On-line monitoring of the viscosity in dextran fermentation using piezoelectric quartz crystal

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Abstract
A novel viscous sensor utilizing AT‐cut quartz crystal to monitor the viscosity of fermentation broth was developed. The sensor system was constructed from the piezoelectric quartz crystal fixed to the cell, exposing only one side of the quartz crystal electrode, an oscillating circuit, a peak level meter, and a personal computer. In order to investigate the characteristics of the sensor system, a sensor signal relating to the resonant resistance of the quartz crystal was measured using dextran solutions with different molecular weights. The linear relationship was obtained between the sensor signal and the (ρη)1/2 of the liquid, where ρ and η are the density and viscosity, respectively. The sensor signal was dependent not only on the viscosity of the liquid but also on the molecular weight of dextran, because dextran solution shows a non‐Newtonian property. The sensor system was applied for the on‐line monitoring of the viscosity in dextran fermentation. A good correlation was observed between the sensor signal and the viscosity value measured with a rotational viscometer for the fermentation broth. Little bubbling effect and agitation of the sensor signal were observed, showing that this system can be utilized for viscosity monitoring in a bioprocess.