barry wrote:i used to use a 1/4" probe in my roaster as a bean temp probe. i logged lots of data and looked at lots of curves. one thing which always bugged me was the turning point on my roaster (the shift from decreasing bean temp to increasing bean temp) was well over 2 minutes, when most experts i'd talked with said the turning point should occur around 90 seconds. hmmm... i was convinced my roaster was radically underpowered. a while back, i put in a couple of more probes, and decided all the probes in the roaster should be of the same type/size, so i replaced the 1/4" probe with a 1/8" probe. guess what? my turning point shifted to just over 90 seconds. so, did my roaster suddenly gain power? no, of course not. the thermal mass of the probe decreased, so the system lag was reduced. if i stuck a bare wire bead probe in there, the turning point might change yet again. so, the problem wasn't with the roaster, but with the measurement system.
cafeIKE wrote:The maximum temperature is the Maximum the beans are exposed to.
Carl Staub wrote:"Severe damage occurs to the cell walls of the matrix at distributed temperatures above 446 degrees F and bean surface temperatures over 536 degrees F The actual temperature values will change due to varying levels of other constituents. Second crack, associated with darker roasts, is the fracturing of this matrix, possibly associated with the volatilization of lignin and other aromatics. Under controlled roasting conditions, the bean environment temperature should never exceed 536 degrees F. A wider safety margin would be achieved by limiting the maximum environment temperature to 520 degrees F. These temperature limits minimize damage to the cell matrix and enhances cup complexity, roasting yield, and product shelf life. "
from Basic Chemical Reactions Occurring in the Roasting Process by Carl Staub
another_jim wrote:The environmental temperature is the hottest temperature the beans experience, their immediate heat source. The speed of the roast is determined by the difference between that temperature and the current bean temperature multiplied by a heat flow factor. The higher the airflow, the better the heat flow, and the lower the required ET-BT delta to maintain roast speed. If your environmental temperatures get very higher, they affect the outside of the bean, worst case charring them, best case deepening their degree of roast relative to the bean centers. In essence, the lower the required ET for a given profile, the more evenly the bean is roasted.
A fast roast, all other things being equal, will be better than a slow roast, since less of the aromatics are cooked off. Howver, things are rarely all-else-equal. In particular, a roaster has a speed limit for a given charge weight determined by the required environmental temperatures. In a drum, lower charge weights can give you faster roasts for the same ET, and it's your call how many extra roasts you want to do go with lower weights.
from Managing environmental temperature in the roaster
another_jim wrote:My goal was to hold the MET between 250C and 260C (490F to 510F) throughout. I started at the 375C drop in, 7.75 amp and 4.5 air setting recommended for 225 gram roasts. This turned out to hold the MET very nicely at the target without requiring any changes. After the second crack started, the MEYT started to climb, and I had to slowly run the airflow up to 7.5 to keep it steady. This produced a 5 minute roast finish, and an overall roast of 17 minutes. In any case, using air to hold the MET steady is a simple and effective profiling strategy.