Development of a new bioreactor for kojic acid production using Ca-alginate immobilized fungal cells

Abstract

Conidia of Aspergillus oryzae were immobilized of Ca-alginate gel beads and then incubated in a nutrient medium to yield an immobilized biocatalyst producing kojic acid. The growth behavior of mycelial cells in the matrices and the physiological properties of the immobilized biocatalyst was found to greatly affect the kojic acid porduction in immobilized cell cultures. Thus, three major factors investigated were as follows; the preparative conditions of making the immobilized gel beads, the environmental conditions of cultivating the immobilized cells, and then the configuratin and operation of a bio-reactor. When mycelial cells was grown to be confined within the surface of the gel beads and segregated each other by gel structure, the catalytic activity of immobilized cells was maximized. In situ growth of the immobilized cells produced kojic acid in a linear proportion during the cultivation, and then accumulated 83g/L of kojic acid in a shaking flask arter 22 days. The crystal formation at the concentration above 83g/L determined the batch production period, because of the difficulty of separating the crystals from the immobilized gel particles. The immobilized cell culture repeated by replacing with fresh media decreased the procuction rate of kojic acid as the number of regeneration increased, and its stability was abruptly destroyed in third cultivation with the appearance of carverneous center inside the gel beads. When the immobilized cells were incubated in an airlift bioreactor with external loop, the mycelial growth in gel beads was highly dependent on fluid flow pattern which was governed by a baffle located in the bottom section of riser. The spiral fluid flow induced by the baffle resulted in the mycelial growth to be confined within the bead surface as in shaking flask culture. The dual sparging with air and pure oxygen effectively released the oxygen limitation in the immobilized cell cultures. Especially, the small variation of flow rate of pur...