RESUMEN
The structure of standard and stabilized calcium pectate gel (CPG) beads has been examined by scanning (SEM) and transmission (TEM) electron microscopy. A two-stage crosslinking procedure with polyethyleneimine (PEI) and glutaraldehyde (GA) led to the formation of a more compact layer on the bead surface. On the other hand, the stabilization procedure did not significantly change either gel bead interior or morphologic properties, vitality and biotransformation activity of immobilized bacterial cells (Nocardia tartaricans) against cis-epoxysuccinate as well as yeast cells (Trigonopsis variabilis) against cephalosporin C. The structure of these cells within the calcium pectate matrix remained unchanged. Moreover, the two-step chemical stabilization of CPG containing T. variabilis or N. tartaricans had a favourable effect on storage and operational stability at semi-continuous and continuous processing in stirred batch and packed-bed reactors. The most valuable effect of stabilization was the fact that the hardened CPG comprising the cells N. tartaricans resisted, for a long time (360 days and more), the destructive effects of the product (such strong sequestering reagent as L-(+)-tartaric acid) at high concentrations (up to 1 M). Non-hardened CPG was destroyed after 21 h. The reference materials, hardened and non-hardened calcium alginate gels (CAG), were destroyed over 3 h or 30 min, respectively.
