When handling microbial cultures that become increasingly viscous during cultivation it is very important to keep the liquid moving «in phase». For more information and a video see Dr. Shaker’s hint no. 5. There is no need for complicated measuring devices; the condition of the cultures can be determined by accurate observation of the rotating flasks on the shaker tray.
The «out-of-phase» condition is reached when the medium is swirling erratically on the bottom of the flask. This has negative effects on mixing and gas-liquid mass transfer. To ensure sufficient oxygen supply, a higher centripetal force must be applied to keep the liquid moving with increasing viscosity.
The amount of centripetal force depends on shaking speed and shaking diameter as demonstrated in the following:
Therefore to increase the centripetal force, the shaking diameter D and in particular the shaking speed n, must be increased.
This means that for viscous shaken cultures a shaking diameter of 50 mm or preferably 70 mm and a shaking speed of 200 rpm or even more is recommended.