Effect of total roast time?

I’ve been around the Bullet for a very long time, but it is only in the past few months I have really begun to experiment with different roasting techniques. Of course there are a great many variables to play with in each and every roast, but one I’m particularly curious about is the total roast time.

I am mostly roasting Ethiopian beans to just past the beginning of first crack, and searching for ways to maximize both sweetness and acidity. With 500g charges, my roasts have ranged from less than 7 minutes to over 10. In many cases, I find the flavor in the shorter roasts to be preferable – more bright but still packing sweetness. I once managed to roast away all the acidity in a roast by extending the roast-- the effect was so dramatic that my friend who brought some of the beans to an expo called me up to ask what had happened to all the acidity! He thought he must be brewing it incorrectly… hah.

So what effects have you guys noticed when – all other things being more or less equal – the roast takes a bit longer to get to where its going?

How do you vary the roast times? With a different pre-heat temp, power settings throughout the roast, or fan speeds?

As you said, there are a lot of factors to play around with and that affect a roast’s flavor profile. And there are factors within the bean too: water, density, sugar content, fermentation.

Acids develop early into the roast and are broken down as temperature and duration increase, while water evaporates and starches first develop and caramelize.

Scott Rao writes about DTR, development time ratio, the ratio of time after first crack to the entire roast. I would suggest reading his book/blog on the topic.

Ethiopian beans, specifically those naturally processed, are hard to replicate from roast to roast even on large cast iron roasters with lots of heat inertia.

Agreed! Nailing a roast for EYC natural beans is very rewarding, but difficult to replicate in my experience. Those are my favorite flavors as the OP described: acidity, sweetness. I prefer without smokiness, and ideally with bright fruit flavors (especially blueberry or strawberry). I have found that stopping the roast as first crack dwindles out seems to give the best results, although many times I do end up with some astringent acidity and less sweetness than desired. I wish I knew how to fix that, without losing the fruitiness. At times I would try to correct this on subsequent roast by extending the time beyond the end of first crack, or trying to prolong first crack by lowering temp input and increasing fan speed in various combinations. These efforts have not usually helped to preserve the acidity or fruitiness/sweetness, and resulted in a smoother coffee and occasionally some chocolate notes or flatter coffee. Sorry if I’m not best descriptors here. I think one downfall of the bullet is insulation and detecting the sound of first crack, which for me is difficult. Seem that the peak of the ROR curve before it begins to decline is important to achieve best outcome and to be able to perceive the sound system of first crack in my experience. I unfortunately have been “victim” of overbuying green beans and have stockpile of old crop that I cannot just throw away. It is harder to hear 1C sounds on the past crop I’ve noticed as well. I’ve read some of that stuff by S Rao, about smoothly declining ROR. I’m hoping that the new sensor will help facilitate better odds of nailing the roast in the manner by making minor adjustments. I would like to see implementation of a predictive ROR graph (like with Artisan), Software PID, manually creating a graph and program the roaster to follow the curve automatically, and support for third party sound detecting software to automatically detect 1C (was never able to implement this for Artisan with publicly available software).