August 18, 2021

Hot Air roasting versus Kiln roasting

My adventure with coffee roasting involves a "Fresh Roast SR800" machine. After using it for a few weeks, I began to wonder about the differences between "roasting" beans with a flow of hot air versus more traditional roasters that use a flame and rotating kiln.

It would seem to be all about how heat is transfered. People like to say that with hot air roasting, almost all of the heat is transfered by convection. With a rotating kiln, most (perhaps 80 percent) of the heat is transfered by convection. Conduction from contact with the walls of the kiln account for the rest. With roasters that use a basket like kiln (like the Behmor), radiative heating (transfer via infrared) can also play a part.

Consier this from a Wikipedia article:
Two main types (of commercial home roating machines) exist: the fluid-bed or fluid-air roaster, and the drum roaster. The fluid-bed roaster heats the beans faster, retaining more of their desirable acidic flavor compounds. However, the resulting roasted bean is tougher, harder to grind, and less of it ends up as "mouth feel" or body in the coffee beverage. The drum roaster takes more time so the acidic compounds are less in evidence, having evaporated somewhat to yield a mellow-tasting beverage. The roasted bean is softer and easier to grind, and more of it contributes to the body of the beverage.

Interestingly, some of the biggest and most expensive commercial machines are air bed machines. They roast fast, allowing a greater throughput. The word is that a big air bed machine can roast the same size batch in 6-8 minutes, as compared to 15-20 minutes. With one exception, these big machines aren't used by "specialty" roasters. Rather they are used in factories, for the production of (of all things!) instant coffee. It is all about speed and uniformity. So this commercial use is hardly an endorsement for those seeking quality coffee.

The big exception is the Sivetz roaster. Expense precludes most small shops from using it.

Note that "fluid bed" doesn't imply the use of any kind of liquid. The fluid is hot moving air. Do a search on "fluidization" to learn about how particles can be suspended in a moving liquid.

The Sweet Marias article talks about what I am most interested in, namely how differently coffee tastes when drum versus hot air roasted.

Have any comments? Questions? Drop me a line!

Tom's coffee pages / tom@mmto.org