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Stalled roast?
mtbizzle
Earlier today I roasted two batches of a natural Yirgacheffe in my popper - see the 2nd roast here:

(click to enlarge)

i.imgur.com/IpMRjjl.jpg

I know that naturals often roast unevenly. Still I am surprised by the range of colors - black to brown to light brown to tan orange.

Does it seem like some of these beans stalled during the roast - the lighter ones? I've been trying to extend roasts (popper roasts so fast) and have been roasting w/ a fan nearby. In any case, the colors don't seem normal to me, and I haven't had much success roasting this coffee, so I'm curious if I can learn from someone else here.
Edited by mtbizzle on 05/04/2020 7:56 PM
Roast: Kaldi wide, SR800, various other projects
Grind: Niche, Bunnzilla WIP, Commandante
Favorite equipment/approaches: Decent, La Pavoni, iced prismo aeropress (Tom's recipe), Hario switch, Hoffman-style french press
renatoa
Popper was my first roast machine, and abandoned after some months for what I considered that times to be the main weakness, too narrow roast chamber led to insuficient beans vertical circulation, without external help, like a spoon.
If you let the beans untouched, the lower layer will never go to surface, at least during dry phase.
Because there is a significant heat gradient between bottom of can and the upper area, uneven roasting is not a surprise.
Later I found that things can be improved be leaning the machine a bit, making the air circulation asymmetrical. This way the lower layer beans have a shorter way to escape to surface, and a convection style vertical movement of beans start, making the heat exposure more even.
Other solution is to shuffle the beans with a wood spoon, until they dry, become lighter, and start to "boil" by themselves.

Another check you can do, specific to naturals, get some of the darkest beans you see in the plate, and rub a bit with fingers, could be the bean skin that didn't "chaff'ed". If this skin is gone, the bean color underneath is same or close as the rest of the stack.
renatoa
A more significant mod you can do is to change the can slots layout, to divert the airflow from a mainly toroidal, to a combined vertical and spin movement, as in the ikawa roaster, see picture here:
https://pbs.twimg...7/1500x500

This means extracting the can from the popper, covering some of the lateral slots by a heat resistant tape (aluminium or kapton), then doing another slots in the bottom, operation that requires some skills.
I can imagine how I would do those slots... but you must first exercise on a piece of tin of comparable thickness and strength.
You start by cut a straight line on the bottom, as a partial radius, about 1/3 of the bottom circle radius for the start, using a Dremel disk tool, then I would try to bend the tin by pushing with a spoon, to create that paddle slot shape. If success, you can try to extend the slot length to a dimension that would not affect the structural strength of the bottom in the long time... you might use the ikawa proportions of slot dimension vs bottom circle as a guideline.
Then proceed with next slot...

Some days later edit... found today a machine that seems to feature natively the bottom slots as Ikawa:
https://images-na...L1010_.jpg

... but... apparently, agitation seems worse, I see even totally stationary beans in the bottom center, check this video at 1:30:

https://youtu.be/...aWZ6M?t=90



wth !

This example show once again what significant difference is between a popper FB and a fountain style FB...
Edited by JackH on 05/19/2020 8:17 AM
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