Production of pullulan using an oscillatory baffled bioreactor

Pullulan is a well-known extracellular polysaccharide produced by the aerobically growing yeast-like fungus Aureobasidium pullulans (A pullulans). Pullulan has a wide range of commercial and industrial applications, eg it is used in adhesives, in laminates or nylon-like fibres and fabrics, and in various foods as a low calorie ingredient. Pullulan itself is produced and deposited on the outside of the fungal cells, and because of the low oxygen permeability of the pullulan, this layer acts as a barrier for the transfer of oxygen from the medium to the fungus. This could be detrimental to the survival of the cells and, in turn, restrict the cultivation of the pullulan. In pullulan fermentation it is therefore desirable to break up and remove this layer of pullulan adherent to the cells. The conventional impeller-driven vessels often exhibit a gradient in mixing capabilities, with high intensities at and near the impeller but low intensities in peripheral regions. Such gradients increase significantly with the scale. The existence of the mixing gradient in a given vessel can potentially inhibit the process of pullulan generation in regions distant from the impeller, leading to reduced productivity. The oscillatory baffled bioreactor (OBB) offers enhanced and uniform mixing at very low shear rate compared with conventional mixing vessels; this would provide better control over the cultivation of pullulan. In this paper, the production of pullulan in the OBB is reported and compared with the fermentation process carried out in a conventional stirred tank fermenter. (C) 2003 Society of Chemical Industry.