There are two basic methods of roasting coffee. The most common method is the drum roasting method. Nearly all commercial roasters are drum roasters. Conversely, most home roasters are Air Roaster or a Fluid Bed Roaster. A fluid bed roaster uses a stream of hot air to roast the coffee beans. The air is of sufficient force to cause the beans to circulate or to swirl which gives the beans the appearance of being a “fluid bed.” Current technology has not devised an efficient means of creating a fluid bed roaster with sufficient capacity to roast commercial quantities of beans.
Of course, it’s a hopeless discussion which one produces better coffee, since we are talking about a process where the final arbiter of the result is the tongue of an individual who has a particular preference. . Drum roasting has has artistry on its side. It’s like a chef with the freshest ingredients who simmers his sauces slowly, or faithfully oils their cast iron skillet and commercial drum roasters are big, attractive, expensive, old-world-European-looking machines. So in essence it is a tradition.
Air roasting has less intrinsic charm, unless you are really into hair dryers. “But the roast is easy to observe, the process is “clean” because there is no effluence from atmospheric gas burners, and some variables of the drum process.” (Aug – Sept 2003: Drum Roasting vs. Air Roasting?)
In either case, a heat source is necessary to roast beans. Quite simply, the roasting process is a process by which heat is transferred from the source to the coffee beans. The important portion of the coffee bean is the internal temperature and not the surface temperature of the bean. This is significant in determining the flavor that is brought out in the coffee. This becomes important because the two basic methods of roasting bring the bean to this point at different rates of heat transfer.
The significant difference between drum roasting and air roasting is in the rate of heat transfer. “Air roasting has a much higher transfer rate (some say as much as two times) than drum roasting” (Aug – Sept 2003: Drum Roasting vs. Air Roasting?). It is this difference in time and temperature that creates a difference in flavor because the chemical change in the bean is achieved differently.
So what happens to the taste of coffee:
“While the differences are many, the primary difference, and the one most noticeable to the coffee drinker is that fluid bed roasted coffees tend to have a higher acidity” (Aug – Sept 2003: Drum Roasting vs. Air Roasting?). This means that fluid bed roasted coffees have a flavor that is often described as being “brighter” than drum roasted coffees. This means the aroma is very detectable. What this means in the cup is that it has a lighter mouth feel and less body.
Essentially the reverse of this is true for drum roasted coffees. “Drum roasted coffees generally have more less distinct flavors in the cup, but have a much richer and fuller mouth feel” (Aug – Sept 2003: Drum Roasting vs. Air Roasting?). Drum roasters do have a greater ability to bring out the nuances of flavor.
Bottom line, drum roasting is akin to slow cooked meal, complex flavor levels, but time consuming end product is for a connoisseur. Air roasting is like making instant noodles, quick and efficient, end product enjoyed by all.