Elaboration of polysaccharide from sucrose by fermentation with Pullularia pullulans

Abstract

Recently, microorganisms have been recognized as powerful tools for many synthesis processes. The extracellular synthetisation of bacterial polysaccharides is a good example. The fungus Pullaria pullulans has been shown to synthesise the dextran-like polysaccharide pullulans from sucrose. The present research is aimed to study many of the related aspects of the elaboration of polysaccharide from sucrose by Pullularia pullulans. The experiments are performed on laboratory-scale shake flasks and on small scale batch fermenter (7-liters) to gather the necessary informations for large-scale pilot-plant design and scale-up problems, and for the continuous production in futur research. The comparison between two preliminary scales shows that the yield of polysacharide is slightly higher and the production rate is faster in the batch fermentor. The yield is very high in general, around 70% (wt) of initial sucrose after five days in shake flasks and 85% (wt) after four days in the batch fermenter. The viscosity of the fermented broth may become extremely high, up to 42,000 cp, but a high viscosity corresponds to very low yield of polysaccharide. The fermented broth rheological behaviour changes from newtonien at the beginning to the pseudo­plastic behaviour during the fermentation process. Kinetics models of microbial cells growth and cf polysaccharide production are established respectively as, (formules voir document) where R₈, R₉, R₁₆ are parameters depending upon the fermentation conditions, and δ۬ is the production lag time. A 2⁵⁻³ factional factorial experiments design performed on batch fermentor shows that when the range selected, the effects of temperature, aeration rate, agitation speed and initial pH are not significant and the initial concentration of sucrase alone affects considerably the production of polysaccharide