Characteristic behavior of viscosity and viscoelasticity of Aureobasidium pullulans culture fluid

Measurements of dynamic viscoelasticity and steady flow viscosity were made for culture fluids obtained by cultivation of Aureobasidium pullulans IAM 5060 with initial pHs of 6 and 7 and for exopolysaccharide (EPS) solutions obtained by removal of microbial cells. The molecular weight of EPS of the pH 6 culture is about 850,000, and that of the pH 7 culture is much larger. In the present study, the complex viscosity shifted to a considerably larger value than that of the steady flow viscosity. This differs from the Cox-Merz experience law which claims that there is a similarity between the angular frequency dependence of complex viscosity and the shear rate dependence of steady flow viscosity. On the other hand, the dynamic viscosity at small strain amplitude practically corresponded to the steady flow viscosity. The storage modulus of the pH 7 samples decreased markedly with a strain amplitude of more than 0.2, indicating that a dense network structure was formed. The linear region of the plot of viscoelasticity against strain amplitude was wider for the pH 6 culture, reflecting the stability of the network structure in this sample. The viscoelastic level of the EPS solution was a little greater than that of the culture fluid. This suggests that the viscoelasticity of the culture fluid is mainly caused by network structure forming among dissolved polysaccharides.