The onset of First Crack (FC / 1st crack) is an important control point in roasting coffee. Rapid development af the taste takes place from here onward. First crack is when the beans expand and make a noise like popcorn or the crack of a thin branch.
But all the beans do not crack right at the same time. First one single crack, then quiet, then another. But at some point, a lot is cracking. So when should you call the exact time of the start of FC ? Was it a 9:15 or 9:40 minutes ?
Another thing that makes it difficult is the background noise: the noise of the roaster and the beans hitting the drum. So it’s difficult to hear the cracks.
Try listening to this sound bite … Make your own notes before reading on:
I asked in a roaster forum to listen and say when they would say FC started (click on the picture to see a bigger size):
Around 0:15 into the sound bite, I was also shortly wondering: “is it starting now ?” But then it silenced, so no.
Then there is one clear snap around 53 seconds in. But I need more of a roll of cracks to say that it has started.
So I would also say around 1 minute.
I have made a podcast episode about listening for First Crack and counting Devellopment time from there. It is episode 5 in the podcast Coffee Roasting Navigated. Also in episode 11 we listen to different sounds of First Crack.
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.
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When you can’t hear the cracking
How well you can hear the cracking also depends on the roasting machine. Some roasters are so insulated that it is impossible to hear.
This is what I have heard that 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.
Listening devices
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 noises as well) . On this roaster I took out the trier and fitted the stethoscope on.
Photo by Morten Münchow, CoffeeMind.
Here is another device: earmuffs with a tube. This photo is from the roastery Holy Bean in Denmark. Bo Nielsen got them from CoffeeTools.
Sound analysis
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
The main problem seems to be to get microphones that can withstand the heat from the roasting process.