Planar extensional viscosity of corn meal dough

The application of the entrance pressure drop method to evaluate the extensional viscosity of corn meal during extrusion-cooking was investigated. An approximate planar extensional flow was generated by forcing the dough from a circular channel barrel into a slit die. The pressure drop along the contraction was calculated to obtain the entrance pressure drop ΔP_en. Both Cogswell's and Binding's analyses were applied to estimate the apparent planar extensional viscosity η_pa from ΔP_en. The magnitude of η_pa from Cogswell's analysis was found to be much larger than that predicted by Binding's analysis. In addition, the severe extension-thinning behavior exhibited by the fluid introduced problems in the calculations of Binding's analysis for some cases. Calculation and interpretation of the Trouton ratios have to be done carefully when extrapolation of the viscosities is required. Moderate to large entrance corrections were obtained, with lower values obtained for the lower moisture contents and temperatures of extrusion. In all cases the entrance correction decreased with increasing flow rate.