I notice that iROR drops, sometimes dramatically, shortly after first crack starts. By dramatically I mean from 6.7 degrees C/min to 1.4 over the course of 40sec. I would expect, due to what I understand is an increase in internal bean temperature resulting in first crack, that RoR would actually tend to increase at first crack, not decrease. Can someone explain what is taking place. I’m open to an education here.
From what I’ve seen from your roasts, it’s just the result of what you’re doing. A lot of changes to fan, power and drum will do that. The bean probe lags behind the IBTS and registers the drop a bit later. And yes, the temperature will rise, but not with so many changes. I might be wrong and there could be something else, but from what I’ve seen, it seems to me that you’re over managing your roast and are too concentrated on the curve. If you haven’t already, you could check out this course by Morten Münchow, which is free for Bullet users.
Pretty much what I expect as 1C is about to get started. I currently roast only one variety of greens so 1Cs (400.x°) is pretty consistent for my purposes. I’ve modified my Recipes to bump power from P6 → P9 at IBTS = 394°. That leads the start of the heat loss because it takes time for the inductive heating to increase the temp. Then I adjust power from P9 → P4 at IBTS = 408° followed by one last power adjustment from P4 → P3 at 413°. No further power adjustments after that as the roast coasts out of the 1C area and wait for 2Cs at about 440.x°.
We had this discussion here a few years back and the consensus (iirc!) was that there is moisture loss as the greens crack and that heat is carried out of the beans with that moisture. Maillard process begins about 371° which is well ahead of where I have to adjust power to (kinda!) keep I-RoR on a constant slope.
Works for me but as always… YMMV.
Edit- Btw… well spotted. I haven’t seen this I-RoR sag/decline mentioned here for at least 2 or 3 years.
you are right, Braca. I do obsess over the curve and that causes me to futz with the fan speed in order to keep things going in the right direction. I don’t consider roasting to be set and forget, I try to pay attention every step of the way but maybe I’m too reactionary. It bothers me that the infrared sensor readings are so jumpy. I do try to watch the trend more than the individual spikes and dips.
And thank you Bab, I do have to realize that changing temp setting does come with a certain lag in effect. This is a hot metal drum that doesn’t cool down easily. I have to plan for temperature reduction well in advance of when I want it to take effect.
First crack is the release of the moisture within the bean during roasting (very similar to cooking popcorn). If you think of a human sweating in a breeze, it makes sense that the temp will drop (evaporative cooling). Many opinions as to whether you should adjust the power to make “the curve” stay along the previous trend or not. Some people say this only makes a pretty curve. Others say it makes great coffee. Others say that the proof is in the brew, and to do what tastes good THERE. Nonetheless, the temp drop you are noticing is to be expected. Up to you to decide what to do with it. This link talks about it a little more. Didn’t do a tremendous amount of searching to find this link, so don’t put too much weight on it…