For oxygen to be transferred from a gas bubble to an individual cell, several independent
partial resistances must be overcome
* 1 resistance within the gas film to the phase boundary
* 2 penetration of the phase boundary between gas bubble and liquid
* 3 transfer from the phase boundary to the liquid
* 4 movement within the nutrient solution
* 5 transfer to the surface of the cell
For fermentations carried out with single celled organisms such as bacteria and yeasts, the resistance in the phase boundary between the gas bubble and the liquid is the most important factor controlling the rate of transfer.
Microbial cells near gass bubbles may absorb oxygen directly through the phase boundary and the rate of gas transfer to such cells is increased.
In cell agglomerates or pellets, the O2 transfer within the agglomerate can become the limiting factor.
The mass transfer of oxygen into liquid can be characterized by the oxygen tranfer rate
(OTR) or by the volumetric oxygen transfer coefficient (kLa). These values have
been thoroughly examined as a critical parameter for bioreactor function. The oxygen
transfer rate and the volumetric oxygen transfer coefficient are dependent on the
* the vessel geometry: diameter, capacity
* mixing properties: power, impeller configuration and size, baffles
* aeration system: sparger rate, geometry, location
* the nutrient solution: composition, density, viscosity
* the microorganism: morphology, concentration
* the antifoam agent used
* the temperature
The baffles serve to disrupt the vortex pattern that develops around a single-shaft
impeller rotating in an u