Sunday, April 1, 2018

Rheology of the Fermentation Broth

Rheology of the Fermentation Broth 

The rheology of the fermentation fluid during gellan gum production exhibits strongly pseudoplastic behaviour, even at 0.1 % (by mass per volume). 
The initial broth viscosity is that of Newtonian fluid with a viscosity close to that of water, but the broth rapidly becomes non-Newtonian with strong shear thinning properties.
This pseudoplastic behaviour during the exopolysaccharide accumulation phase is also common in the production of other microbial polymers and described by power law model:
· h = t/g = kg n–1 

where h stands for broth viscosity, t is the shear stress and g is the shear rate. The model has two independent
parameters: the shear thinning index n (equal to 1 for Newtonian fluid and decreasing to 0 with increasing degree of shear thinning) and consistency index k. Dreveton et al. (28) found that the value of the index n dropped quickly from an initial value of around 1 to 0.30 within 9 h of culture and remained constant thereafter.
The consistency index, k, increased steadily with polymer concentration. The power law is, however, valid
only if the stress t exceeds the critical value tc (which is extremely difficult to determine). The yield stress, t0, is
another important factor that describes the shear stress at the very beginning of the pseudoplastic behaviour,
corresponding to the first non-zero shear rate. The square root of this parameter was found to be linear function of
the gellan concentration. 

Dreveton et al. (28) reported that viscosity of fermentation broth during production of gellan gum depends on media constituents. 
Growth without organic nitrogen source resulted in a broth of low consistency and intense shear thinning behaviour. Fermentation parameters were also reported to influence the viscosity of fermentation broth. Viscosity of fermentation broth appears higher at higher agitation rate.