2.5.1. Stirring and mixing
The transfer of energy, nutrients, substrate and metabolite within the bioreactor must be brought about by a suitable mixing device. The efficiency of any one nutrient may be crucial to the efficiency of the whole fermentation.
For the three phases, the stirring of a bioreactor brings about the following:
Dispersion of air in the nutrient solution
Homogenization to equalize the temperature and the concentration of nutrients throughout the fermentor
Suspension of microorganisms and solid nutrients
Dispersion of immiscible liquids
0.5 m/sec. Nutrient solutions can be subdivided into two groups according to the way they behave when stirred: viscous solutions with Newtonian and non-Newtonian properties; and viscoelastic solutions, in which normal liquid-state properties are not observed in stirred vessels. The viscosity, the ability of a material to resist deformation, is the most significant property affecting the flow behavior of a fluid. Such behavior has a market effect on pumping, mixing, heat transfer, mass transfer and aeration.
There are only a few examples which fall into the second group, e.g. polysaccharides and certain antibiotic fermentations. Most fermentation solutions fall into the first category. Uninoculated solutions and bacterial cultures often behave as simple Newtonian liquids.
With many mycelial organisms, changes occur during the fermentation not only in the
amount of mycelium, but in the characteristics of the nutrient solution. Substrates are
taken up during metabolism and the proportion of undissolved substrates is reduced. At
the same time, metabolites are excreted, thus affecting the viscosit