The charge temperature is the temperature reading from the bean probe at the moment you release the batch into the roaster. It is common to assume that the charge temperature determines how the early part of the roast progresses. In reality, the heat transfer in the early stages of the roast depends on the total thermal energy of the machine, which is determined by your BBP.
If your BBP involves holding the machine at a high set temperature for several minutes and then letting the machine cool until it reaches charge temperature, the machine will contain a lot more thermal energy than if you had cooled it down during the BBP for several minutes before bringing it back up to charge temperature.
Same charge temperature, different thermal energy. The standard BBP (blue line) cools the machine between batches, and therefore the machine has less thermal energy at the beginning of the roast. Idling the machine at a higher temperature (red line) between batches increases the thermal energy of the machine. Even though the charge temperature is the same, the higher thermal energy leads to a faster roast.
The charge temperature is less important than the overall BBP for controlling your roasting. For this reason, Scott recommends choosing a reasonable charge temperature for your machine and batch size and sticking with it. If you establish a reasonable charge temperature to begin with, it’s not necessary to change it to manipulate the flavour of your coffee or adjust it for different varieties.
Making changes to the charge temperature affects the entire roast curve and might require you to redesign the entire roast profile from scratch. It is much easier to charge at a consistent set temperature and then adjust the gas settings to achieve your desired result.
A Reasonable Charge Temperature
A good starting point for choosing a charge temperature is to charge at the same temperature at which you end your roasts,