Method for operating an agitating device and a digester wherein the digester (1) is filled with a substrate (7) and wherein an agitating device (10) controlled by a control device (50) is disposed in the digester (1). The following process steps are carried out. a) A target load curve (60) is lodged in the control device (50) b) the control device (50) prescribes a target speed of rotation (61) c) the control device (50) operates the agitating device (10) at an actual speed of rotation (71) corresponding to the prescribed target speed of rotation (61); d) the control device captures an actual measurement value (81) which is characteristic of the torque of the agitating device (10) at the actual speed of rotation (71); e) the control device (50) derives from the actual measurement value (81) an actual characteristic value (91) of the applied torque of the agitating device (10); f) the control device (50) compares the derived actual characteristic value (73) against the target characteristic value (63) of the substrate (7) resulting from the target load curve (60) at the prescribed target speed of rotation (61); g) the control device (50) controls the agitating device (10) in dependence on the result of comparison.