Effect of ingredients on the rheological properties of extruded corn meal

A slit rheometer, mounted on a square cross section barrel extension, was attached to the exit of a single screw extruder to facilitate the simultaneous measurement of steady shear viscosity (η_sh), apparent planar extensional viscosity (η_pa), and the first normal stress differences (N₁). The hole pressure method was used to determine N₁ and the entrance pressure drop method with Cogswell’s analysis for abrupt planar contraction flow was used to determine η_pa. The effect of slot dimensions on N₁was investigated using low density polyethylene. Increasing the depth and width of the slot by 50% produced no significant change in the hole pressure. N₁ predicted using the Higashitani-Pritchard-Baird (HPB) equation compared favorably with extrapolated N₁ values obtained on a cone and plate rheometer. Addition of salt and sugar to corn grits affected the rheological properties of the melt, as observed by the changes in η_sh, N₁, and η_pa. Of the three material functions, N₁ and η_pa were more sensitive to the addition of ingredients. Corn grits exhibited thinning behavior in shear and planar extensional flows.