It’s a little hard to advise as you say the roast is bad but not what your target is and how your results differ.
What’s your batch size?
Are you preheating? To what temp and how long?
What are you doing at the end of the roast in terms of managing temperatures, deciding when to stop the roast and how long it takes from then, to room temperature beans?
How long are you resting the coffee for before tasting?
Do you know how old your green beans are? Are they still green or are they mostly pale? Link to the product page?
Hearing the crack is easiest beside the exhaust.
I tend to turn my extractor fan off momentarily when the air temperature is approaching 240c.
Once I hear the consecutive pop pop pop, I can whack up the extractor for when it’s needed most.
An automotive stethoscope or suchlike might be a last resort if you really can’t hear the cracks.
I have some thoughts based on experiences with a Gene Cafe, high altitude beans and my partner’s Barista Express Impress.
The grinder you’re using will limit you to the darker end of medium at a 1:3 ratio, if you want a 1:2 it’s going to have to be dark… I can’t say how dark because that’s not something I care to explore.
When I say darker end of medium, I mean preheating to 250c for ~15mins (whilst removing defect beans, I don’t think it needs that long) sticking at 250c/max air temperature for as fast a roast as a Gene Cafe will deliver on a 250g batch (~4min maillard/browning phase) taking the air temp down to 232c once first crack starts, stopping 2:30 after first crack and cool around 1min after that.
The roast I just described, in a 1:3 ratio shot, with the built in Sage grinder, can be dialed in to be balanced but couldn’t find a way to go lighter without changing the grinder or basket.
If I were making a light roast I’d keep everything the same but end at 230c and stop 1:10 after first crack.
Roast to your taste, equipment and beans; try changing one variable across several roasts and compare them side by side: end temperature, lowering air temp 30seconds after FC, smaller batches, different time past FC.
Not all beans can handle the early heat, likely the Brazilian beans you have are low density. There is merit in using a smaller dose, 120-140g, pre-heating/starting at 190c and increasing to 250c 2mins into the roast and the profile will be similar from there.
At the risk of information overload, the roasts I’ve described are to quicken the maillard phase as much as the roaster allows.
A shorter maillard should equate to retaining origin characteristics and acidity.
Perhaps you don’t want this, you can start a roast with a cold machine for the other extreme and it’ll take significantly longer, some would argue this makes the coffee taste more generic; if the beans are poor quality, this could be desirable.
While a longer maillard will contribute to making the beans a little darker, brittle and easier to extract, given the same time past first crack…
…Ultimately it’s the time past firstly crack and end temperature making the biggest contribution to roast level.
One challenge with the gene cafe is you don’t truly know the end temperature of the beans. You can know they’re roughly 160c when at full yellow and perhaps a shade over 200c at first crack and understand the exothermic reaction taking place at that moment which usually means lowering the air temperature to stop the temperature shooting up when you need it to be increasing very slowly.
One nice thing about the Gene Cafe is if you use it in the most simple way possible, you will get a naturally declining rate of rise / ideal roast curve as the beans get closer to the air temperature; just need to be very attentive right at the end.
I’ll stop there.
Let us know what you’re doing and how you want the results improved.
