Microbial cells are more readily rendered nonviable by the combined action of air sparging and mechanical agitation than by either action along. A. bubble breakup/coalescence model that incorporates the cell-bubble encounter rate, bubble breakup rate, and death probability is proposed to describe cell inactivation in the presence of bubbles maintained through the joint action of agitation and air, which is continually fed into the impeller stream region via passive vortex entrainment from the surface above or via active sparging from below. Experimental results obtained from a fragile algal (Ochromonas malhamensis) culture are consistent with the model prediction. In particular, the specific cell death rate is linearly related to the specific bubble interfacial surface area. It is shown that cells exhibit sparging-sensitive characteristics when agitation is mild, but become sensitive to surface vortexing when agitation turns vigorous enough to introduce air entrainment. Experimental data obtained from different stirrer sizes are in good agreement with the model.
(c) 1992 John Wiley & Sons, Inc.