Michael Sivetz invented fluid bed roasting in the 1970’s. Green beans are placed in a tall cylinder suspended above heating elements. Air is drawn from the bottom, over the heating elements, and through the beans. The heated air flows through the beans, causing the beans to move in the cylinder in a fluid motion, much like a giant hot air popcorn maker. Roast temperature is measured in the middle of the bean sprout, providing a very accurate measurement of what is actually happening with the beans. The person roasting has infinite control over temperature, and speed with which the beans reach and/or hold temperatures, resulting in distinct roast characteristics and roast uniformity.
With a drum roaster, green beans are placed in a rotating cylindrical drum. Heat is applied either directly under the drum or through the center through a conduit. Heat can come from either electric heating elements or gas flames. Temperature is measured intermittently either via the flame/electrical element temperature and/or the ambient bean temperature within the drum. If too much heat is applied or if the speed of the rotating drum is set too high the centrifugal force will push the beans against the drum and transfer too much heat onto a section of the beans, causing tipping and/or scorching.
In either case, as green coffee beans heat up, the moisture within the bean will expand, causing the bean to swell. Doing so results in chaff, the thin brown skin on top of the bean, to loosen and separate from the bean, much like the red skin on a peanut. In a fluid bed roaster this chaff is blown up the cylinder and is removed from the beans through the venting system. In a drum roaster, the chaff remains, and is incorporated into the roast. Mr. Sivetz maintains the chaff in a drum roaster carbonizes, forming carcinogens, though this theory has yet to be scientifically proven.
The biggest benefit offered by the Sivetz fluid bed roaster, is its ability to sense the digital bean temperature accurately with a thermocouple. Hence with this information, the degree of roast is known at all times, as well as allowing easy settings for different degrees of roast and offering accurate reproduction of roasts. This is not easily done on a drum roaster, when intermittent probed bean samples and operator eye judgments are used.
Another advantage stems from the greater efficiency in heat transference that is possible with hot air roasters. The beginning stages of coffee roasting are focused on driving moisture from the bean; this moister content can be as much as 12% by weight. The bean can not actually be roasted until the moisture is removed, so this step is critical. When the coffee bean still contains this moisture, prolonged exposure to high heat results in "cooking" rather than "roasting" of the bean. The longer it cooks, the greater the formation of acids such as acetic and quinic, which can be quite irritating to the stomach. Since the fluid bed roaster can reach higher temperatures in half the time of typical cylindrical roasters, the time for these acidic compounds to form is greatly reduced. I have many customers who drink my comfortably, without the previous acidic stomach irritation they previously endured.
I had the good fortune to roast with a Sivetz roaster for over 6 years with a previous company. Regrettably, Michael passed in 2012 and his family has shut down the business. Used Sivetz roasters rarely come up for sale. Fortunately, other manufacturers are incorporating his technology into their designs. I’m delighted to have found a company that has created a fluid bed roaster to my specifications.