Temperature dependent steady and dynamic oscillatory shear rheological characteristics of Indian cow milk () ghee.

Rheological characteristics of ghee were investigated at 18, 24, 30 and 36 °C. The steady shear properties were evaluated by varying the shear rate from 0.01 to 100 s and the dynamic shear properties were studied by varying strain and frequency sweep from 0.01 to 100% and 0.1 to 100 rad s, respectively. At the four selected temperatures, the ghee samples displayed non-Newtonian shear thinning behavior with flow behavior index () ranging from 0.224 to 0.911. As the shear rate increased from 0 to 100 s, the values of dynamic viscosity decreased from 54 to 8.14, 20.01 to 1.05, 1.33 to 0.295, and 3.02 to 0.0025 Pa s at 18, 24, 30 and 36 °C, respectively. Out of four rheological models (Power-law or Ostwald-de Waele, Herschel-Bulkley, Casson, and Bingham model) fitted to the shear rate and stress data, the Ostwald model was found to be superior in predicting the shear rate-stress data at 18 °C, whereas Ostwald-de Waele and Herschel-Bulkley models predicted all the data points over the temperature range of 24-30 °C, as observed by the values of coefficient of determination ( ), standard deviation (), and relative deviation percentage ( ). The value of activation energy ( ), as calculated from Arrhenius type equation, was found to be 1.98 × 10 kJ mol over the entire temperature range. The study also revealed that the magnitudes of dynamic shear viscosity (*) were higher than those of the steady shear viscosity () at the four temperatures, indicating that the Cox-Merz rule was not applicable to the ghee samples.