The insert (13) has mixing elements (16) extending in a radial and axial manner. The mixing elements have a modifiable effective impact surface. The mixing elements or a part of the mixing elements are arranged in a foldable movable manner. The mixing elements are aligned transverse to an extension of mixing arms. The mixing elements are enclosed between horizontal running, vertical distant and circular disk type board parts, whose rotational axis is a rotation axis (x) of a stirring device (6). The mixing elements are designed in a screw thread type.