Design of immobilised dextransucrase for fluidised bed application.

Immobilisation of dextransucrase from Leuconostoc mesenteroides NRRL B-512F in alginate is optimised for applications in a fluidised bed reactor with high concentrated sugar solutions, in order to allow a continuous formation of defined oligosaccharides as prebiotic isomalto-oligosaccharides. Efficient design of fluidised bed immobilised biocatalyst in high density solutions requires particles with elevated density, high effectiveness and both thermal and mechanical stability. Inert silica flour/sand (Mikrosil 300) as supplement turned out to be best suited for increasing the density up to 1400 kg m(-3) of the alginate beads and generating a stable expanded bed without diffusional restrictions. Kinetic investigations demonstrate that low effectiveness of immobilised enzyme due to close association to dextranpolymers (dextran content of enzyme preparation >90%) is compensated by reducing the particle size and/or by decreasing the dextran content. A low dextran content (5%) is sufficient to immobilise and stabilise the enzyme, thus diffusional limitation is reduced essentially while operational stability is maintained. Fluidisation behaviour and bed expansion proved to be appropriate for the intended application. Both calculated and measured expansion coefficients showed good agreement for different conditions.