drgary wrote:Does Boot give the reasons behind his suggestion?

I just took a quick look at the post and the reasoning behind decreasing the drum speed later in the roast appears to be to slow down the transfer of heat (i.e., reduce convection).

Willem Boot wrote:Varying the rotation speed of the roasting cylinder can help in slowing down or accelerating the heat exchange.

Seems like the quicker and more effective way to slow down the heat transfer would just be to reduce the heat and, particularly in larger or insulated roasters, to increase airflow along with reducing heat to bring in more cool air to help overcome the thermal mass of the roaster.

I signed up for the promo roasting courses that they have put out and found them so simplistic that they were a complete waste of my time! After that experience I heavily discount the validity of anything they say.

mfortin wrote:Here is the link for the video:
https://coffeecourses.com/coffee-roaster-drum-design/

I would guess it's a slip of the tongue and move on, because it doesn't make any sense to me.

My assessment is the same as keno's. If the purpose is to slow down the heat transfer rate or to alter the convection/conduction ratio, it seems to me that there are other more effective ways (reducing the heat and the increasing the airflow) than the variable drum rpm control. Personal anecdote, a couple years ago I pondered over the drum speed question and didn't customize my sample roaster with the optional variable rpm drum motor. It was an "informed" decision, but in the hindsight, I don't know if I made a "wise" choice for my upgraditis.

keno wrote:My roaster has adjustable drum speed. Here is how I've been using that feature, but I haven't done any very scientific assessment. On larger batch sizes I'll use a slightly higher drum speed to ensure sufficient agitation, but I don't change the drum speed throughout the roast. On smaller batches I use a lower speed. And for smaller batches, in particular where thermometry readings are potentially hypersensitive due to the reduced batch size, I tend to increase drum speed throughout the roast as a substitute for increasing airflow which would tend to cause large fluctuations in readings. Drum speed changes for small roasts are just a gentler way to increase

The instruction manual that came with my roaster suggests the opposite in regards to drum speed. "You should use higher RPM if the batch size is smaller; lower RPM for larger batch size. The recommended RPM for 450g of green coffee is 62/min."

This seems to be the case when removing green coffee with the tryer. At 60 RPM with 1/2lb charge, the tryer is not catching anything; an increase in drum speed resolves the issue.

I don't see any potential benefit of reducing the drum speed at the later part of the roast as reducing speed will means more contact time of the beans to the drum. you certainly don't want the beans to touch more to the surface of the drum when it's 190 or 200 degree celisus and will only lead to increase bitterness.

Increasing drum speed at the later part of the roast is a more logical reason but i'm afraid you're going down the rabbit hole. There has been no study or result whatever on this info since increasing it during the roast will alter the vortex of air moving in the drums, change the pressure of the roast to the beans and possibly change the way airflow move. It's actually involve a more complicated process and it's effect will depends on the roaster design