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

Rheological characteristics of Desi 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^-1 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^-1, respectively. At the four selected temperatures, the ghee samples displayed non-Newtonian shear thinning behavior with flow behavior index (n) ranging from 0.224 to 0.911. As the shear rate increased from 0 to 100 s^-1, 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 (R ), standard deviation (SD), and relative deviation percentage (R ). The value of activation energy (E ), as calculated from Arrhenius type equation, was found to be 1.98 × 10⁶ kJ mol^-1 over the entire temperature range. The study also revealed that the magnitudes of dynamic shear viscosity (η*) were higher than those of the steady shear viscosity (η) at the four temperatures, indicating that the Cox-Merz rule was not applicable to the ghee samples.