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M. J. MCCARTHY¹, R. L. Powell², J. A. Fort³, D. M. Pfund³, and D. M. Sheen³. (1) Department of Food Science and Technology, University of California, One Shields Avenue, Davis, CA 95616-8598, (2) Department of Chemical Engineering and Material Science, University of California, One Shields Avenue, Davis, CA 95616, (3) Fluid Dynamics Group, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, WA 99352

Performance of processing systems and the quality of many liquid food systems depends upon the viscosity of the food. These liquid food systems are generally heterogeneous and involve a multiphase mixture of solids, liquids and, often, gases. These multiphase foods are most often opaque and it may be difficult to take a representative sample that can be tested off-line. The goal of this work is to develop on-line ultrasonic-based sensors for the measurement of fluid food viscosity. The principle of the technique rests with the conservation of linear momentum for steady pressure driven flow in a tube. Here, the pressure drop in the tube determines the local shear stress. A simultaneous measurement of the velocity and a robust technique for differentiating it provides the local shear rate distribution. Evaluating the shear rate and the shear stress at points along the tube radius yields the shear stress and shear rate relation. The superiority an experimental technique based upon this principle is that a single measurement of the velocity profile and the pressure drop provides the viscosity over a wide range of shear rates - up to two decades in practice. This permits the accurate characterization of complex liquids including slurries. Implementing this technique for slurries requires measurement methods that provide for the accurate velocity determination over the entire tube radius and that the system can work in the presence of particles and with opaque systems. We have developed a technique based ultrasonic Doppler velocimetry (UDV). The instrument is capable of making measurements of velocity in tubes. We have shown that these measurements can reproduce the parabolic velocity profile found for Newtonian fluids. We have also been able to make accurate measurements of the velocity of fluids that exhibit power law behavior with the concomitant blunted velocity profiles. UDV measured viscosity data agree well with results from traditional viscometers above 10 s^-1.