Production of the Polysaccharide Pullulan by Aureobasidium pullulans Cell Immobilization

This review examines the immobilization of A. pullulans cells for production of the fungal polysaccharide pullulan. Pullulan is a water-soluble gum that exists structurally as a glucan consisting primarily of maltotriose units, which has a variety of food, non-food and biomedical applications. Cells can be immobilized by carrier-binding or entrapment techniques. The number of studies utilizing carrier-binding as a method to immobilize A. pullulans cells appears to outnumber the investigations using cell entrapment. A variety of solid supports, including polyurethane foam, sponge, diatomaceous earth, ion-exchanger, zeolite and plastic composite, have been employed to immobilize pullulan-producing A. pullulans cells. The most effective solid support that was used to adsorb the fungal cells was polyurethane foam which produced polysaccharide after 18 cycles of use. To entrap pullulan-producing fungal cells, agents such as polyurethane foam, polyvinyl alcohol, calcium alginate, agar, agarose, carrageenan and chitosan were investigated. Polysaccharide production by cells entrapped in polyurethane foam, polyvinyl alcohol or calcium alginate was highest and the immobilized cells could be reutilized for several cycles. It was shown that the pullulan content of the polysaccharide synthesized by cells entrapped in calcium alginate beads was low, which limits the method's usefulness for pullulan production. Further, many of the entrapped fungal cells synthesized polysaccharide with a low pullulan content. It was concluded that carrier-binding techniques may be more effective than entrapment techniques for A. pullulans cell immobilization, since carrier-binding is less likely to affect the pullulan content of the polysaccharide being synthesized.