Adequate and continuous supply of oxygen to the growing cells of the microorganism is a crucial factor. The dissolved oxygen profiles usually require special attention during process scale-up due to various reasons, such as maintaining the specific power input and a turbulent flow on both scales, keeping the air-flow proportional and the pressure identical. Maintaining a turbulent flow on both scales is more difficult to control than the other three requirements.
If the viscosity, usually in the range of factors of 30 - 2000 more viscous than water, increases during the fermentation process, the Reynolds number may become too low resulting in a large decrease of oxygen transfer. A general solution to this problem is diluting the process, which affects the productivity. The dissolved oxygen (DO) profiles in 25,000-L fermenters show relatively high concentrations of oxygen in the well-mixed Rushton turbine regions, while low to very low values were measured in the radial and axial planes away from the stirrers. This may lead to relatively dead zones in even larger-scale fermenters where oxygen depletion may reduce the productivity of the process.
Recently, progress has been made to improve oxygen transfer by introducing novel gas injection nozzles and using pure oxygen. Intensified oxygen mass transfer can be achieved by injecting gas into liquid via these nozzles at supersonic flow velocities, forming very tiny gas bubbles (size range of 100 um) and giving enhanced interfacial area and higher gas pressure in the bubbles on commercial scale.I tend to think much more about keeping oxygen out of reactors. [There are three sides to the fire triangle: heat, oxygen and fuel. The typical 2000 gallon reactor is full of flammable fuel, i.e. solvent, so keeping the oxygen level close to zero is important.]
Fermentation is about getting oxygen to the bugs so they can make product, so keeping oxygen levels up is important. It looks like fermentation processes run into similar issues as other agitated processes -- if the solution becomes too viscous, you can't get oxygen to the bugs and your process slows. Also, it appears that agitation can only do so much to make sure that oxygen is well-distributed throughout the reactor. (I don't think I'm wrong to say that a Rushton turbine has a relatively high power number, so you would it expect it to deliver the most mixing capability for gas/liquid systems*, so that tends to indicate the problems associated with oxygen distribution in fermentation...)
(I wonder if this is a problem in beermaking? I am going to guess "no", but I don't know very much about beermaking processes...)
*Note: IANAChE, i.e. I Am Not A Chemical Engineer.