I've been suspecting a particular roasting anomaly for a while and think I've narrowed down the issue.
This is a typical medium roast profile:
I actually use this as a template for medium roast Central and South Americas. I let it go to 400* for a full medium, or drop sooner for a lighter roast. When I get lazy I'll use this profile and modify my heat settings when I want something different. I can tell by the BT curve whether I'm getting to 400* in 25% post 1C or 20%.
Anyway...being in the biz, I have the "luxury" of roasting the same coffees over and over again under lots of conditions. I am able to formulate a good game plan to achieve most roasting goals.
The other day I roasted 8# of a fantastic Colombia Yellow Bourbon that I've been working with for a few months. I have roasted dozens of batches, most on the light side. I use the above profile and turn down the heat a bit more before 1C and drop around 18% at 390*. It makes for a very bright and juicy drip. I will also take this coffee to 400* and drop around 20% for a more carmelly flavor profile. It is great both ways.
So I bagged up a few bags for retail and brought in a bucket for brewing at the bar. As my norm, I always taste the first cup of every roast. This time I was stunned to find it almost undrinkable. I tried grinding finer...much worse. I went all the way up to the coarsest setting on my R120 and it was drinkable, but not sweet and still had a hint of methyl alcohol.
Unfortunately, I did not save this roast profile, but I do remember it. I got distracted and did not turn down the pre-1C heat from 30% to 20% where I should have and the BT ran away a little. Not badly, no big RoR hump, but I dropped at the in-between temp of 395* in 20% DTR. I did not think anything of it. It was certainly within the range of what had previously produced great coffee.
But something developed at 395* that was not there at 390 or 400 that was so offensive I dumped 4# of brew coffee and tossed 4 bags in the garbage.
I have not read anything about this phenomenon as yet. I believe Jim touched on something similar in one of the light roast threads.
Anybody have a similar experience?