Updated: November 7, 2016
The flick and crash is a roasting term coined by Scott Rao in his book titled The Coffee Roaster's Companion. The term is part of his second commandment of roasting and it defines an event in the that can occur around the time of first-crack and again around second-crack. It can have many causes, which we'll explore later in the post. The crash is an event that normally happens immediately after the flick.
Caveat emptor; even Rao states that these are guidelines.
Please don't take the word "commandment" too seriously. One may transgress some of these rules harmlessly on occasion. As with a certain other list of commandments, however, if you make the habit of ignoring the rules, you might end up in a bad place1.
There are many different ways to roast coffee and many ways to achieve the desired roast profile. Understanding what the flick and crash events are, what causes them, what effects they have on the roast, and ways to avoid them will improve your roasting skills, even if you don't always avoid flick and crash. There are many things going on in the roasting environment that we still don't understand. Roasting is a very complex event. As roasters, we should always be prepared to have our theories disproved and always be receptive to new theories—with an appropriate level of skepticism of course. Do your own experiments. Roast, cup, and analyze each batch. If you find you like the flavors caused by the flick and crash, then by all means, let your roast flick and crash. The truth is always in the cupping results and what you—and your customers if you are selling—prefer.
Interpreting Roast Data
The image above is from Artisan Roaster Scope, an open-source, free roasting software. I use Artisan to interface between my laptop and my HotTop roaster. Using the software as well as an Arduino microcontroller I've installed in the roaster, I am able to control the roaster from my laptop and as I make control changes, Artisan annotates the graph to indicate the exact time and temperature of the control change.
For a bit of explanation on what all is represented in the image above; the red line indicates the air temp inside the drum, which I have labeled the environment temperature (ET). The dark blue, increasing line indicates the temperature read from within the mass of beans rolling around inside the drum, which I have labeled the bean temperature (BT). The light blue, (mostly) decreasing line indicates the rate-of-rise (RoR) of the bean temp.
The RoR indicates the change in BT over time—it's one indication of roasting momentum. If the RoR is decreasing while the BT increases, the RoR indicates a slower increase in BT over time—a decrease in momentum. If the RoR is increasing and the BT is also increasing, the RoR indicates a faster increase in BT over time—an increase in momentum.
You can view the settings I use in Artisan to track RoR here. RoR with Artisan is largely an "after-the-fact" analysis tool. It's too noisy to view and be of any use during the roast.
In the image above, the flick starts around the 8-minute-mark of the light blue line. It is the sudden increase in the rate-of-rise. It indicates that the roast momentum is no longer dropping as it always should in order to conform to Rao's guidelines.
The underlying cause of the flick can be several things; too much momentum leading up to first crack (FC), too much heat in the circulating air within the roaster drum, too much heat in the metal parts of the roaster itself, especially the drum, the beans have gone exothermic and are adding heat to the environment, etc. The most basic explanation is there is too much heat and you risk losing control of the roast.
In the image above, I believe there is too much momentum and FC is introducing more heat into the environment and the combination of the two caused the flick. I did not do a good job of anticipating the sudden increase in heat caused by the exothermic reactions of FC. When the beans 'went exothermic,' the added heat pushed the momentum in a positive direction.
Again in the image above, looking at the light blue, decreasing line, the crash is the steep drop immediately after the flick. Like the flick, it can be caused by a number of things; the beans no longer producing enough heat to significantly affect the momentum and/or control changes made in response to FC.
In the example above, the crash was likely caused by a combination of things; the end of significant exothermic activity in the beans as well as control changes I made to the roaster; an increase in fan speed and a decrease in power.*
Avoiding The Flick and Crash
And now we get to the good part; how to avoid the flick and crash.
The image above represents a new roast, performed right after the previous roast as I try to avoid the flick and crash. In both roasts, when the ET hit 390°F I started steadily decreasing the power, indicated in the image by the blue circular annotations with Px to indicate the new power level setting. In the previous roast (roast #327), the changes were less frequent and further apart. In the second roast (#328) I made more frequent changes to the power. For roast #328, I got the power down to 65 just before FC and in #327 power was at 75 just before FC, a 14% difference in applied heat. I also made an additional adjustment to fan speed in #328. For roast #327, fan speed was at 75 until the onset of FC. In #328, I set the fan speed to 85 at 7:51, in anticipation of FC.
The results of the changes moved the flick and crash later in the roast. I still had a bit of a crash just after the onset of FC because I continued to set the fan to 100 immediately at the onset of FC. But moving the flick and crash is progress!
For the next roast, number 329, I focused on ramping up the fan speed sooner, before the onset of FC. At the 7:30 mark I set the fan speed to 85. Everything else is pretty much left the same and the results are clear; a big difference in the RoR after the onset of FC. I still have a crash around 9:45 and that is probably caused by the gradual drop in power I was making after the onset of FC, which I was doing to stretch the roast in order to match the previous roast's profile.
My recommendations for finding out how to avoid the flick and crash on your roaster:
- Minimize the number of changes you make to the roast, i.e. don't change your drop temperature and also change your heat and airflow application.
- Do back-to-back roasts of the same bean to further minimize variables.
- Cup the coffees and add the cupping data to your roasting data (do this with every roast).
In the example above, I violated two of those recommendations: roast number 327 was an unknown bean (some spare beans I had knocking around) and in roast #328 there were additional fan adjustments right at and again just after the turning point (TP). These fan adjustments were Artisan alerts I configured improperly—automated roasting gone awry.
The techniques above will help you discover how to avoid the flick and crash on your machine, in your environment. The basic principle is that the flick is a sudden increase in RoR caused by excessive heat and the crash is the opposite; the sudden decrease in RoR caused by a reduction in applied heat. Both events can be countered by making gradual control changes prior to the onset of FC and then immediately after FC. You want to set the stage for the rapid changes that happen after the onset of FC so that your RoR remains stable and is consistently declining and once FC happens you want to account for the effects of exothermic reactions that come and go rapidly.
1. Rao, The Coffee Roaster's Companion, Scott Rao, 2014, p. 42↩
Update November 7, 2016: Expanded RoR section. Added quote from Companion.
- The HotTop I use is an electric roaster and the element is slow to react. It routinely takes 30 seconds for control changes to completely manifest in the roaster.
Continue Learning: Looking for more how-to articles or articles about roast science? Then check out this page for more!