Fluid-bed destoners are the best option.
Fluid-Bed DeStoners are the only destoners that do not break beans. They are very small, frequently placed on a counter in a café. The stones fall into a container and the stones remain behind for collection.
Fluid-bed destoners work by floating the roasted beans across a bed of air fluidizing them into an almost liquid state. As in water the light beans float to the end of the screen while the heavier stone sink to the bottom.
What to Consider when buying a fluid-bed destoner.
Easy to remove stones without disassembly.
Some destoners require disassembly to remove the stones. There are even promotional videos for them showing them being disassembled while placing the hopper and other parts on the floor. Not a good choice for food-grade equipment. If you are not going to put the parts on the floor, where will they go? Select a stoner with a hinged lid that opens for quick access to remove stones.
Reduced retained beans with a single gate.
Beans in a destoner with the stones after the batch has run are called retained beans. They exist in all destoners. Fluid beds have at least one gate to hold the stones back. That is where you will find the retained beans. Fluid-beds with more than one gate have more retained beans. Each inch of gate retains beans. A single gate destoner can have as little as five inches of gate. A conical destoner with multiple gates may have more than twenty inches of gate.
Avoid a Fluid-Beds on an angle that may allow stones to pass.
When a fluid-bed is on an angle, as with a cone, beans gain forward momentum, and it raises the center of gravity of the stones compared to the gate. That means the stones are more likely to pass over the gate. There are three ways to correct for too much angle on the bed. Add more gates, increase the height of the gates, and hope the stones are removed before they make it to the last gate. If all else fails, advise the operator to run the beans through the destoner several times. That results in copious amounts of residual beans and missing stones. Not a workable solution for most roasters. The best solution is a single gate with a fluid-bed at less than a three-degree angle.
Avoid Fluid-Beds with stones that are hard to find.
A short single gate results in fewer retained beans and stones in a small area near the gate. The stones are easy to find in a small area. Fluid-beds with multiple gates make it hard to find stones scattered in a large amount of retained beans. A scrapper can be provided to wipe the retained beans into the container once the stones have been removed from a flat bed with a single gate. See photo above.
Avoid DeStoners that recommend running the batch several times.
It is true that all destoners miss stones. Stones that are near the density of the beans cannot be separated. They are normally light concrete stones that are not likely to cause damage to grinders. Running through the destoner again is not likely to remove them either. If they are the same density as the roasted beans they may never be separated.
Do You Need a Rare Earth Magnet?
Large pieces of metal are the easiest to separate from beans. The chance that a screw or nut will pass through a destoner is exceedingly small. So why buy a magnet? Some of the lite concrete stones have metal in them. It is also common for metal dust to be in coffee. Many roasters with magnets in their green coffee systems catch magnetic stones and dust. When you add a rare earth magnet to your destoner you may find light magnetic stones and dust over time.
Conclusion
When buying a small destoner consider where you are going to use it, how you are going to remove the stones, and how well it will remove the stones. Remember you never save money by buying the wrong machine.
