Effect of dissolved oxygen concentration and pH on the mass production of high molecular weight pullulan by Aureobasidium pullulans

The effects of DO and pH on the mass production of pullulan with high molecular weight and the morphology of A. pullulans ATCC 42023 were evaluated. A. pullulans showed a maximum production of pullulan (11.98 g/l) when the initial pH of the culture broth was 6.5 in a shake-flask culture. In a batch culture, the mixture of a yeast-like and mycelial cell forms was found at a pH of 4.5, and the maximum production of pullulan (13.31 g/l) was obtained. However, a high proportion of high molecular weight pullulan (M.W. >2,000,000) was produced at a pH of 6.5, with a yeast-like morphology. The maximum pullulan production yield (51%) was obtained at a pH noncontrol (initial pH 6.5) and DO control (above 50%) condition. Pullulan degrading enzyme was activated when the pH of the broth was lower than 5.0 and the portion of low molecular weight pullulan was increased. The formation of a black pigment was observed at an initial stationary phase, at 40 h of fermentation. Therefore, the fermentation should be carried out in a pH noncontrol (initial pH of 6.5) and DO control (above 50%) condition, and should be harvested before reaching the stationary phase (around 40 h) for the production of high molecular weight pullulan.