Micro bubble aeration in stirred bioreactors represents a suitable method that allows sufficient oxygen transfer without the necessity of intense stirring. However, understanding of micro bubble behavior in real culture media remains a crucial factor in the interpretation of possible cell/bubble interactions and the scale up of bioprocesses. The present study focuses the impact of various stirrer configurations, sparging rates, as well as 2 different bioreactor scales on micro-bubbles in SF-900 If insect culture medium, which represents a suitable example for Plutonic F68 supplemented culture media commonly applied in cell culture processes. Obtained results for the first time demonstrate a coalescence determined increase in bubble Sauter diameter d(32) from 187 to 211 mu m with increasing stirring intensities. The development of an empirical model reveals d(32) infinity V-0.4(P/V)(0.4), were P represents the stirrer induced power input and V the medium volume. The results were interpreted a consequence of micro-bubble entrapment in turbulent flow structures. Additional studies on the volumetric oxygen transfer coefficient k(L)a demonstrate a strong impact of axial flow impellers and indicate that increases in k(L)a can be attributed to an improvement of the gas-liquid oxygen transfer rather than an increased specific gas surface area clue to bubble breakup. (C) 2014 Elsevier Ltd. All rights reserved.