I found some time today to roast on the Bullet, using 454g batches and preheating the drum to 230C as suggested above. I had intended to do only one batch, but since the results were so disappointing I did two others. The first batch, unfortunately, was the beans I was hoping to learn how to roast on this roaster. These are Ethiopian DP Humbula Beku (?SP) recently sourced from SM and Coffeeshrub (may still be on their list, I don’t know). The first crack, at least, is basically inaudible so I went with my most usual onset 1st crack temperature from my Diedrich IR-1 Roaster, which is around 375 F. I took the beans to 419F, which where I usually end my roasts, which on the Diedrich, at least with the beans I use and at my altitude for the Diedrich is around 5850 feet of elevation (1783 meters). This is normally at the end of 1st crack but before onset of 2nd.
Ethiopian DP Humbela Buku 454g 5/24/2020
The end product weighed 371g, for a loss in excess of 18%
I should add at this point that I have never had a good roast product (to my taste) where the weight loss from roasting has been less than 15%, or more than 17%, or 16% +/- 1%. That doesn’t mean that every roast with a loss in this range is good, but rather that a weight loss of 16% +/-1% is a necessary but not sufficient requirement, at least for espresso. Perhaps if I roasted coffee for use other than as espresso, or if I preferred really darkly roasted beans, I’d have a different set of parameters.
After this experience I decided to tack on a couple of extra roasts. I have an unrelated real estate business relationship with a fellow who owns a coffee roasting business that I have never frequented and about which I have no opinion. I happened to meet up with him this past week and he gave me a tour of his roasting plant and once he knew that I roast my own coffee, he gave me a couple of pounds of 3 different green coffees in his inventory. I know nothing about these particular coffees and have never tasted them roasted by his hands. So I have no expectations, good or bad, about what I might accomplish with these beans. I decided to roast two of them, if for no other reason than to get a little bit more experience with the Bullet.
The next bean I roasted was his Guatemalan Mira Linda, which was obviously wet processed. I decided to take it to a lower temperature based upon the weight loss I got from the Ethiopian DP coffee above. I terminated the roast at 415 F, which is only 4 degrees F short of what I took the first Ethiopian to. This coffee lost 13.6% of it’s weight (starting weight 454g, ending weight 392g), which was similarly unsatisfactory in my view. Here’s the roast:
It was kind of astonishing to me that 4 degrees F, perhaps 30 seconds of a roast at the end, could change the weight loss from roasting so much. Of course, it is a different bean, so we may be comparing apples to oranges.
The third roast was an Ethiopian Sidamo, not otherwise specified, which was obviously WP by appearance. I decided to split the difference in final roast temperature, so I took it to about 417 F. This coffee roast product weighed 385g, or a weight loss from roasting of 15.2%, which would put it into my self-defined potentially acceptable range. Once again, it’s a different bean and comparisons among these 3 roasts might be fraught with potential error.
Here’s the roast profile:
I haven’t done this sort of an experiment with my Diedrich, but I would be surprised if a difference of 4 degrees F among 3 roasts, from 415F to 419 F, which would take 30 seconds, maybe, would produce a range of weight losses from roasting between 13.6 and 18.3%. That just seems huge to me.
What seems much more likely to me is that the thermometry I’m observing is not reflecting the reality of what is going on in these coffee beans I am roasting. Perhaps the bean temp measurement is not accurate for the way that I roast. Perhaps I am being too “hands on” in my roasting, trying to control the heating element and the fan with fine control that looks like I’m accomplishing something but in fact it is all observational noise, i.e. the signal to noise ratio is not good.
One thing that was obvious however is that preheating the drum to 230C definitely made the roasts go faster than they did before when i preheated to around 180 C. These roasts all finished about a minute to 90 seconds faster than my typical roasts on the Diedrich, which doesn’t necessarily make them bad, just different.
I will start to taste these coffees on Day 2, 2 days from now. My assumption is that the first and 2nd will be nearly undrinkable, and that the last might be drinkable if the coffee used was worth roasting, something I don’t know at this point.
If anyone has any comments on this flight of ideas, they are welcomed, and thanks in advance!