Rheology of yogurt during pipe flow as characterized by magnetic resonance imaging

Flow behavior of yogurt during pipeflow was investigated using a magnetic resonance imaging (MRI) viscometer. The technique utilizes a velocity profile and independent pressure drop measurement to evaluate rheological parameters. The yield stress and slip velocity were obtained directly from the MR images; the Herschel-Bulkley consistency index, K, and the flow behavior index, n, were obtained through curve fitting procedures for product temperatures of 25C and 35C, over a shear rate range of 0.5-40 s(-1). At 25C, the average value of the yield stress was 14 Pa; the values of K and n were 5.6 Pa S-n and 0.36, respectively. As the wall stress increased from 15 to 42 Pa, the slip velocity increased linearly from 13 to 54 mm s(-1). At 35C, the average value of the yield stress was 10 Pa; the values of K and n were 1.1 Pa s(n) and 0.88, respectively. As the wall stress increased from 12 to 31 Pa, the slip velocity increased linearly from 14 to 59 nun s(-1). These rheological parameters and slip velocities are relevant to the postfermentation cooling process performed in a continuous heat exchanger.