Evolution of the specific power consumption and oxygen transfer rate in alginate-producing cultures of Azotobacter vinelandii conducted in shake flasks

The evolution of the specific power consumption and the oxygen transfer rate occurring in shake flasks was studied in cultures of Azotobacter vinelandii, which synthesize alginate, a polymer producing non-Newtonian fermentation broths. Power consumption increased exponentially during the course of the fermentation (up to 1.4 kW m−3) due to an increase in the viscosity of the culture broth. This increase in the viscosity was associated with alginate concentration and the mean molecular mass of the polymer, which reached a maximum of 550 kDa after 50 h of cultivation. At the end of the fermentation, when the viscosity and alginate concentration reached a maximum, a slight drop in the power consumption was observed. This was probably because the fluid was in the “out-of-phase” state due to the high viscosity. In order to explain the “out of phase” behavior, the actual shear rate in the shake flasks should be close to 90 s−1. Although the on-line measurements of oxygen transfer rate showed clear differences in the different filling volumes evaluated, both biomass growth and alginate production were very similar in the three conditions tested. Analysis of molecular mass distributions of the alginates suggests that, in the shake flasks cultures, DOT conditions can be more homogeneous than those present in a stirred fermentor without control of DOT and pH.