Exothermic. Endothermic. Cooling. And how energy is transferred when we roast coffee beans.
When we roast coffee, gases build up within the beans. The beans eventually break, releasing both the gases and a cracking sound. This happens twice, signaling different stages of roasting (read all about it in the 1-Minute NomNom “How to be a crack at roasting coffee“).
The two cracks also signal the transfer of energy into and out of the beans. These are known as exothermic and endothermic reactions. Exothermic reactions transfer energy out; endothermic reactions take in energy.
For example, condensation on a cold surface, changing from water vapor to water droplets such as on the ice cream bar above, is an exothermic reaction (because it loses heat energy). When the ice cream itself melts though, changing from a solid to a liquid, that is an endothermic reaction (because it takes in heat energy).
The roasting of coffee beans has exothermic and endothermic reactions alternating with each other. When we first start roasting, it is an endothermic stage as the heat that is applied is taken in by the beans.
As we continue heating to 175°C/384°F till 196°C/347°F, it becomes exothermic where heat is transferred out of the bean. The steam built up breaks through to give us the first crack. Not only is heat and an audio crack released, but the bean also expands and releases aromas that we normally associate with coffee.
Subsequently, this combination of endothermic and exothermic reactions repeats itself. As we continue roasting the beans beyond the first crack, it is once again endothermic.
It then becomes exothermic at 225°C/437°F: The bean structure collapses, and other gases like carbon dioxide and nitrogen dioxide are released, giving us the second crack. At this stage, the beans also start to look shinier, as the oils from inside the beans are also released, coating the beans.
This second exothermic stage is very brief. Hence once we have released the second cracking, we should stop roasting and start cooling the beans (either with air or water). If we do not, the beans might burn and carbonize (i.e. convert into carbon).
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