the subject invention will now be described in terms of its preferred embodiments . these embodiments are set forth to aid in understanding the invention , but are not to be construed as limiting . the invention relates to an agitator for use in a bioreactor . the agitator has at least two agitator blades ( 1 ) that have segments with distal ends . the segments of the agitator blades are arranged radially to the axis of rotation ( 2 ) and inclined at an angle of attack in the direction of rotation relative to the axis of rotation ( 2 ). the angle of attack of the segments may change continuously or stepwise from 25 ° to 35 ° at the distal end of the upper segment to 55 ° to 65 ° at the distal end of the lower segment of the agitator blade . it is preferred that the angle of attack changes from 30 ° at the distal end of the upper segment to 60 ° at the distal end of the lower segment . the agitator may have two to eight agitator blades ( 1 ) on one plane , with four blades preferred . the agitator blades ( 1 ) may consist of several segments ( 5 ), and the angles of attack of each segment may be different . it is preferred that the angles of attack of the segments of an agitator blade be the same as the angles of attack for the segments of each additional agitator blade , although , as previously mentioned , the angle of attack of one segment of an agitator blade may be different than the angle of attack of a different segment of the same agitator blade . depending on the size of the bioreactor , the agitator blades ( 1 ) can be arranged in a plurality of planes above and below one another . one to eight planes are expedient . preferably , the agitator blades ( 1 ) are fixed to a blade mount ( 3 ) which serves to receive the agitator shaft ( 4 ). if appropriate , the blade mount ( 3 ) is joined to the agitator shaft by fixing means . such a blade mount ( 3 ) having agitator blades ( 1 ) fixed thereto is termed agitator element hereinafter . the agitator elements therefore preferably have a radially inner hollow ring region which encloses the agitator shaft ( 4 ), to which ring region two to eight agitator blades are attached at regular intervals from one another . an agitator blade ( 1 ) may consist of equally large segments ( 5 ), with different angles of attack . the angle of attack of the distal end of the upper segment may be between 25 ° and 35 °, and the angle of attack of the distal end of the lower segment may be between 55 ° and 65 °. preferably , an agitator blade ( 1 ) may consist of three equally large segments ( 5 ), with different angles of attack , where the angle of attack of the distal end of the upper segment is 30 °; the angle of attack of the distal end of the middle segment is 45 °; and the angle of attack of the distal end of the lower segment is 60 °. the agitator has a diameter equal to the diameter of a circle concentric to the axis of rotation and encircling the agitator blades . the ratio of the agitator diameter to the bioreactor internal diameter ( in accordance with din 28131 , the ratio d / d , agitator diameter / reactor internal diameter ) of a single agitator blade may be in the range from 0 . 3 to 0 . 5 , preferably from 0 . 35 to 0 . 45 . each agitator blade has a height , and the ratio of agitator blade height to agitator diameter ( d / h ) may be 0 . 1 to 0 . 3 , and preferably 0 . 1 to 0 . 15 ( see fig1 ). the shape of an agitator blade may be rectangular , with the edges and corners rounded . the agitator blade could also have the shape of a cylindrical section or be bent once or several times to achieve the inventive angle of attack . the shape of the bioreactor is not critical . usually , a cylindrical vessel is used . essential parameters for evaluating efficiency and performance of an agitator are the power input [ w / m 3 ], oxygen transport coefficient [ k l a ( 1 / h )], mixing time [ s ] and cell growth [ cell concentration and vitality ]. the oxygen transport coefficient k l a is determined according to the following formula : k l ⁢ a = otr ( c o 2 * - c o 2 ) k l a : oxygen transfer coefficient otr : oxygen transfer rate [ mol /( l · h )] c * o2 : equilibrium concentration of oxygen at the phase boundary c o2 : concentration of oxygen in the interior of the suspension [ mol / l ] the k l a value can be determined by various methods . it has been found that the measurement range is method - dependent . in the present invention , the “ dynamic method ” was used to determine k l a . zlokarnik , m ., rührtechnik — theorie und praxis , springer verlag , heidelberg , new york , 1999 . a stirred - tank fermenter with a working volume of 10 l is used for culturing a cho cell line . to produce the culture medium , the individual constituents are supplemented in heated purified water ( type 2 ) in a sterile vessel . the media osmolality ( 0 . 29 osmol / kg ) is set , as in the experimental determination of the k l a value , by nacl . the ph of 7 . 1 can be set via addition of correction agents . the drive unit consists of a bench construction having a suspended electric drive motor . this is a 0 . 75 kw direct current motor having a range of speed of rotation of 0 to 1500 rpm . to form the supply unit , all apparatus , connections and fittings which are required for providing and removing steam , cooling water , wastewater , compressed air , carbon dioxide , nitrogen and correcting agents are combined . these include the heating system , gas mixing station and the pressure control valve . in addition , the electrical energy supply is included in the supply unit . the bioreactor used has a height - to - diameter ratio ( h / d ) of 2 . 0 . the reactor is constructed with dished base , flat lid and longitudinal inspection glass . the heating is performed via a jacketed heat exchanger ( v = 3l ). in the vessel there are four baffles having a width of 0 . 