The onset of First Crack (FC / 1st crack) is an important control point in the roast of coffee. Rapid development takes place in the taste from here on. First crack is when the beans expand and make a noice like the crack of a thin branch.
But all the beans do not crack right at the same time. First one, then quiet, then another. But at some points lots are cracking. So when should you call the time ? Was it a 9:15 or 9:40 minutes ?
Another thing that makes it difficult is the background noice: the noice of the roaster and the beans hitting the drum. So its difficult to hear the cracks.
Try listen to this sound bite … Make you own notes before reading on
Some beans have a more clear FC start than others. A late FC (like more than 13 minutes) will tend to make a weaker crack. Low density beans tend to make weaker crack.
I asked in a roaster group on Facebook (click on the picture to see a bigger size):
Around 0:15 into the sound bite, I also was thinking: “is it starting now ?” But then it silenced, so No.
Then there is one clear snap around 0:55. But I need more of a roll of cracks to say that it has started.
So I would also say around 1 minute.
When you can’t hear the cracking
How well you can hear the cracking is different from roaster to roaster. Some roasters are so insulated that it is impossible to hear.
This is what I hear roasters then do:
-> smell the beans in the trier. At first crack comes a smell of vinegar.
-> look at the beans in the trier. When they crack they jump around.
You can also use some listening device to get the sound. On this photo I listen with a stethoscope. It only really works through air – and not on metal or glass window (because that will transmit all the other noices as well) . On this roaster I took out the trier and fittet the stethoscope on.
Here is another device: earmuffs with a tube. This photo is from the roastery Holy Bean in Denmark. Bo Nielsen got them from CoffeeTools.
Research has been done on the sounds during coffee roasting by P.S. Wilson. In his article “Coffee roasting acoustics” he concludes:
> Near the end of the roasting process, sounds known as first crack exhibit a higher acoustic amplitude than sounds emitted later, known as second crack. First crack emits more low frequency energy than second crack.
Link to article: http://asa.scitation.org/doi/full/10.1121/1.4874355
But I hear that the problem is to get microphones that can withstand the heat from the roasting process.