So I found this to be astonishing…
On October 30th I did this roast no doubt one of the best roasts I have done recently. The balance in this cup was perfect. It produced a cup that just satisfies — you know when you get this just right. It was that kind of roast.
On November 11th I did this roast. Same bean, same temperature settings, same charge but a very different result. Maybe not quite bad enough to throw out but I would have considered that if it was not the only batch I had for the week. The dominant flavor in this roast was orange pith (NOT peel — pith, not good at all). You can tell the coffee is pretty good underneath but really dominated by this harsh/bitter taste.
Here is a chart comparing the bean mass temperature profile:
And a summary of the two roasts:
The good (really good) roast reached first crack about 1:18 sooner than the poor roast. Overall roast times were about the same (right to first snaps of 2nd crack) but the good (really good) roast spent about 52 seconds longer between first and second crack. The difference: The good roast was the second in a series for the day — the roaster was substantially pre-heated. From this test I have to say this makes a huge difference. Does this slight difference in roast profile explain the huge difference in taste?
[EDIT] — OOPS the data in the table and plot above show the max bean mass temperature instead of the min bean mass temp. This is important because of the large temperature swing that occurs as the roast chamber revolves. Once during each revolution the themocouple is exposed causing the max reading to be about 35 to 40 degrees higher than the min temperature (when the beans cover the sensor). I think the min value is closer.
The chart and table below is correct…