The drum about Fluid-Bed Roasting & the Green Bean Coffee Roaster
To understand the mechanics of fluid bed roasting, you need a few engineering basics.
A mass of distinct individual parts can be handled as a fluid within the right environment. This is referred to as fluid bed mechanics. Individual coffee beans within a fluid bed become a fluid mass when each is lifted and rolled in a stream of hot air.
A Green Bean Coffee fluid bed, roasting machine uses a glass chamber to suspend the green beans with a vented screen over a combustion chamber that has gas heated, fan forced air driven through it. The overall design balances air, bean load, and temperature, so that the flow of hot air will levitate green and roasted beans.
At Green Bean we sell two models with differently sized roasting chambers. The differing volumes determine the bean mass (1.2Kg or 600g), air, and temperature requirements. A specific amount of hot air raises the green beans from room temperatures through roast temperatures. Hot air comes from gas burners and is fan driven up through the beans, separating the beans from the chaff and collecting any foreign material at the same time.
Roast times are between nine and eleven minutes, depending on degree of roast. Cooling times are less than six minutes minutes. So you can comfortably expect to get at least three roasts per hour from your roaster.
Because the roast process depends upon convection, the roast chamber preheats quickly. To get a consistent roast you need to add the correct amount of green beans put into the chamber for each roast. There is a designed balance among weight, airflow, roasting temperature, and roast time. The stated capacity of each size of machine is not variable by much, or the balance is defeated.
If the load is too little, the bean mass is so light that it degenerated as airflow increases and bean movement becomes chaotic. The roast goes so long that the beans are grossly uneven. If the amount is too large, the mass is so heavy that upward air pressure from the blower and combustion is overcome. The bottom layer cooks rapidly, without air coming in to spread heat through the mass. The result is charred beans.
The Green Bean Roaster is the only roaster that allows you full 360-degree access to watch the roasting process as it takes place.
The darkness of the roasted coffee is determined by the red LED number on the control panel – from “0″ to “9.” “0″ will be a very light roast, and “9″ will produce an extremely dark roast.
Different coffees will reach different degrees of darkness at the same setting, so you will have to learn the appropriate setting for each type of coffee you will be roasting. This, of course, is a matter of taste.
The settings represent different finish temperatures for the roast. “0″ sets the machine to finish the roasting cycle at 193 degrees C, and each number up from there represents a 3.5 degree C increase in the finish temperature. You can find out what setting is correct for the type of coffee you are roasting by a couple of different methods. One is to start at a setting of “4″ and, by trial and error, do roasts until you find what number is right.
A more systematic way of doing this would be to set the machine to roast level “9″, then roast the unknown coffee until it looks right. Then, hold down both “arrow” keys at the same time. The LED window will scroll off the temperature in degrees C. You can then use the “Gas” switch to cut off the roast and start the cooling cycle. For the next roast, you can simply set the machine to terminate the roast cycle at the temperature that you observed.
The profile set, you start the Green Bean Roaster and watch the beans tumble and dance as the fan driven air brings them up to temperature.
The temperature is increased slowly as we want the beans to lose a certain amount of moisture. You’ll notice that a decimal point will appear in the LED display during the roast cycle. This indicates that the burner is actually operating. The burner will have a number of six-second off pulses during the roast, as microcomputer adjusts the roast rate. But if it is off for more than six seconds, you know the roast has concluded, and that the machine is in the cooling cycle.
Because the roast cycle is controlled by temperature rather than time, the results of the machine are very repeatable, although there can be some slight variation in colour if the machine is very cold at the start of the roast.
There is no need to let the machine “rest” between roasts. It is built to withstand continuous operation.
In Green Bean fluid bed machines, the balance among air volume, temperature, and bean weight creates the fluid bed, and it sets limits for roasting to the desired temperature.
Roasting goes only to the selected temperature. Once the terminal temperature is set, there is little to do during roasting. When the gas combustion chamber switches off, the fan continues to force room temperature air through the roasted coffee beans. This fast removal of heat “sets” the flavour in the beans and also prevents over roasting.
It’s important to note that unlike a drum roaster you have no need to remove the beans from the roaster to cool the beans. The Green Bean roaster will automatically switch off when the temperature reaches 36 degrees Celsius (body temperature).
With the right amount of green beans, roasts at specific temperatures will have similarity, roast after roast.
If you’d like more (or less) detail, on the roaster and it’s merits relative to the competition please feel free to contact us for a chat.
Thanks, and happy roasting!
