DEVELOPMENT OF BIOREACTORS FOR PRODUCTION OF MICROBIAL POLYSACCHARIDE

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There are many useful microbial polysaccharides. The broth is higly viscous. Therefore it is difficult to separate microbial cells from the broth which yields difficulty in industrial production of the microbial polysaccharides. In this study, we cultivated Aureobasidium pullulans to produce pullulan, which is one of the microbial polysaccharides composed from glucose. To solve the problem accompanied with downstream processing of pullulam broth, we attempted to develop a bioreactor equiped with a membrane separation unit.The results obtained are aummarized as follows1) The broth of pullulan from A. pullulans was cross-filtered with a module of thin channel type made of polycarbonate resin. It was composed from a straight path rectangular channel with a dimension of 24 mm in width and 98 mm in length. The channel depth was 2 mm. The operation condition was optimized to obtain a high permeation flux and high transmission of pullulan. The sucrose content in the medium and its pH were mos … More t important factors affecting the flux and transmission. With the decrease in the initial sucrose content in the medium, the flux and transmission became higher. The control of the medium pH during pullulan fermentation affected considerably the performance of filtration. When the pH was controlled to 6 with the initial sucrose concentration of 1 %, the flux reached 2x10^<-4> m/s and pullulan transmission was 100 %, which enables to conduct a continuous production of pullulan by holding microbial cells from the broth while feeding fresh medium.2) Two bioreactors equipped with a separation unit of microbial cells from the broth were prepared : one was a fermentor made of stainless steel at which bottoma microfiltration membrane with a nominal pore diameter of 5mum was set and in the other bioreactor a crossflow membrane module was placed outside the fermentor and the medium was circulated with a rotary pump (bioreactor equipped with a crossflow filtrator). When the broth was controlled at pH 6 feeding a fresh medium, a continuous production continued for about 40 h using a bioreactor equipped with a crossflow filtrator. After about 40 h the permeation flux became extremely low since the morphology of the cells changed to filamentous-form from yeast-like form which increased the specific resistance of the cake to be formed on the membrane. Therefore, it was difficult to continue. It is necessary to find out an optimum reaction condition such as composition of the feeding medium. Less