Heat Reduction and ROR Curves
A lot has been said about ROR curves. Nerds, fanboys, and contrarians are all guilty of talking past each other in this issue.
The truth is that your ROR curves will depend highly on 2 things: your Peak ROR and how dark you are roasting. The famous dramatically-declining ROR curves we've all seen online require a very high Peak ROR, a relatively fast roast, and a light drop. Development time may be significant (15-20%). A rapidly declining ROR after First Crack is necessary to achieve a longer development time for a light roast.
Beside that, the data is usually smoothed, you don't get to taste any of these coffees, and you have no idea if your probe, placement, etc, is comparable. But there are some excellent and necessary principles to apply when it comes to reading ROR curves and how you reduce heat. Here they are.
1) You will not increase your heat setting after Dry End.
2) Your ROR will decline over the time of the roast. Whether you use a dramatic decline or a subtle decline is not itself a predictor of your roast quality.
3) Your ROR will not increase at any point after Peak ROR, unless roasting very dark.
This 3rd one requires some nuance and exceptions. Depending on your probe readings, you may see lots of dips and valleys. You may see a slight uptick in the ROR just before First Crack. You may see it tick, crash, and resume its place. These things can be indicators of something wrong with your roast, or more likely they are just anomalies that you must deal with given your machine, probe and software. I've heard many good roasters tell me that they thought they had problems with "crash and flicks" until they switched to another roaster and all the issues went away. They didn't suddenly get better at roasting, but they probably did get better probes.
When is it a flick and crash?
1) When you notice your heat getting away from you leading up to First Crack, so you compensate by reducing heat, then after First Crack all your heat is gone, so you recompensate by increasing heat. That is the most likely scenario.
2) During the Maillard (MAI) phase, it's possible to miscalculate the momentum you've already built into the bean and it's environment. Heat can begin to increase beyond your intention, and this flick can cause some flavor issues (although truly, small flick will not cause a totally flat coffee). Heat and ROR increase during MAI is especially damaging to the beans flavor development. After Dry End, you will reduce heat and reduce the ror to allow proper flavor development.
3) It is also a common problem that heat gets out of control during development after First Crack. Reduction in heat or increase in airflow at least 30s-1m before First Crack should help with this easily. In some roasters, the bean's momentum developed throughout the roast is enough to carry it thru development, depending on how dark you want to go, even with the heat completely off (although you may need to back off airflow to trap that heat).
When is it not a flick and crash? When you notice a consistent flick, crash, and resumption of the expected ROR trending curve— on every bean, on every roast, right at First Crack.
So-called “flat” ROR curves, as long as they do have some kind of gentle decline, and do not increase especially during MAI, are perfectly acceptable and can produce great coffee. They may be necessary if you cannot achieve a high Peak ROR (a moderate Peak ROR may be ~34, a higher Peak ROR may be 40+ degrees F/min), or if you want to roast into Second Crack. A controlled increase in ROR just before Second Crack is sometimes expected, to rebuild momentum to get all the beans thru that Second Crack at somewhat the same time.
In our next post, we will overview the different phases of roasting and how they affect your final coffee product.