Computer Driven Roasting

Since upgrading my HotTop drum roaster to the “latest and greatest” B2k+ controls, I now have a USB port that plugs into my laptop computer for both recording roasts, and designing roast profiles that I can then send to the roaster. I still manually charge the beans and monitor the roast, and can make adjustments on-the-fly if I wish, but I can also run a roast in “full auto mode” and as long as I drop at the same environmental temperature, things will stick pretty close to the original plan, within a few degrees.

Roasting with this level of control is not only incredibly convenient for producing large batches consistently, it also dramatically shortens the learning curve on becoming a coffee roaster. This is not to imply that one can roast “by the numbers” it just means that besides smell, sight, and sound, there is additional information that you can rely upon. I know that when my BT hits around 300°F, I should be at or near the End of Drying point in the roast. And somewhere in the neighborhood of 365°F, I can expect first crack to begin. These are important benchmarks in the development of a roast, and knowing these cues helps give proper attention to the smells, sights and sounds that traditional roasters had to rely upon before thermocouples and temperature logging software became available.

I relate this directly to my past experience as a “color separator”. When I started out running digital color scanners, you really only got one or two shots at getting things right, because color separations took a couple hours to complete and there was an associated material cost. To properly produce an image, I had to take dozens of readings by analyzing each image with a microscope before committing to a decision and hitting the “scan” button. Now that images can be viewed on calibrated computer monitors, the process of color and tone correcting images is dead simple. What used to take hours to accomplish and years to learn, I can now teach someone over a period of hours, and accomplish results in a minute or two.

The image on this post describes the stages of a roast, starting with the Charge, when beans are dropped into the roasting drum, followed by the Turning Point, where the environmental temperature and the bean temperature equalize and the beans start to show a rise in temperature. You see that this chart shows a Turning Point at 212°F. That is not the actual bean temperature. At that point, the beans are only slightly above room temperature, having just been added to the roaster. It is an average of the Environmental Temperature at the “far end” of the drum from the heating elements, and the bean mass as it tumbles over the thermocouple.

The Drying phase is indicated between the Turning Point and the Drying End marks. During this period, the beans are slowly rising in temperature, and moisture is being driven off. Green beans, when they go into the roaster, have around 10%-12% moisture in them. Much of that moisture evaporates during the drying phase. The smell of the beans reminds people of wet grass on a hot day at the beginning, and dry grass being raked up at the end.

Once the drying phase is completed, interesting things begin to happen. There are chemical reactions inside the bean caused by the rise in temperature and the breakdown of cellulose, these reactions create new compounds, which in turn react and change as the roasting process progresses. When beans reach First Crack, the remaining moisture inside the bean has expanded to a point where it fractures the bean’s structure, and the moisture is then driven off. The roasting process can be stopped at this point, or continued. The characteristic flavor of the roasted beans will change every few seconds at this point, with more fruit and sour notes dominating early in the roast, and more cocoa and smokey notes dominating the later roast stages.

If you continue roasting, carbon dioxide with build up pressure and produce a Second Crack, which is a point at which most of the sweeter sugars have been caramelized and the bean is getting close to being exhausted of its flavor components. At this stage, most, if not all of the early flavor notes are no longer present, and other flavors dominate. Roasts should not continue long after this stage, as once the flavor components have been burned off, the only thing left is the burned cellulose, which can combust, starting a fire inside the roaster.

The roast that resulted from the program shown was delicious, though lately when roasting the same beans, I have been slowing down the roast progress after First Crack end, rather than letting it start to climb toward Second Crack, with great results.

The blue squares and grey diamonds show the heating element and fan speed changes made during the roast. I have been working toward making the fewest number of changes to produce the best possible roast, which is why you don’t see much movement in these settings. While the graphs show me what is happening to the beans and the roasting chamber, these are the elements I am controlling, so it’s important to have these settings so that I can evaluate the roast and determine any changes I might want to make on the subsequent batch.

Well, I didn’t mean for this post to be Coffee Roasting 101, but I hope you don’t mind. I wanted to share my thought process and explain why computer-driven roasting is such an advance for craft coffee producers.

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