I finally had the chance to do a roasting session on my Diedrich IR-1 yesterday, after a 3 month pause, mostly to be out of my main residence during the height of the coronavirus pandemic (it having been located in what was a “hot spot”). One of my justifications for buying the Bullet for my “business house” was that I was going to be stuck there for a while during the epidemic. I will also use this post as an attempt to get back to the original topic of this thread.
Yesterday I roasted 2, 2lb batches, of each of 3 different DP Ethiopians, for espresso, on the Diedrich, for a total of 6 back to back roasts, all taken to around 419 F bean temp on the Diedrich, which with that roaster at my altitude of about 5850 ft means the coffee makes it through 1st crack almost to the end but doesn’t enter 2nd crack. First crack was generally started around 375 F, after about 9 minutes 30 seconds, and then the roast was carried on for 4 to 4 mins 30 seconds further after which the beans were dumped.
As is my practice for many years, once I get within 10 degrees F of initiating FC, I reduce the gas heat input, and once FC actually starts, I further reduce the gas input to a great extent, plus change the ventilation to 80% through the drum (maximum you can get with the damper in this roaster). Once the rate of rise (ROR) slows precipitously, I gingerly increase the gas heat input and reduce the ventilation through the drum also, the latter only if needed. I paid particular attention to the bean temps over the next ~4 minutes of each roast, until the beans were dumped.
Unfortunately, I do not have any way to record the bean temps in real time or to put them on a graph, as the thermocouple in the drum goes to a digital read out but there is no way to interface with it as the roaster does not have an “electronics package.” Nonetheless, it is pretty easy to describe what happens as it is consistent and repeatable across all of the roasts I did with the 3 different coffees yesterday.
Once FC starts on this roaster, the bean temperature rises very rapidly as the roast becomes self-supporting, or “exothermic.” Even with the gas heat turned down to a minimum (in my case this is going down from a maximum of around 6.75 WC (Water Column Inches) to 2.0 WC, e.g. reducing the gas input by more than 2/3, plus increasing the drum ventilation from 50% to 80%, the beans very rapidly go from ~375F to 395F in the space of about 1.5 minutes. This is at least as fast a ROR as the beans are going through in the minute or two before the FC starts, although perhaps a bit slower than the ROR earlier on in the roast. Only after about 1.5 minutes of greatly reducing heat input and increasing drum ventilation, does the ROR slow considerably, and in some cases it slows down A LOT.
As I have mentioned earlier on in my posts on this thread, I intentionally slow my roasts down after onset FC to draw them out to 4-5 minutes after onset FC, which I have found works well for single origin Ethiopians used for espresso. I had some roasts with poor results on the Bullet attempting this profile, however it is now fairly obvious to me that what happened on the Bullet is not what happens on the Diedrich with the strategy I employ.
On the Bullet, whose drum doesn’t have a great deal of “thermal mass,” when you cut the heat input and you increase the fan speed, you get an almost immediate response in the “rate of rise” ROR, whereas in the Diedrich IR-1, this is delayed by about 1.5 minutes to my observation. So, IF the “ROR crash” as quoted here from Scott Rao is in fact a real detriment to the quality of some roasts, it also probably matters WHEN that ROR crash occurs. From my own informal observations on the Bullet, it seems that if the ROR crash occurs right after onset of FC, this is not good for roast quality. IF the ROR crash occurs later, in this case delayed by about 1.5 minutes, then it does not have the same effect.
Finally, bringing this back to the original topic of this thread started by Stuart: I would not compare roasting on the Bullet to roasting on a high-thermal-mass gas roaster. You may very well be able to obtain similar results, but the experience is totally different. The gas roaster is presumably designed to roast in a certain way, which the thermal mass of the drum dictates. It’s kind of like driving a Sherman Tank through a blizzard; you don’t need to overthink things, the tank is going to keep on going its way and nothing on the ground is going to stop it. The Bullet, on the other hand, is nimble and more like a sports car with summer tires on it, you can make it do what you want it to do, but that is going to take a lot of attention on your part. A commercial gas roaster is a very solid piece of equipment; the Bullet, in comparison, feels rather flimsy. One advantage of the Bullet is that at least in theory you could find a good profile and then just execute the whole profile automatically through the software. There are certainly big gas commercial roasters that can do that also, but you are talking an enormous amount of money for a gas roaster that can do that.
On the whole, I much prefer the Diedrich gas roaster to the Bullet, however I am more used to it, and I can’t say that with experience the Bullet isn’t capable of getting very similar results.