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Alginate is a copolymer of β- d-mannuronic and α- l-guluronic acids. Distribution of these monomers in the alginate structure is one of the important characteristics that affect the commercial value of the polymer. In the present work, the effect of agitation speed in the range of- rpm on alginate production by Azotobacter vinelandii ATCC was investigated at a dissolved oxygen tension of5% of air saturation. Experiments were conducted in a fermentor operated in batch mode for h while the production of biomass and alginate, the consumption of substrate and the change in culture broth viscosity and monomer distribution of the polymer were monitored. Results showed that the growth rate of the bacteria increased from0. to0. h by the increase of mixing speed from to rpm. On the other hand, alginate production was found to be the most efficient at rpm with the highest value of4. g/l achieved at the end of fermentation. The viscosity of culture broth showed similar trends to alginate production. Viscosity was recorded as. cP at rpm while it was only4. cP at rpm. The MM- and GG-block contents were almost equal in most of the culture times at rpm. On the other hand, GG-blocks dominated at both low and high mixing speeds. Knowing that GG-blocks make rigid and protective gels with divalent cations, due to the higher GG-block content, the gel formation potential is higher at rpm as well at rpm, which might originate from the unfavorable environmental conditions that the bacteria were exposed to.
