Thursday, March 18, 2010

Chemical reactions vs. physical changes at 1st pop

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?


Anonymous said...

Good post. From what I've read the release of moisture during first crack actually has an impact on the environmental temperature of the drum. It was explained to me that the introduction of moisture is responsible for the slowing effect.
Detour Coffee Roasters

Cafe CK said...

You're blowing my mind! Love it.

The Coffee Collective said...

Might be the moisture!? I guess it would then be the water vapour that would have been trapped inside the rigid structures of the bean before 1st pop, that is released as the porosity of the bean is increasing around the pop. It would still indicate that it is not slowing down of exothermical chemical reactions that is the reason for the slower temp. development at this point - right!?

fooducation said...

The fact that the early processes in roasting are endothermic is rather easy to explain:

- heating up of the beans (especially the water in the beans)
- evaporation of the water in the beans
- other chemical processes demanding energy (browning processes etc.)

The first two of these demand quite a lot of energy; water has a very high heat capacity (it takes much energy to heat up water), but evaporation is even much more energy demanding. It takes 7 times more energy to convert water att 100 degrees to vapour (still at 100 deg) than it takes to heat the same amount from 99-100 degrees.

During the second step, 1st pop/crack, the beans expand and lose weight. The following is my educated guess: remember how your average fridge works? A compressed gas expands through a nozzle resulting in cooling of the inside. In case of a single coffee bean: water vapour suddenly expands as the beans expand. This results in an increased volume, and consequently reduced pressure. As temperature, pressura and volume of a gas all are dependent on each other, this a slower temperature development is not surprising at all.

Tips for further reading:'s_law

As you suggest, the extra energy needed for vapourising the liquids inside the beans might also be an important factor (see above). I guess this depends on how much liquid is left inside the beans as well as whether the porosity of the bean is increased at this point, letting out these liquids.

Finally, pyrolysis is an exothermic process and might, somewhat simplified, be coined as "burning". This is of course exothermic (as you would notice every time you light a match). Hence, the system will heat up more rapidly than what can be explained by simply the energy you put in.