Mass transfer resistance of sterile plugs in shaking bioreactors

One of the mass transfer resistances for the gas exchange of shaking flasks is the sterile plug. The gas exchange through the sterile plug is described by an extended model of Henzler and Schedel [Bioprocess Eng. 7 (1991) 123]. Based on this model, a new method was developed to obtain the mass transfer resistance of various sterile closures. It consists of measuring the water evaporation rate of the shaking flask and is therefore very easily applied. Sterile plugs made of cotton, wrapped paper, urethane foam and fibreglass and caps made out of aluminium and silicone have been examined. Instead of the oxygen transfer coefficient (k(O(2))), which is commonly found in the literature, the carbon dioxide diffusion coefficient (D(CO(2))) is used to describe the mass transfer resistance of the sterile plug. The investigation revealed that this resistance is mainly dependent on the neck geometry and to a lesser extent on the plug material and density. The gas exchange of aluminium-caps was not reproducible.