Characterization of kappa-carrageenan gels used for immobilization of Bacillus firmus

In this study, aimed at a biochemical and physical characterization of kappa-carrageenan gels used for entrapment of Bacillus firmus NRS 783 (a superior producer of an alkaline protease), effects of carrageenan concentration, gelation temperature, initial cell loading, and strength of the curing agent (KCl) on the properties of cell-free and cell-laden gels were examined. The physical properties of the differently prepared gels that were examined included density, free volume fraction, mechanical strength, and change in gel volume during gel curing. The biochemical characteristics studied included viability of gel-entrapped cells, cell leakage from cell-laden gels, and cell penetration into cell-free gels. For the range of carrageenan contents investigated [between 2% and 5% (w/v)], the mechanical strength of the gels with/without KCl curing was observed to increase with an increase in carrageenan content of gels. The mechanical strength of each gel increased substantially upon extensive curing. Free volume fractions in excess of 0.8 were observed for all gels. Of cells that were viable prior to immobilization, 90-92% remained viable after formation and extensive curing of gels for cell-gel mixtures prepared at 45 degrees C. Attempts at prolonged storage of cell-laden gel beads at 0 degrees C as stock cultures of immobilized B. firmus were unsuccessful due to a significant decline in cell viability during such storage. On the basis of the cell leakage studies, the average pore sizes of 2%, 3%, 4%, and 5% gels were deduced to increase in the following order of carrageenan content (w/v): 4%, 3%, 2%, and 5%. Commensurate with the decrease in the average pore size (or the increased tightness of the gels) with increasing carrageenan content, both the extent of cell leakage and the extent of net cell penetration decreased with increasing carrageenan content for the first three gels. Owing to non-uniform distribution of free space and much larger pores, the extent of net cell penetration in 5% carrageenan gels was considerably low, while the extent of cell leakage in 5% carrageenan gels was an order of magnitude greater than the extents of cell leakage in the other three gels.