Liquid films on shake flask walls explain increasing maximum oxygen transfer capacities with elevating viscosity

In biotechnological screening and production, oxygen supply is a crucial parameter. Even though oxygen transfer is well documented for viscous cultivations in stirred tanks, little is known about the gas/liquid oxygen transfer in shake flask cultures that become increasingly viscous during cultivation. Especially the oxygen transfer into the liquid film, adhering on the shake flask wall, has not yet been described for such cultivations. In this study, the oxygen transfer of chemical and microbial model experiments was measured and the suitability of the widely applied film theory of Higbie was studied. With numerical simulations of Fick's law of diffusion, it was demonstrated that Higbie's film theory does not apply for cultivations which occur at viscosities up to 10mPas. For the first time, it was experimentally shown that the maximum oxygen transfer capacity OTR_max increases in shake flasks when viscosity is increased from 1 to 10mPas, leading to an improved oxygen supply for microorganisms. Additionally, the OTR_max does not significantly undermatch the OTR_max at waterlike viscosities, even at elevated viscosities of up to 80mPas. In this range, a shake flask is a somehow self-regulating system with respect to oxygen supply. This is in contrary to stirred tanks, where the oxygen supply is steadily reduced to only 5% at 80mPas. Since, the liquid film formation at shake flask walls inherently promotes the oxygen supply at moderate and at elevated viscosities, these results have significant implications for scale-up.