As promised in our last post, we repeated the drum speed experiment with a larger batch of green coffee beans (20 oz this time vs 12 oz last time). Our goal: learn whether or not higher RPM on the drum produces better coffee. This time, we used our DR Congo Muungano bean.
Scroll down to learn more, and contact us if you enjoy the post.
The Short Answer
As expected, yes, faster drum speed produced better coffee on the larger batch...and the difference was more pronouced than it was on side-by-side 12 oz batches. Both batches displayed the gentle acidity that is the hallmark of East African washed coffees. Like last time, the higher-RPM batch had a slightly more complex aroma and taste.
So, why does the higher RPM batch taste better? Pretty simple: more even roast. The faster drum speed agitates the beans more, which increases the chances that each bean is exposed to the same kinds of heat,* on all sides, for the same amount of time.
Here's another way to think about it. At lower speeds, the beans at the center of the bean mass are more likely to get "trapped" in the middle, away from the heat source. Thus, they will end up less roasted than the beans at the outside.
This is especially important for lighter roasts, since being trapped increases the odds that those beans do not absorb enough heat to get beyond the grassy stage.
*Coffee can be roasted with 3 types of heat: conduction, convection and radiant, which is true for both commercial roasters and home appliances.
- If you are roasting on a cast iron skillet, that's conduction: the heat is transferred from the heat source - flame - via contact with a solid object, the pan.
- When heat is transferred via fluid, such as air, instead of a solid, that's convection. For those of you roasting green coffee beans in a popcorn popper, you're using convection.
- And when the heat source emits waves of energy (think about your hand hovering over an electric burner), that's radiant heat. That's how the Behmor that we used in these experiments works.
If you really get into the nitty-gritty, you're often roasting with at least 2, if not all 3, types of heat simultaneously. We'll get into that in a future post.
And in the spirit of being comprehensive, technically, coffee could also be roasted a fourth way: microwaves. More on that in yet another future post. (Hint...not recommended.)
And a final note, for all you scientific pedants out there...Yes, we know that what we're actually describing is methods of heat transfer.