Mechanical mixing and biological deactivation: the role of shear stress application time

The macroscopic effects of mechanical mixing on the biological activities of Saccharomyces cerevisiae cells employed in batch fermentations of dilute starch hydrolysate solutions are studied. The rotational speed strongly affects the overall product yield over 300 min⁻¹. Also the fermentation kinetics is markedly influenced by agitation intensity, in that the process appears to be limited by substrate diffusion from the bulk to the cell inside up to 450 min⁻¹, whereas cell metabolism becomes of greater significance over this threshold. A new model is also presented to describe the decrease in specific productivity with the increase in either the mixing intensity or the exposure time to shear stress. The present results suggest the existence of different types of simultaneous mechanisms for metabolic activity inhibition and/or death linked to mechanical mixing.