another_jim wrote:Thanks for this antidote to ROR blather (the main reason I've stopped posting a lot on home roasting -- I gave a much simplified version of these equations some time ago, and was ignored. I hope people will pay more attention to these)
So the real question is what does happen in drum roasters to create a faster rise in temperature during the first crack. My KISS answer is that the first crack happens. This is a release of superheated steam. In an air roaster, where the volume of airflow per bean is very high, this steam heating is lost. In an drum roaster, where the volume of airflow per bean is low, the super heated steam affects the thermometry.
For almost a decade I have used the term "less endothermic" when describing 1st crack, whilst all around me claimed the beans became exothermic. When using a CBR1200 roaster, clearly I needed less heat but not "no" heat to continue at 1st crack. Fair enough but you are putting cold air in people say.
I have dumped beans into an insulated container a little into 1st crack, they stop roasting instantly
I've killed the heat to the roaster (drum roaster), a little before 1st, just enough to they just get into 1st, reduced the airflow to nothing....and 1st crack stops.
These things would be different, especially the last one if coffee was exothermic at 1st crack. In fact, if it was in most drum roasters when you turn the heat off at 1st, the beans would not only continue to rise with the residual energy being imparted to them by the heat left in the roaster, but would continue way beyond the normal 6-7C rise that you see. If they were indeed exothermic, they would give a lot more heat than that to the roaster.
I believe there are 4 things happening.
1. There is a phase change of water as it bursts out of the bean
2. There is a structural change as it cracks, reducing bean density
3. There is a size change as the beans expand
4. Airflow within the drum/roaster changes
All of these things make the beans absorb more heat and absorb it faster from the environment (the roaster)....hence we have to turn the heating elements off in a drum roaster (or burners off). The probes in the bean mass then start registering an increasing temperature. My external probe, the one just outside the perforated drum, but in the casing it rotates in, shows a much higher temperature...reflecting the energy still in the roaster, this one often stays static, or reduces slightly.
My roaster takes 25 minutes to warm up, the heating elements being off for a few minutes, isn't exactly going to cool it down much.