Time-dependent viscosity of stirred yogurt. Part II: tube flow Original Research Article

Pressure drop/flow rate data at 5°C for stirred natural yogurt was obtained using a tube viscometer with tube diameters of 1.86 and 3.34 mm and tube lengths of 0.25, 0.5, 1.0, 1.5 and 2.0 m. When subjected to a range of laminar flow rates in the tube viscometer, yogurt demonstrated flow behaviour associated with thixotropic fluids. Measured pressure drops were compared to results predicted analytically using time-independent power law models to characterise initial and equilibrium viscosity in the shear rate range employed. These viscosities were determined using a conventional rotational viscometer. Pressure values measured experimentally were generally less than those predicted using the initial power law viscosity model and were consistently greater than those predicted using the equilibrium power law viscosity model. While pre-shearing the samples gave rise to a reduction in the values of pressure drop per unit length of tube, the experimental pressure values obtained were bounded by values predicted using the initial and equilibrium power law viscosity models. To allow for time-dependent effects, the power law indices, K and n, were modified to account for tube residence time. For all tubes used and in the range of flow rates employed, predicted pressures drops using the modified power law equation were closer to the experimental values (error 0–41%) than the corresponding pressure drops predicted using the initial (error 0–129%) or equilibrium (error 37–83%) power law viscosity models.