Fermentation broth rheology during dextran production by Leuconostoc mesenteroides B512(F) as a possible tool for control

Dextran is a polysaccharide produced commercially by Leuconostoc mesenteroides B512(F) growing on excess sucrose. The high viscosity of the resulting broth makes process analysis and control extremely difficult. In an attempt to overcome this the broth rheology was monitored with time and correlated with other process variables. It was determined that the viscosity and pseudo-plasticity increase through most of the fermentation, but then go through a sharp maximum immediately prior to completion of dextran synthesis and sucrose consumption. This was attributed to release of dextran molecules from the cell wall, reducing the size of polymer aggregates in solution. As such, the change in rheology provides a marker for the completion of the fermentation. The analysis requires no pretreatment of the broth, and it is envisaged that an on-line viscometer, determining apparent broth viscosity at a single shear rate, could be used for process monitoring and control. This would enable early detection of operating upsets, as well as avoid errors introduced by manual sampling and analysis, ans permit the optimum point at which to harvest the broth to be selected more precisely. © 1993 Springer-Verlag.