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Bioprocess Biosyst Eng (2013) 36:911925 DOI 10.1007/s00449-012-0825-y
ORIGINAL PAPER
The effect of hydrodynamic stress on the growth of Xanthomonas campestris cultures in a stirred and sparged tank bioreactor
F. Garcia-Ochoa E. Gomez A. Alcon
V. E. Santos
Received: 7 May 2012 / Accepted: 5 September 2012 / Published online: 26 September 2012 Springer-Verlag 2012
Abstract The specic growth and the xanthan production rates by the bacterium Xanthomonas campestris under different shear levels in shake asks and in a stirred and sparged tank bioreactor have been studied. The shake ask has been used as a reference for studying the shear effects. An effectiveness factor expressed by the ratio of the observed growth rate and the growth rate without oxygen limitation or cell damage was calculated in both modes of cultures. It was observed that the effectiveness factor was strongly dependent on the operational conditions. A strong oxygen transfer limitation at low stirring rates, indicated by a 54 % decrease in the effectiveness factor was observed. In contrast, at higher stirrer speed, cell damage was caused by hydrodynamic stress in the turbulent bulk of the broth, yielding again a decrease in the effectiveness factor values for stirrer speeds higher than 500 rpm. Cell morphological changes were also observed depending on the agitation conditions, differences in morphology being evident at high shear stress.
Keywords Mechanical stress Shear stress Shear rate
Cell damage Xanthan gum production Xanthomonas
campestris growth Oxygen transfer rate Oxygen uptake
rate
List of symbolsa Specic interfacial area (m)
Cj Concentration of compound j (kg j m-3)
CO2 Dissolved oxygen concentration(mol m-3 or % saturation)
D Stirrer diameter (m)
DL Oxygen diffusion coefcient in liquid phase (m2 s-1)
d Maximum inner diameter of ask (m) d0 Shaking diameter (m)
dc Average cell diameter (m)
E Energy (J)
g Gravitational acceleration (m s-2)
k Consistency index in a power-law model (Pa sn)
kL Mass transfer coefcient (m s-1)kLa Volumetric oxygen mass transfer coefcient (s-1)
mO2 Dissolved oxygen consumption coefcient (molO2 kg X-1 s-1)
N Stirrer speed or shaking frequency (rps or rpm)n Flow behaviour index (-)ne Modied Newton number or Power number for shake asks ()
NP Power number ()
NPh Phase number dened according equation (31) ()
OTR Oxygen transfer rate (mol O2 m-3 s-1)
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