A biomass cleaner (10) of a harvesting machine, comprises at least a first sieve (11) for sieving biomass and having one or more first sieve apertures (13) that arc openable and closeable in dependence on control commands an air fan (16) the speed of which is adjustable in dependence on control commands and one or more control devices that generate the said control commands, the first sieve (16) defining a support for biomass that is sieveable via the sieve and the biomass cleaner including one or more pressure sensors (17) for generating one or more signals that are related to the pressure of air in the vicinity of the biomass. The or each pressure sensor is operatively connected to supply the said signals to a control device that derives therefrom further estimates of the sieve losses and/or the MOG content of the biomass, as defined herein, during operation thereof. The control device defines two control loops (41, 42) a first of which is a relatively fast control loop (41) that controls the adjustment of the speed of the air fan (16) and the second of which is a relatively slow control loop (42) that controls the degree of opening of at least the apertures of the first sieve (11).
