30E-3 |
In-line viscosity measurements of yogurt using selected viscometers |
J. H. LEE, Division of Food, Biological & Chemical Engineering, Taegu University, 15 Naeri Jinryang, Kyoungsan, 712-714, South Korea, T. A. HALEY, Department of Food Science, Purdue University, Food Sci. Building, W. Lafayette, IN 47907, and O. H. Campanella, Department of Agricultural and Biological Engineering, Purdue University, AGEN Building, W. Lafayette, IN 47907. Viscosity is an important factor to influence the final product quality and therefore, it has to be monitored and controlled. Knowledge of the flow behavior of the product during food processing is essential to develop cost-effective processes and they are important information for the proper design and operation of food material handling equipments. Our objectives were to evaluate the performance of selected commercial ln-line viscometers, namely an oscillating sphere viscometer, a tube viscometer and a coaxial cylinder viscometer for their ability to characterize the rheological properties of yogurt and compare the results from off-line rheometry. The flow rate for each fluid was varied at discrete values of approximately 3.5 or 16.2 L/min within a range of 4.8-19.0 or 23.2-88.2 L/min for Exp. 1 and Exp. 2, respectively. Data were collected when the flow rate was both increased and decreased for the Exp. 1 while the flow rate was only increased for the Exp. 2. For the coaxial cylinder viscometer, the RPM was varied at discrete values within a range of 20-200 RPM. Before changing to a new speed setting, a sample was taken for off-line rheological analysis. The results of tube as well as coaxial cylinder viscometers indicated a shear thinning behavior of yogurt with yield stress. While, the results of the oscillating sphere viscometer underestimated the flow curves generated from off-line rheometry. When the shear stress was measured or calculated with increasing shear rate, the shear stress values were higher than those determined with decreasing shear rate showing hysteresis phenomena. These phenomena were more evident with tube and coaxial cylinder viscometers. These results suggest that these in-line viscometers can successfully used to monitor the rheological properties of yogurt with proper calibration. Additional characteristics of each instrument that should be considered when choosing an instrument for in-line application are presented.
Session 30E, Food Engineering: Rheology and texture
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