I doubt it is as simple as “gas is better;” it may have little to do with the actual fuel source, other than the obvious observation one has with most gas ranges or cooktops vs. electric ones; it’s easier to fine tune gas and it comes on strong really quickly, compared to electric. At first blush I’d say that the differences are due to the thermal mass of the roasters and their drums, combined with the thermometry that each roaster offers. In the Diedrich the drum temperature may be the same or nearly the same as the air temperature measured by the probe, and the drum is so heavy and carries so much heat that the presence of the beans doesn’t reduce the drum temp very much. This may not be the case in the Bullet and in the earlier phases of the roast the drum may be cooler than the air temp, helping to make the bean temp less than the measured air temp that is imputed to be the bean temperature. I’m guessing here.
It could be that using other parameters than the reported bean temp would lead to a better roast product.
I should add that I go between two houses, one my primary residence in the mountains, and the other in the big city where my work takes me. The Diedrich is at the primary residence, and when I am there I tend to have a lot more free time, hence can use a roaster fairly easily. The Bullet is at my work residence and when I use it my time is limited hence I don’t do a lot of roasts and don’t do them very often.
Two of my most recent roasts might be illustrative. Both were with an extremely fruity, small beaned, DP Ethiopian I bought from SM/Coffee Shrub, Humbela Buku (?). I have produced delicious and enticing results on my Diedrich but nothing good on the Bullet with this bean. Both of the Bullet roasts I am describing had essentially the same profile, other than that one of them was carried to a higher final temperature. The parameters are below:
(1) Drum preheated to about 180 C (~355F).
(2) Steady rise to first crack at about 9:30 minutes at around 375F, which was largely a guess since this bean’s first crack is just about inaudible. On my Diedrich I hit onset FC a bit earlier, maybe 8:45 to 9:00, but this is hard to accomplish on the Bullet.
(3) I aim to end the roast just before 2nd crack commences, within 4 to 4.5 minutes after onset FC, so total roast time is on the order of 13:30 to 14:10: These parameters are achieved by adjusting the heating element intensity and the fan on the fly, mostly by increasing the heating element intensity to near maximum and the fan to near minimum before onset FC, and thereafter moderating temps and increasing fan speed so as to draw out the time between onset FC and end of the roast.
(4) the first roast finished at around 419 F (215C) and looked quite light in color for what it should have been, showed no oil, and lacked flavor and was hard to grind 3-10 days later in actual use. My go to grinder is an HG-1 hand grinder with Mazzer Robur 83 mm burrs in it, and you can tell how hard a bean is when you grind it by hand! Weight loss from roasting was about 16.5%, which is similar to what I get on the Diedrich. The difference was that the beans in no way resemble the beans I roast in the Diedrich that lose 16.5% of their weight in the roasting process, they seemed very under-roasted.
(5) I took the second roast 6 degrees F higher, to 425F (218.3C) finishing bean temp, on account of the results I got on the earlier batch. I never roast this dark normally, this was for educational purposes given the results above. The beans came out very much darker than I am used to, and by day 3 had some oil spotting. Grinding them was like grinding charcoal, and the taste was similar Weight loss from roasting was a whopping 19.5%.
These are only two of the roasts I have done but they illustrate the results I am getting. There is no way that accurate thermometry would show a 6 degree F difference between these two batches, with weight loss percentages of 16.5 vs. 19.5 %. There is no way that observed thermometry and roast timing throughout the roasts would be similar, other than for 6 degrees F, and maybe 30 seconds in overall time difference, and in one case produce beans that were very undeveloped, and in the other that were like mini-charcoal briquettes.
That’s where I am at the moment.