RHEOLOGY PART FOUR: APPLYING RHEOLOGY TO FERMENTATION

In most, if not all of fermentation broth, we are dealing with Non Newtonian fluid. The Non Newtonian nature is due to the composition of a fermentation broth which is not uniform and complex. The fermentation broth often show complex interactions of solid, liquid and gas phases.
To make things worst the rheology of the fermentation broth is always changing as a function of time and with the progress of the fermentation process.
It is more difficult to control and optimize a fermentation process if it is a Non Newtonian fluid! Things would definitely be easier if the fermentation broth is a Newtonian fluid. (But then again there would be no bread, cheese, yogurt, fish sauce and many more fermentation products!)
The main impact of Non Newtonian rheology is that it affect mixings and mass transfers of heat and oxygen and prevent efficient homogenous composition to occur.
We all know that in rheology it is the study of fluid deformation and flow under pressure and the relationship between stress and strain. Through simple observations we can see how difficult it is to mix and aerate viscous fluid. Each rheological type will give different mixing profile.
This has led to the classification of various classes of Non Newtonian fluids such as
1-viscoplastic fluid,
2-bingham fluid,
3-pseudoplastic fluid,
4-dilatant fluid
Non Newtonian rheology curves can be made up of various types. Most of these rhelogical curves are graphs where the x- axis is shear stress and the y- axis is shear rate
The rheological graphs are interesting not only in comparing between the Newtonian and the Non Newtonian but also the varying properties even among the various Non Newtonian fluid
It is interesting to note generally that all Non Newtonian fluids show some similarity in relationship with Newtonian fluid reflecting the effect of shear stress on shear rate. There is roughly a direct linear relationship (with variations) between shear stress and shear rate.
Differences only that Newtonian fluid adhere strictly and very linear and start at point zero of the axis.
1 Viscoplastic and Bingham starts only after certain level of shear stress. This means that the fluid will NOT respond immediately to applied stress and will only react after reaching the critical power point
2 Pseudoplastic and dilatants start at zero point but are curved in their shape. This mean that the fluid will respond immediately to the power or energy input. This is similar to Non Newtonian. However their response will be different in that it is not a linear relationship between stress and shear rate
3 Viscoplastic, pseudoplastic and dilatants are curved in their shapes This means that these fluids react in their yield behavior under stress differently.
So what does these observations mean in fermentation?
These rheological graphs will tell us how to respond efficiently with the type of broth being fermented.By understanding the various rheological changes that occur in the fermentation broth we can:
1 Try to achieve uniform homogenization and optimum mass transfer
2 Try to optimize energy usage in mixing of the fermentation broth
In carrying out the fermentation, we are using the impeller to mix the broth. Energy is transferred and dissipated to the broth by the impeller system. The impeller is in simplicity the shear stress being enforced upon the broth.
The effect of the impeller or mixing on the broth will result in the flow or turbulence of the broth. The broth will respond by exhibiting stress yield properties such as thinning out of the broth to improve mass transfer processes.
So if we know the rheology of the fermentation broth it will help us to adapt to obtain very efficient fermentation by adjusting our mixing regimes. This is especially so when the rheology changes with time and conditions.




Type rest of the post here.