Mechanisms of flocculation with chitosan in Escherichia coli distintegrates: effects of urea and chitosan characteristics

• Four different chitosans with different charge densities and different molecular weights were used for investigation of the mechanisms involved in selective flocculation of cell debris particles in E.coli disintegrates. It was found that the main mechanism for flocculation is a "non-equilibrium" bridging process in which a very efficient removal of cell debris particles can be achieved with highly charged chitosans. The high molecular weight (6.5-6.6x105) chitosans produce very large and shear resistant flocs suitable for filtration as well as settling and centrifugation. The low molecular weight (1x105) product forms smaller and more shear sensitive flocs best suited for centrifugation. There are small differences in flocculation dosages depending on molecular weight, but large differences are found with significant changes in charge densities. A decrease in the deacetylation degree from 93 to 39% increased the flocculation dosages by 100 to 150%. A low molecular weight chitosan gave a much broader flocculation region than that of a high molecular weight. Flocculation by addition of urea revealed a hydrogen bonding capacity of chitosan toward cell debris particles which was not involved in chitosan´s interaction with proteins or nucleic acids. The purification of the enzyme -galactosidase could be increased by a factor of 3.7 when using a two step flocculation procedure. The enzyme yield was 82% and the enzyme solution was essentially free of both nucleic acids and cell debris particles.

Nyckelord

Cell debris

chitosan

Escherichia coli

flocculation mechanisms

ß-D-galactosidase

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