Production of pullulan form aureobasidium pullulans and rheological properties of its culture broth

Abstract

Pullulan is an $\alpha$-glucan wherein maltotriose units are repeatedly polymerized via $\alpha$-1,6 linkages on the terminal glucose and have many good properties to be applicable. For the efficient production of pullulan from Aureobasidium pullulans, the study on optimization of culture medium was carried out and the composition of optimized culture medium were as follows; sucrose 5\%, yeast extract 0.1\%, $K_2HPO_4\; 0.5\%,\; NaCl\; 0.1\%,\; (NH_4)_2SO_4\; 0.06\%,\; MgSO_4 \cdot 7H_2O\; 0.02\%$. The optimal culture temperature of $\underline{Aureobasidium}$ $\underline{pullulans}$ IFO 4464 was 27$^\circ$C. The initial pH of culture medium appeared important factor for the production of pullulan. The lower the initial pH became, the more cells were obtained but the less pullulan was obtained. The effects of the rate of aeration and agitation on the production of pullulan were investigated. In highly aerated and agitated systems(600 rpm, 2.0 vvm) the maximum production yield and productivity of pullulan were obtained, 60-66\% and $0.290g/l \cdot hr$, respectively, which were higher values than those of poorly aerated and agitated system (200rpm, 1vvm), having 40\% and $0.090g/l \cdot hr$, respectively. The rheological properties of culture broth of pullulan fermentation were investigated. The culture broth obeyed the power-law, $\tau = K(\gamma)^n$ and showed pseudoplastic behavior. As time passed, it became more pseudoplastic in proportional to the accumulation of pullulan. When the maximum production of pullulan was obtained during the fermentation process, the minimum non-Newtonian index, n, was obtained, around 0.2. Consistency index, K, increased with the increase fermentation time up to 7 days. In order to investigate the relationship between concentration of pullulan and kinetic parameters of powerlaw, two equations were introduced. $\ln K(t) = \ln \tau_0-n(t)\ln\; \gamma_0,\; K = AP^B$, where $\tau_0,\gamma_0$ A and B are constant. Experimenta data fit...