The invention relates to a method for operating a stirring device and a fermenter. The fermenter (1) is filled with a substrate (7), and a stirring device (10) which is controlled by a controller (50) is arranged in the fermenter (1). The following method steps are carried out: a) a target load curve (60) is stored in the controller (50); b) the controller (50) specifies a target rotational speed (61); c) the controller (50) drives the stirring device (10) at an actual rotational speed (71) which corresponds to the specified target rotational speed (61); d) the controller detects an actual measurement value (81) which characterizes the torque of the stirring device (10) at the actual rotational speed (71); e) the controller (50) derives an actual characteristic value (91) for the torque applied to the stirring device (10) from the actual measurement value (81); f) the controller (50) compares the derived actual characteristic value (73) with the target characteristic value (63) for the substrate (7), said target characteristic value resulting from the target load curve (60) at the specified target rotational speed (61); and g) the controller (50) controls the stirring device (10) dependent on the result of the comparison.