Kinetic analysis of a high viscossity polysaccharide, methylan, production from methanol by methanol by methylobacterium organophilum

Abstract

For the develpment of the fermentation process for a high viscosity exopolysaccharide, Methylan, production by Methylobacterium organophilum from methanol, the effects of environmental factors and fermientation variables on cell growth and Methylan production were investigated in fedbatch culture and kinetic modelling was also attempted to illustrate the fermentation data. The concentrations of cell and Methylan were maximum at 30$^\circ$C and pH 7. The high temperature and high pH such as 36$^\circ$C and pH 8 reduced significantly the viscosity of Methylan solution. Howere, high aeration rate and agitation speed and longer fermentation time were favorable for the enhanced viscosity of Methylan solution at the same concentration. It means that environmental factors affect significantly the degree of polymerization of Methylan. Effect of the concentration of ammonium ion was investigated in fedbatch cultures. Cell growth increased proportionally to the initial concentration of ammonium ion in the medium. However, Methylan production was inhibited remakably by the high concentration of ammonium ion. Therefore, it was necessary to feed continuously the nitrogen source for the higher production of Methylan. In order to determine the feeding method of nitrogen source, fed-batch cultures using media with different C/N ratios (methanol/ammoniumion) were carried out. The higher values of C/N resulted in the lower cell growth rate and the higher production rate of Methylan. For the increase of Methylan concentration, the C/N ratio must be changed from lower value to higher value (only C fed), after cell growth reached the desired level. By feeding ammonium ions within the desired level (usually 0.45 g/l) during the exponential phase, Methylan concentration if fed-batch culture could be increased up to 12.5 g/l. Cells grew continuously through the gaseous nitrogen assimilation even after the ammonium ion supplied to the medium was depleted. At this phase, molybdenum play...