Rheological behavior of blueberry and red raspberry juice concentrates were studied with the objective of defining suitable mathematical models for use in evaporation and other processing procedures. The flow properties were determined for juice with solids content of up to 65 degrees Brix and temperatures between 20 and 60 degrees C. The two juice products are predominantly Newtonian over the range of temperature and solids content studied. For a given solids content, raspberry juice showed a slightly higher activation energy than blueberry juice (for example, at 65 degrees Brix the values are 41.2 and 39.1 kJ/mol, respectively). With juices containing 10% and 55% dissolved solids, flow instability (Taylor vortices) occurred in the concentric cylinder gap when shear rates reached 180s(-1) and 1000s(-1) respectively. Determination of laminar flow boundaries is important for accurate characterization of flow behavior of fluid foods for processing applications. The mathematical models obtained in this study would be useful for spot determination of juice viscosity during evaporation. (c) 2004 Elsevier Ltd. All rights reserved.