaster with a 50 kg-capacity at Zarraffa’s Coffee in Queensland, Australia. Photo by Coffea, published under a Creative Commons licensce

Thanks to the short roasting times and efficient heat transfer, fluid-bed roasters are ideal for industrial-scale roasting. The limited contact between beans and surfaces means the roasting temperature can be rapidly changed, even in large roasters, by simply changing the incoming air temperature. Fluid-bed roasters can roast fragments and broken beans that may get stuck in or fall through small gaps in drum roaster designs (Eggers and Pietsch ).

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