Scott Rao on The Flick - Page 10

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
Mile High Roaster

#91: Post by Mile High Roaster »

[creative nickname] wrote:I've seen a wide variety of complicated suggestions for what should be done with airflow since I started roasting. I've also rarely seen convincing explanations of why those suggestions are best. It seems to me that if the air setting was high enough to begin with, there is no reason why leaving it alone should lead to a smoky roast. I certainly haven't noticed that kind of flavor defect since I started leaving airflow alone (and I always run it at 50% or higher). I also feel like leaving airflow alone leads to temperature data that is easier to interpret in a valid and reliable way. But I will admit that I have never compared two otherwise similar roasts, with one having constant airflow and the other increasing, to try to isolate the impact of that variable by itself. Maybe we should start another thread? Or else make airflow effects the focus of a future R&L thread?
I built my own drum roaster and have been experimenting a lot with airflow, trying to conceptualize the basis for adjusting higher or lower during the roast.
Lately, I have been pursuing the idea that airflow in a solid drum plays a prominent role in controlling conductive heat transfer, from the standpoint that with any decent sized batch a large number of beans are pinned against the drum under a dog-pile of other beans; higher rates of airflow seem to allow the beans to cook very, very evenly toward the end of roast when temperatures are very, very high-- when high temperature gradients can easily lead to uneven cooking like scorching or instantaneous over-roastng of the bean's surface. So, early in the roast, high airflow is less necessary because the beans are generally not as sensitive to scorching.

The other wrinkle is that, in my experience, hammering the beans with a direct blast of hot air in the name of "increasing convection" causes some harsh, hollow flavor defects. The reasons for this happening are not entirely clear, but my experience is like this time and time again.

What this has lead me to is using very low RPM (16 RPM with an 8" drum) in combination with very high airflow (just below sucking beans out of the drum at highest setting- 100%), adjusted upward incrementally (25% to yellow, 50% to just before first, 100% to end of roast). What I aim to achieve here is to reap the benefit of the scorch-free roasting environment that a highly convective environment can offer, combined with doing my best with low RPM to keep the beans from being lofted up and directly ravaged by the hot airstream, as well as reserving the highest levels of airflow for later stages of the roast when scorching is more of a concern.

The results? The roasts done this way develop extremely evenly with a nice, controllable profile via ET setpoints with perfect surface appearance. Coffee tastes really good to me.

What I'm really beginning to consider now is that the "smokiness" folks often attribute to low airflow roasts is incurred not from contact with roast smoke, but from uneven, scorched bean surfaces due the lack of even heat transfer that a highly convective environment can facilitate, and which depends upon higher airflow.