There are two basic methods of roasting. This is true of both commercial roasting operations and home roasting. The most common method is the drum roasting method. Nearly all commercial roasters are drum roasters. Conversely, most home roasters are fluid bed roasters. 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.
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. The internal (core) temperature of the bean must be brought to the point where it causes the first exothermic reaction – an event known as “first crack.” First crack begins to occur at about 400 degrees Fahrenheit. From this point to approximately 435 degrees Fahrenheit the beans absorb heat and increase in core temperature. At approximately 437 degrees Fahrenheit the second exothermic reaction – second crack – takes place.
The process occurring between first and second crack is known as pyrolisis and this is a chemical change in the bean itself. 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.
The significant difference between drum roasting and fluid bed (hot air) roasting is in the rate of heat transfer. Fluid bed roasting has a much higher transfer rate (some say as much as two times) than drum roasting. It is this difference in time and temperature that creates a difference in flavor because the chemical change (pyrolysis) is achieved differently.
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 (remember, acidity is not necessarily a bad thing in coffee). This means that fluid bed roasted coffees have a flavor that is often described as being “brighter” than drum roasted coffees. Another significant difference is that fluid bed roasted coffees tend to remain harder. This means they grind more consistently, but it also means there are fewer solubles. 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 muted (less distinct) flavors in the cup, but have a much richer and fuller mouth feel. Drum roasters do have a greater ability to bring out the nuances of flavor because they are much easier to control as to air flow and heat. In drum roasters the heat source can be increased or decreased with an almost immediate effect since much of the roasting is done by conduction (direct heat) and not convection (hot air). Most fluid bed roasters have little control over heat and virtually none over air flow (because the air has to be kept flowing at a sufficient rate to keep the beans circulating). This control in the drum roaster gives greater control over the chemical change that takes place within the coffee bean.
The bottom line is that air roasters create brighter more acidic coffees with less body while drum roasters create more muted coffees with greater body. Your personal flavor preference will likely determine which method you would prefer, but honestly, have you ever met a premium coffee you did not like?