My hypothesis is that sometimes it's a matter of how the internal moisture pools within the cellular matrix of the beans. And when heat is applied the areas with less moisture will often get black or burnt parts on them. This can also be caused from beans with a lot of defects.
Unless you have burnt marks on the flat surface of a very significant number of beans, and this can be caused by a lack of drum rotation and beans merely getting scorched by sitting for a bit with heat applied. But this in unlikely unless you do something strange with your roaster.
I'm starting a roasting business in central-eastern Africa, in Burundi.
I used a good quality coffee (AA). Some of the beans were burnt on one face. The drum seems to turn normaly. So I thought about the power of the burner. What do you think about it ?
It sounds like mabye the roaster is too hot...and/or the drop temperature is too high.
How about giving us some minimal information to work with, because at the moment all we can do is guess: e.g.
1. Batch Size of roaster and batch size you are roasting to get these probs (is your batch much smaller than the roasters capacity e.g. 10 kg in a 30kg roaster will be challenging!)
2. Drop temp
3. 1st crack time
4. 2nd Crack time (if roasting to second)
5. Total roast time
6. Some information on temps during the roast and max temp
7. You say the beans are AA are they all a uniform size
Are you reducing the heat input at 1st crack, or just pouring on the heat till second and then dumping. How dark is the roast
As you can see you aregiving such little information, it might be difficult to help you.
Indeed I roasted a smal quantity in a preheated drum. something like 10-12kg.
First crack temp was about 200°Celcius but I did not really care about temp and time, I looked at the color of the beans and waited for first crack. shortly after first crack I removed the beans from the drum.
It is strange because two days before I roasted the same quantity, same quality (AA uniform beans), the result was fine although the roast was too dark for the local market.
I'm doing some tests and should start to sell in the local market within 2 weeks.
You are roasting far to small a quantity in a 30kg roaster. This means unless you are very very good at roasting, you are going to get the results shown in the picture. I know you don't care much about temperatures, but with those small batch sizes in relation to the 30kg capacity of the roaster, the whole dynamics of the machine is changed.
Forgive me for saying, but it also sounds as if you are quite new to roasting?, because just getting to first crack is not as important as how you get there and if you got that much scorching shortly after 1st crack, I would make a guess that first came in about 7 minutes!
Try roasting a 25 or 30kg batch, if you have not done so before, ensure that the drop temperature is not too high (e.g. 160-180C) and the heat applied at a rate to take you to first in around 11-12 minutes.
At 1st try and reduce the heat input a little (don't stall the roast though) and with an AA bean I would guess that you want to be around 2 - 2.5 minutes after 1st when you end the roast
If you are new to roasting you MUST record the information about each roast in a log.
1st crack time
2nd crack time
Total Roast Time
The temperatures (drop, 1st, how you manage the heat)
Also with a big roaster (30kg) preheating the roaster before you can roast will take around 50 mins, even if the roaster seems up to temperature before that, you won't roast very well or consistently. Even after warm up the first couple of batches will be different to the ones that follow.
If you want to get an idea of whats being recorded by amateur (not professional) roasters, look here:
Great posts from davec. Our very own nw java posted his roasting log for all to review. Unfortunately his site is currently down. I have a similar log, if you want to use it just PM me with your e-mail address and I will e-mail it to you.
10 minutes roast is most likely too short. Which brand of roaster do you have?