Studies on the production of exopolysaccharide pullulan from aureobasidium pullulans

Abstract

For the production of exopolysaccharide pullulan with Aureobasidium pullulans, the physiological factors influencing the production of exopolysaccharide, rheological properties of fermentation broths, development of new alternative carbon sources, and mutagenesis of A. pullulans with MNNG were investigated. The utilization of various sugars was examined. Among them, sucrose gave the best result for the exopolysaccharide production, 27.5 g/l from 5\% sucrose. The specific growth rate and specific production rate of exopolysaccharide were inhibited by the sugar concentrations higher than 0.29 M for glucose and maltose, 0.14 M for sucrose. By a fed-batch fermentation, 58.0 g/l of exopolysaccharide was obtained from total 10 \% sucrose. The cell growth and exopolysaccharide production were greatly influenced by the pH and nitrogen content of fermentation broth. Especially the morphology of cells was dependent on the pH of fermentation broth. Around pH 3.0, cell growth was maximum and mycelial growth was predominant. The exopolysaccharide biosynthesis seemed to optimal about pH 4.5. But the maximum production of exopolysaccharide was achieved under the initial pH 7.5 without pH control. The air supply to cells was necessary to maintain or accelerate the overall metabolic activities of cells for the exopolysaccharide production. Rheology of fermentation broths changed from Newtonian to non-Newtonian fluid by the accumulation of exopolysaccharide and well expressed as power-law equation. About 10\% in the viscosity of fermentation broth was attributed by the cells. The cell growth, exopolysaccharide production, and substrate consumption were generally expressed by the kinetic model equations. Especially the rheological parameters had the relationship with the concentrations of exopolysaccharide produced. As alternative carbon sources, the utilization of starch hydrolysate and inulin was examined. From 5 \% starch hydrolysate (DE 75), 17.0 g/l of exopolysaccharide was ...