A biomass cleaner, of a harvesting machine, includes at least a first sieve for sieving biomass and having one or more first sieve apertures that are openable and closeable in dependence on control commands. The biomass cleaner also includes an air fan, the speed of which is adjustable in dependence on control commands. The biomass cleaner has one or more control devices that generate the control commands. The first sieve defines a support for biomass that is sieveable via the sieve, and the biomass cleaner includes one or more pressure sensors for generating one or more signals that are related to the pressure of air in the vicinity of the biomass. At least one pressure sensor is operatively connected to supply the 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.