While roasting coffee at lot of different chemical reactions is going on in the bean. These reactions are very complex and are responsible for the development of coffees flavour. Some of the chemical reactions in roasting are endothermical (demanding energy to proceed) others are exothemical (producing energy while proceeding). I recall to have heard that proportion between endothermical and exothemical reactions are changing forth and back during roasting. Roughly speaking the Endothemical reactions are dominating the start of the roast. whereas the exo. reactions are dominating the end of the roast. But during the middle of the roast the proportions should change forth and back.
For a while we have noticed that when 1st pop is at is full speed (around 188-192C bean temp on our roaster) the bean temperature has been increasing slower than both before and after this point. I have believed that this was due to the exothermical reactions slowing down at this point. With the rather rough equipment we have to measure the proportions between the two types of chemical reactions (the bean termocouple), this particular point is in fact the only indication of what might be exothermical reactions slowing down while roasting that we can see.
Intuitatively it seems rahter straight forward if the exo. reactions would just increase contiuesly as more energy is put into the roast and not be slowing down. I know from what I have heard that this is not true, but could it be that what is slowing down the temperature development at 1st pop is not the slowing down of exo. reactions but rahter the energy needed for the physical expansion of the bean at this point? Might this be similar to the extra energy needed two vaporize a liquid? if you add a constant heat two a liquid the temperature would increase constantly also, but at the transition phase the enegry added would be used to changes phases from liquid to gas therefore you would not have a constant temperature increase at this point. Might it be a similar physical property that is responsible for the slower temperature development at 1st pop and not slowing down of the exothermical chemical reactions?