This morning, Jan van der Weel and I were on the phone, discussing ways to ensure that the several roast profiles we want to try out are executed smoothly so we can evaluate the effect of the small differences in tasting. The toolbox offered by Artisan is very helpful here.
|Rate of Rise almost touching zero at 7:00 and also barely at 8:00|
Often, you can see a sudden change in the heat development around the onset of First Crack. At that time, the pressure inside the million or so cells of the bean has increased so much that a lot of things begin to happen. Steam, carbon monoxide and carbon dioxide begin to escape out of the weak spots in the enlarging bean cells and hundreds of compounds that will be decisive for the taste of the coffee are composed, decomposed and in some cases recomposed, each in their own pace.
The temperature probe sitting inside the mass of beans that are rolling and bouncing against it passes on as truthfully as possible what the temperature is, but in these turbulent moments around the start of First Crack, it is up to us to interpret the sudden ups and downs in degrees Celsius.
In my perception, the bean body is not getting cold all of a sudden with the splash of hot steam and gas that it begins to emanate. Just like people breaking out in a sweat in the hot air sauna are not suddenly cooling down but merely regulating their inner balance with a new process.
Rate of Rise
Still, one would want to have enough manual control or a clever enough PID system to cruise calmly through these turbulent moments.
I particularly try to keep the bean mass rising in temperature at all times, albeit at a steadily declining rate. Therefore I need to avoid touching a value of zero Rate of Rise.
The targeted Rate of Rise in the phase starting with the onset of First Crack is important then. In my observations, the Rate of Rise can momentarily drop 3ºC and if at that moment the targeted Rate of Rise is 3ºC or less, the bean mass will temporarily not heat up but stay at a level temperature or even cool down for a few seconds before the bean body temp is rising again.
That would mean that for a brief period in time, the driving force of all the development inside the bean is taken from the heat energy of the bean and not from the roaster.
The books and other sources I have available so far indicate that this should be avoided and it makes sense. First Crack is not like fireworks where you ignite the process and let things happen, stepping back.
So even as I try to design a roast profile that gradually diminishes the Rate of Rise toward the end, I want to have the Rate of Rise around the onset of First Crack as low as possible but high enough to remain safely above zero when the 2-3ºC are momentarily cut off of it.
|Designer to the rescue: tweaking the profile plan|
With the mouse one can freely move any of these points around and observe what effect this will have on the rest of the profile. I know from the previous roast at what temperature my Yirgacheffe beans go into First Crack (202ºC). I also know that the roast color was Tonino #99, so I can afford to roast a little darker and I can therefore raise the Drop temperature two degrees, pulling up that Drop marker on the profile. I then shift the Dry End marker to allow a smooth curve without any bumps or sagging trends and as I quit the Designer, the profile is shown as if it was an actual roast, including the projected Development Time.
|Dress Rehearsal for roasting: checking the phases|
|Entering the wish list for the PID, Setting target Value for temperature in ºC in minutes:seconds|
Then it's time to execute and there is very little for me to do but observe the "autopilot" running the planned trajectory. The red line shows the plan for the actual moment, the brown line in the background shows the complete design and the dark brown line logs the actual bean temp as measured by the robe. I only need to do a series of very small manual adjustments to the air intake of the roaster, because in the beginning, the small heavy beans need all the flow there is to get moving and as the roast procedure goes on, the beans get drier and bigger and gradually less air flow is needed. Too much airflow would blow away too much energy of the heater and the bean mass won't reach the target temperature. Also, too much airflow can diminish the aroma of the coffee.
|Actual roast. Gradual decline of RoR works well, lowest is 2.5ºC at 7:30, 2.1ºC at the Drop|
Rate of Rise as planned
In the logged roast profile above, all went very well. 250g green Yirgacheffe beans in, 216g roasted beans out at 213ºC, weight loss 13.6% and a 27.6% development time. The First Crack turbulence slowed down the Rate of Rise a little but it was 2.5ºC at the minimum there, never touching zero.
More to taste soon!