Physicochemical parameters involved in the interaction of Saccharomyces cerevisiae cells with ion-exchange adsorbents in expanded bed chromatography.

Expanded bed adsorption (EBA) is an interesting primary technology allowing the adsorption of target proteins from unclarified feedstock in order to combine separation, concentration, and purification steps. However, interactions between cells and adsorbent beads during the EBA process can strongly reduce the performance of the separation. So, to minimize these interactions, the mechanisms of cell adsorption on the support were investigated. Adsorption kinetics of the baker's yeast Saccharomyces cerevisiae on the anion exchanger Q Hyper Z were directly performed under real EBA operating conditions, in a lab-scale UpFront 10 column. The yeast was marketed either as rod-shaped pellets (type I yeast) or as spherical pellets (type II yeast). For both types, a complete series of experiments for determining the adsorption profile versus time was performed, varying the superficial velocity or the pH. In parallel, the surface physicochemical properties of the cells (surface charge and electron-donor and electron-acceptor components) and of the support were determined. First of all, whatever the yeast types, the relation between cell adsorption and bed expansion has been highlighted, demonstrating the important role of hydrodynamic. However, for the type II yeast cells, adsorption increased dramatically, compared to the type I, even though it was shown that both types exhibited the same surface charge. In fact, there were strong differences in the Lewis acidic and basic components of the two yeasts. These differences explain the variable affinity toward the support, which was characterized by a strong electron-donor and a weak electron-acceptor component. These observed behaviors agreed with the colloidal theory. This work demonstrates that all kinds of interaction between the cells and the support (electrostatic, Lifshitz-van der Waals, acid/base) have to be taken into account together with hydrodynamic characteristics inside the bed.