Abstract: A foam breaker for a fermentation reactor has one or more curved rods, preferably hollow rods with open ends (tubes), and an arrangement for so mounting these on the rotor of the reactor that they rotate with the rotor. The rods or the tubes have leading ends or inlets respectively which scoop up liquid of the liquid phase contained in the reactor, and opposite closed ends or outlets respectively at or through which this liquid is slung away or expelled in form of jets which pass through and destroy a layer of foam that forms above the liquid phase in the reactor.