Under normal operating conditions, the liquid in shaken flasks rotates «in phase» with the movement of the shaker table. But under certain conditions, a shaken culture can get «out of phase». This «out of phase» condition is characterized by an uncontrolled swashing of the liquid on the bottom of the flask. Thus, specific power consumption is reduced, mixing is poor and the gas-liquid mass transfer is impeded.
The probability for a culture to get «out of phase» increases with lower shaking diameters, small filling volumes, large and / or many baffles and especially with the viscosity of the culture broth. Also the acceleration of the liquid can elicit «out of phase» conditions. If the shaking frequency is increased relatively quickly; the liquid may get “out of phase”. A low acceleration of shaking frequency keeps the liquid «in phase».
The following video of an "out-of-phase" experiment illustrates both conditions and shows, how the transition can be induced manually:
Two flasks with 500 mL colored water are on the shaker tray at 200 rpm and 25 mm shaking diameter. At the beginning, both flasks are «in phase». The left flask is removed from the tray until the liquid is steady. While placing it back on the shaker, the liquid is quickly accelerated and gets “out of phase”.
Especially when the viscosity of the medium increases with fermentation time, due to filamentous microorganisms, medium components or products, unnoticed «out of phase» operating modes can derogate the results of the whole series of experiments.
Particularly at screening or medium development, strains may be mistakenly selected because of their altered morphology with lower viscosity. Hence, they remain “in phase” and are better mixed and aerated.
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