AZRich wrote:Forgive me if I am repeating someone else's observation. As I see it, you have moisture in the bean which undergoes a phase change into a gas. That water phase change is the whole crux of the discussion. What is the energy implication of this, and is the bean just an innocent vessel which contains and then releases this water/energy? If the bean started with no water content then there would be nothing to argue about, correct? Is this too simplistic?
When you're boiling a pot of water, there's a phase change from liquid to gas. Since it is the hottest water molecules that make the change from liquid to gas escaping steam carries away the hottest water and leave the cooler molecules in the pot until they become hot enough to escape as steam. This is what limits the temperature of the remaining water to whatever the boiling point is at the current atmospheric pressure.
Water inside a coffee bean is contained in the same way that water in a pressure cooker is contained. Since it cannot escape to carry off the hottest molecules it is not limited in maximum temperature the same way water in a pot is. When 1C occurs the fracturing of the bean allows that steam to escape but it may already be hotter than the boiling point of water and I presume is at the same temperature as the solid parts of the bean that it had been in contact with prior to 1C.
Rao explains the drop in temperature as evaporative cooling. Evaporative cooling occurs when liquid water evaporates from the surface of an object because that liquid water requires heat to make the phase change from liquid to gas. With 1C I have a difficult time believing this is the case because prior to 1C the water was under pressure inside the bean and has already been given enough energy to convert to steam. This is what causes 1C in the first place. There is no need for liquid water to steal heat from the bean to become steam as it already has plenty. I think the drop in temperature we see after 1C is more likely a result of measuring air temperature + bean temperature + steam temperature when bean temperature is less than air temperature.
Prior to 1C, the BT thermocouple is in contact with the air inside the roaster and momentary contact with the beans. Since the beans are still heating up I would expect the air temperature to be higher than the bean temperature so I would expect BT to report a temperature lower than the ET reading but higher than the actual bean temperature (since contact with the beans is momentary). When 1C occurs, the BT probe is in contact with air, steam which is at the actual bean temperature, and the beans themselves. I would expect this to drop the reported temperature since the probe is spending more time in contact with stuff that is at the actual bean temperature.