Hey Experts!
Trying to figure out some Colombian honey processed castillos and am coming across a phenomenon that I havnt experienced with any other beans, whereby the B-RoR curve spikes at FC whilst the I-RoR curve continuously declines. Can anyone educate me as to why this might be? More so just for my education than anything else as I know success lays in the cup, but can’t quite figure out why my I-RoR curve looks ok but the B-RoR one is trash. Thanks!!
Well, I have a theory. The BT probe actually measures a combination of bean temperature and chamber air/gas temperature (probably depending somewhat on drum speed). During first crack (and I think a little before 1C as well from ‘silent’ cracks), a lot of CO2 and water vapor is expelled from the beans. This is picked up by the BT probe, but not the IBTS because the IBTS just reads bean temperature. Some coffees do this more than others. I pretty much ignore this and use the IBTS. If it it doesn’t crash or flick, then I figure things are ok. Cheers.
Jim
1 Like
On a related note, here is a link to a Barista Hustle article called, The Evaporation Front. The Evaporation Front - Barista Hustle
In the article there is a graph of the water content of exhaust gases during a roast. This is attributed to Davison (2019). There are a couple of interesting things about the graph that I noticed. The first is the big spike in moisture right around first crack. This is what I was talking about in my previous post about gasses affecting the BT probe. The other thing is the plateau a little before first crack. I have noticed and Rao has noted that the RoR curve during a roast tends to flatten a little before first crack. My theory is that where the moisture curve flattens less heat is being used to evaporate moisture and thus goes into general bean temperature increase (or at least surface temperature increase). The article states that it takes 5 times as much energy to go from 100C water to steam as it does to heat that water to 100C in the first place. That heat goes somewhere. So, the RoR flattens (the I-RoR too). This is an actual bean temperature increase, not just the influence of hot gasses.
Another interresting article is: "The End of First Crack?" also on Barista Hustle. The End of First Crack? - Barista Hustle
Cheers
Jim
2 Likes
Yeah, I think that’s exactly it.
The IBTS number is bean surface temperature. The BT number is the temperature of the metal BT probe. And the BT temperature is affected by the air as well as the beans.
1 Like
Thanks very much for the feedback, I did suspect it was something due to thermal conductivity as water vapor enters the mix around FC. Really appreciate the reference material too! I was getting a little confused as roasting this same bean from a different harvest had very different results with the B-RoR and I-RoR following each other closely. One major difference however is these new beans take about an additional minute before they yellow during drying phase, so I’m assuming different densities / water qty etc play a part in the RoR disparities. Seems that using I-RoR / IBTS is a truer representation of what’s happening with the bean post FC.
Again, thanks very much, appreciate the education!