Production of gluconic acid by Aspergillus niger immobilized on polyurethane foam

Production of gluconic acid by cells of Aspergillus niger immobilized on polyurethane foam was studied in repeated-batch shake-flask and bubble-column fermentations. For passive immobilization, various amounts of polyurethane foam and spore suspension were tested in order to obtain a suitable combination for optimal concentration of immobilized biomass. Immobilized cells were sucessfully reused with higher levels of product formation being maintained for longer period (65–70h) than free cells. The highest gluconic acid concentration of about 143 g l^–1 was reached on hydrol-based production medium with 0.3-cm³ foam cubes in the bubble column, where the effect of more suitable aeration and particle volume: medium volume ratio scheme was also investigated.