RHEOLOGY OF FERMENTATION BROTH

The fermentation broth is a very complex soup or solution. Fundamentally, the fermentation broth is the sea of nutrients in which the microorganisms grow, reproduce and 'swim' . The fermentation broth supply the microorganisms with all the nutrients the microorganisms need to grow and produce the various fermentation products.

The fermentation broth too act as the medium for various physical, biochemical and physical reactions to take place. The fermentation broth will be implicated in all the mass and heat transfers that occur within the fermentor, and it will be the medium that holds the fermentation products formed.

The nature and composition of the fermentation broth temporally and spatially will affect the efficiency of the fermentation process. The interactions between the fermentation broth and the various components is complex and affect both directions

WHAT IS IN THE FERMENTATION BROTH?
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At any time the composition of the fermentation broth is complex consisting of anything that ends up in the fermentation broth. This includes:

1 Raw substrates
2 Fermentation products
3 Microorganisms and its derivative components
4 Chemical additives added to the fermentor
5 Gases such as oxygen and other metabolic gases

All three main phases; solid, liquid and gases are present in the fermentation broth and their possible interactions


RHEOLOGICAL PROPERTIES OF FERMENTATION BROTH
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One of the most important singular properties of the fermentation broth will be its rheological or viscosity characteristics. Let us not stick to strictly to the definition and quantification of rheology but rather try to appreciate it from its behaviour and its impact.

We always 'picture' fermentation broth as a thick gooey sticky mixture that is thick and viscous compounded by rising bubbles of gas exploding at the broth surface. Maybe this picture is too dramatic but in a way it is true!

The viscous nature or the rheological properties will affect the mixing regimes of the fermentor.
Viscosity is not a simple but a complex phenomena that is always changing and responding to various parameters. Very rarely can we describe a fermentation broth as following a Newtonian behaviour. In most cases it is a complex combinations of various Non Newtonian behaviour.

This poor understanding of the fluid behaviour of the fermentation broth will affect the efficiency of mixing and liquid circulations resulting in poorly controlled or less economical fermentation process


WHAT CAUSES THE BROTH TO BE VISCOUS?
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The viscousness of the fermentation broth is caused by the interactions of the various components in the fermentation broth. The interactions may occur between the components of the broth and the water or it could result from the interactions between the components themselves. Such interactions in the viscosity of the broth could occur at the level of the ions and molecules which involve the various ionic forces or it could involve at the macrolevel such as between the various biopolymers tangling and sliding with each other. The overall result will be that the fermentation broth will be viscous.

Now let us look at one of the components which make the fermentation broth viscous, that is the contribution of sugars to the viscosity. Sugar or the carbohydrates are the main carbon source in any fermentation media and supply the carbon needed for energy and skeleton structures of the cells and organic compounds

Experience have shown to us that sugar in solution is sticky, but dry sugar is not sticky. The stickiness or viscousness of the sugar in solution is caused by hydrogen bondings which develop between the sugar molecules and water. During the interactions of sugar and water the hydrogens in the water molecules and the hydrogen in the sugar molecules have an attraction for each other. Thus it is the hydrogen bondings that make the sugar sticky!

Thus we see that most of the viscosity in the fermentation broth is caused by the various hydrogen and other ionic bondings


IMPACT OF VISCOSITY ON FERMENTATION
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The most crucial effect of viscosity is that it makes the situation very difficult to achieve proper and complete mixings. This will affect the various mass transfer processes that occur in the fermentor. poor mixings due to high viscosity will also result in the formation of various physical and chemical gradients

Viscosity makes scaling up studies difficult due to the change in behaviour of the fermentation broth such as difficulty in mass heat transfers, solubility of components and gases and mixings at the upper scale of fermentation process