Cell immobilization

Cell immobilization is defined as “the physical confinement or localization of intact cells to a certain region of space with preservation of some desired catalytic activity”.

The advantages of cell immobilization include-

• enhancement of fermentation productivity;
• feasibility of continuous processing;
• cell stability;
• lower costs of recovery and recycling in downstream processing.
  

The techniques can be divided into four major categories based on the physical mechanisms:

• attachment or adsorption on solid carrier;
• entrapment within a porous matrix;
• self-aggregation by flocculation (natural) or with cross-linking agent (artificially induced);
• and cell containment behind a barrier.
  

Cell entrapment is a simple and useful method to provide a simulation of high-density cells used in small-scale bioreactors where a cell recirculation method with e.g. filter and continuous centrifuge is rather difficult. But there is a need for using the cell entrapment method in industry due to additional cost of immobilization material.

Self-aggregation of cells leading to formation of a flock and easy sinking is very useful for cell recirculation, and thus performs cell retaining. Yeast has a natural flocculation capability which is usually employed in the brewing industry. The flocculation of yeast is a reversible, asexual and calcium-dependent process. Lectin-like proteins, so-called flocculins, which stick out of the cell walls of flocculation cells, selectively bind mannose residues present in the cell walls of adjacent yeast cells. Calcium ions in the medium are needed in order to activate the flocculins. Once the yeast forms flocks, the separation of cells from broth can easily be carried out by sedimentation; thus cell recirculation is performed without any force-driven separation, i.e. filtration and centrifugation.