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Characterization of flow behavior of yogurt using MRI |
W. B. YOON and K. L. McCarthy. Food Science and Technology, University of California-Davis, One shield Ave., Davis, CA 95616 Rheological behavior of yogurt is important to characterize processing conditions as well as final product qualities. The particulate gel structure of yogurt causes unusual rheological behavior, such as yield stress and slip. Conventional rheometry requires complex procedures to characterize these problematic properties. Magnetic resonance imaging (MRI) techniques have been used to visualize the flow profiles. Such problematic rheological parameters can be directly obtained from the MRI flow images. The objective of this study was to characterize flow behavior of yogurt with MRI-based and conventional viscometers. A commercial yogurt was used for this study. To obtain off-line rheological parameters, the Bohlin CVO rheometer and a vane viscometer were used. Viscosity was measured in the shear rate range 0.015–100 (1/s) in a bob-and-cup geometry. The vane geometry was used to determine the yield stress. To obtain the images, the MRI measurements were conducted using a SMIS spectrometer connected to a 0.1T electromagnet. A horizontal capillary viscometry (ID=0.95cm, length=1.3m) was centered in the spectrometer. Yogurt showed typical shear thinning behavior with a yield stress. The yield stress from the vane viscometer was determined to be 12.7 Pa. The consistency (8.59 Pa.s), flow index (0.48), yield stress (7.46 Pa) were determined by the Herschel-Bulkley model. The MRI flow images yielded the yield stress (9.21 Pa), consistency (8.86 Pa.s), and flow index (0.29), as well as the slip velocity. The slip velocity was ranged from 1.26 to 6.3 cm/s as a linear function of average flow velocity (1.4-8.6 cm/s) in the capillary. Similar results were observed from both viscometers. The advantage of the MRI-based viscometer is the ability to measure the slip velocity and yield stress of yogurt at any process moment. This implies the potentiality of using a MRI-based viscometer for in-line process control of yogurt manufacture.
Session 42, Food Engineering: Rheology and texture
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