This was a response by Jim in the recent post "Gaging Temperature from Bean Color"another_jim wrote:You went wrong by lowering the environmental temperature during the roast. This happens automatically in a lot of home and self-built roasters with poor on/off temperature controls, but you've been doing it deliberately. It is one of the major roast control errors.
When the temperature around the beans falls, the amino acids and sugars at the surface of the bean, that have developed earlier in the roast by the breakdown of starches and proteins, repolymerize. Starches and proteins have no taste and no aroma, while sugars, amino acids and other small compounds do. So when the temperature drops in the roaster, the coffee flavors die.
Moreover, if you drop the environmental temperature at just the right time, from half way through the 1st crack to just after its end, then you will end up with ugly as well as flat tasting beans. At this stage of the roast, the beans are in their "glass phase," that is their skin is expanding smoothly. The temperature drop is like dropping a hot glass into cold water. The beans stay small, and show cracks, wrinkles and mottling.
These problems are covered up in roasts that go into the 2nd crack or beyond, since the breakdown of the cellulose then expands the beans some more. Moreover, until the end of the 2nd crack, some remaining polymers break down into roasty tasting smaller compounds. So the only remaining hint at what went wrong is that instead of a little fruit and acidity to go with the roast flavors, there'll be none at all.
I think some people get misled by larger drum roasters or unventilated sample roasters needing to cut the heat source at around the first crack. This is not to reduce the temperature inside the drum, but to prevent it from overheating. There is already so much heat stored in them, and so little opportunity for it to escape, that the drum temperature continues rising even when the heat source is off. This is never true for a convection roaster like the Gene, or even most well ventilated drums.
In summary: love the one you're with -- it's better to operate the roaster you actually have than the imaginary Probat you really desire
I've heard many times about stalling the bean mass temp. rise especially during first crack and it's negative effect on the roast. Also we often hear about keeping the roasters environmental temps below 500f for various reasons. But I've not heard much on the negative effects of lowering the environmental temp. inside the roaster. If I'm understanding what Jim is saying, lowering the environmental temp. even if the bean mass temp. has not stalled, will effect the fruited and subtle notes in the roast especially if done around or during first crack. Many homeroasters tend to ramp the temp up approaching first crack and then lower to slow the momentum and to stretch the time between first and second. But now I'm thinking I need to look at my ramp temps rather than applying the brakes so to speak. Also the effect of thermostats in many home roasters and their often large temp. swings between on and off seems to be even more important than I had previously thought (the reason I went variac and by-passed my thermostat that often had a 30 degree swing). I hope we can have a more detailed discussion on this as it seems like an important factor many of us had not considered.