Abstract:
Biocatalytic efficiency of immobilized cells has been known to be significantly higher than that of free cells. Increases in specific rates of substrate uptake and product excretion have been reported for immobilized cells. However, for immobilized (entrapped) aerobic organisms, significant substrate concentration gradients may exist as the polymer gel exerts its own resistance to mass transfer. Cells located at the periphery are most likely to have adequate oxygen and nutrients while cells located relatively further away (central part) are under oxygen and nutrient limitation. This work focused on the reduction of mass transfer limitations in entrapped aerobic cells by entrapping the cells in micro porous beads. Micro porous beads were prepared by mixtures of different ratios of starch or granulated sugar and sodium alginate. They were gelled by dropping in calcium chloride solution and the starch or granulated sugar was leaked out of the bead into the cross linking/curing agent (2% CaCl2.H20), there-by producing micro pores. The effects of sodium alginate and starch or granulated sugar concentrations on the porosity, bead stability, immobilized cell concentration, cell leakage and cell distribution were studied. The results showed that the optimum sodium alginate concentration for both normal and micro porous beads was 2% while optimum concentrations of starch and granulated sugar mixtures were 0.4% and 1.2%, respectively. However, STA represented 2.0% sodium alginate – 0.4% starch micro porous bead while GSA represented 2.0% sodium alginate – 1.2% granulated sugar micro porous bead. The micro porous beads were more stable and gave better cell distribution than the control (2% sodium alginate alone). Generally, beads prepared with sodium alginate - starch mixture were more stable and immobilized higher cell concentration than those prepared with sodium alginate-granulated sugar. The results of alpha amylase and gluco amylase production by Aspergillus niger revealed that with micro porous beads, the amylase activities were significantly higher (p<0.001) than those obtained with the control beads. In comparison 2.84 U/ml/min alpha amylase and 7.27 U/ml/min gluco amylase was obtained for the control, STA gave 6.61 U /ml/min alpha amylase and 11.98 U/ml/min gluco amylase, while GSA gave 5.04 U/ml/min alpha amylase and 10.01 U/ml/min gluco amylase. Good porosity, better stability and high immobilization capacity obtained with micro porous beads show that they are very good carriers for aerobic processes.