1 × d . three 25 mm and two 19 mm ingold ports are let into the vessel wall laterally . depending on the requirements of the measurement control instrumentation of the fermentation , a po 2 electrode , a ph electrode , a temperature sensor ( pt 100 ), a sampling valve ( cv 25 ), a turbidity probe and pco 2 electrode can be used . the agitator elements consist of stainless steel and are fixed via two grub screws on the agitator shaft which is centrally located in the fermenter . the direction of rotation was chosen to be anticlockwise . the number of agitator elements per fermenter was three for the standard disc agitator and two for the inclined - blade agitators . the standard disc agitator consists of six fixed perpendicular discs symmetrically attached to a horizontally arranged disc ( din 28 131 ). a standard disc agitator with a diameter ratio ( d / d ) of 0 . 4 was used . the angle of attack of the paddles of the type 1 inclined - blade agitator was variable . the oxygen input was set for 45 ° ( sbr type 1 ) and for 60 ° ( sbr type 1 60 °). the dimensions were as follows : d : 118 cm blade width d b : 40 cm blade length : 95 cm diameter ratio ( d / d = 0 . 55 ) in contrast to the type 1 inclined - blade agitator , the inclined - blade agitator type 2 was designed with a narrower but longer paddle surface . the paddle is slightly bent at the ends . the position of the bent ends is opposed ( in z shape ). the dimensions were as follows : d : 118 cm blade width d b : 16 / 29 cm blade length : 115 cm blade length without bent ends : 93 cm diameter ratio ( d / d = 0 . 55 ) inventive inclined - blade agitator ( sbr type 3 ) ( fig4 and 5 ): the inventive inclined - blade agitator sbr type 3 is subdivided into three equal - sized segments . each segment was 32 centimeters in length . in contrast to the inclined - blade agitator type 2 , the outer segments were bent in the same direction ( c shape ). the two outer segments were inclined with respect to the central segment in the direction of rotation by 15 °. the dissolved oxygen concentration was determined using a clark oxygen electrode ( metler toledo , inpro ® 6000 ). the ph and temperature were measured in the fermentation system by probes installed in the probe ring . for the temperature measurement , a pt 100 resistance thermometer was used . its accuracy is tested during sterilization and at prescribed time intervals during fermentation by a contact thermometer ( type cs 20 ). the ph was determined by a combination ph electrode from ingold . before installation , this electrode was calibrated with buffer solutions of ph = 4 . 01 and ph = 7 . 0 . for measuring pressure , a pressure paste electrode was used . to determine the mixing times , a turbidity measurement system from aquasant messtechnik ag ( as81 with af44 ) was used . the oxygen transport coefficient was determined by the saturation method . the fermentation system is charged with 10 l of deionized water after installation of the gas - introduction and agitator element under test . all internals ( probes , baffles and riser pipes ) and process parameters ( p = 1 bar , t = 37 ° c ., osmolality = 0 . 3 osmol / kg ) correspond to those in the fermentation . the osmolality is set and checked by means of nacl . after heating and calibrating the po 2 electrode , all of the dissolved oxygen can be removed from the medium by gas introduction with nitrogen ( gas 1 = 0 . 5 l / min , speed of rotation = 250 rpm ). at time point t = 0 , pure compressed air introduction starts and the respective speed of rotation is set . the dissolved oxygen concentration increases to the saturation concentration of approximately 6 . 6 g / l . after three experiments the po 2 electrode was recalibrated , and during long use of the same medium , the osmolality was checked daily . k l ⁢ a = - ln ⁡ [ 1 - ( c o 2 , ⁢ l / c o 2 , l * ) ] t it was determined as a function of speed of rotation and the feed gas volumetric flow rate ( gas 1 ). for these parameters , ranges and steps were chosen which correspond to those of the fermentation . the test liquid used in the experiments was 0 . 15 molar nacl solution ( 8 . 7 g / l ). this has hydrodynamic properties ( coalescence behaviour , oxygen saturation concentration ) similar to the medium . by using the inventive inclined - blade agitator type 3 , a markedly higher increase in k l a value could be achieved , compared with all other agitator systems used , not only with increasing speed of rotation but also with increasing gas - introduction rate . the fall in k l a value which occurs at a speed of rotation of 200 rpm when the inclined - blade agitator type 1 was used occurred in this agitator in the form of a reduced increase . the oxygen transport coefficients achieved by the standard disc agitator at 250 rpm could be achieved with the inclined - blade agitators at speeds of rotation of 100 to 150 rpm ( fig6 ). the mixing time was measured using a turbidity measurement system with milk as tracer ( all other conditions similar to the k l a value determination ). at a tracer concentration of 5 ml / l , a measurement signal of 85 % of the maximum measured value was established . the change in turbidity was followed by the turbidity probe installed in the probe ring and displayed on the compensation recorder . the termination condition for this experiment was a constant measurement signal of 85 %. the time to achieve the desired mixing quality of 95 % is the mixing time . by using the inclined - blade agitator , the mixing times could be decreased by up to 70 %. using the inclined - blade agitator type 3 , the mixing time could be decreased by approximately 40 % compared with the other inclined - blade agitators ( fig7 ). this experiment was performed analogous to the example 1 , but with a stirred - tank fermenter with a working volume of 1000 l . the fermenter was filled with a 0 . 15 mol nacl water solution at 37 ° c . two different types of agitators were compared with respect to their oxygen transfer coefficient , k l a , namely a standard inclined - blade agitator ( sbr type 1 ) and an inventive agitator ( sbr type 3 ). before pure compressed air was introduced into the fermenter , oxygen was removed by the introduction of nitrogen ( 20 l / min , 150 rpm ). afterwards , pure air was introduced at different velocities ( gas 1 = 10 , 12 or 20 l / min ). the results of the experiment using two different oxygen electrodes ( electrode 1 and electrode 2 ) are plotted as “ oxygen transfer coefficient ”- vs -“ speed of rotation ” in fig8 . by using the inventive inclined - blade agitator type 3 , the k l a value could be increased by a factor of between 1 . 5 and 2 . 7 compared with the standard agitator of type 1 .