It is not that easy to repeat a great roast you did some weeks ago, even though you use the exact same settings on the roaster. That is because the roast is affected by other things: how much the roaster is preheated, the batch size, and the surrounding air (see Changing weather).
The next thing is that the roasting process is slow to respond to changes so you have to make changes in advance. It’s like steering a ship.
So, it is a good idea to have control points along the way to see where the roast is headed and to do adjustments at an early stage … to better navigate the roast.
Here are the control points that I (Therese) use :
Turning point (see definition in Roasting Basics): use the temperature or time to see how fast the roast got off the ground. I note down the temperature. If it is lower than normal, the preaheating was too low. But then apply more heat.
115ºC: I note the time when the BT reach 115ºC. Again here I can see how fast the roast is going. If it is too fast, decrease the heat (P-setting).
From here on I prefer to use the physical changes: color change and the sound of First crack. I make notes of the time, BT and ROR.
You can also keep using the BT reading. Just be aware that it is also affected by the airflow (fan setting). I initially used the BT reading as a navigation aid, but I found it more reliable to go by the color changes:
Note: this was with an older BT probe that had a lower temperature reading.
Looking at the ROR level tells you how fast the roast is going. If it is higher than I want at this stage, I have to turn the heat down. From around First Crack, you can also increase the airflow (fan setting) to the point where it cools.
I typically aim for a First Crack start between 7 and 9 minutes. But for some espresso roasts I like it to be later – but no later than 11 minutes. Read more under Taste preferences.
You got to find your own control points and choose your aims.
How Steffen repeated 7 roasts using the PlayBack function in the Bullet software but still needed to adjust along the way.