Coffee beans loose 15 to 25% of weight during the roasting process with little or no effect on the weight of any debris in the coffee. After roasting, the foreign material is 15 to 25% heavier than the coffee beans and can be easily removed by pneumatic means.
Destoners are essentially an adjustable vacuum cleaner. The roasted coffee destoner is designed to lift only material with the density of roasted coffee beans and nothing heavier. In fact many of them will actually drop out broken bit lighter than the selected density.
The process is accomplished by adjusting the excess air entering the vacuum stream such as to just barely lift the coffee beans to the elevated hopper and therefore drop out anything heavier.