A general viscosity model for high moisture extrudates of pea protein isolates/gluten blend

A capillary pre-shearing rheometer, Rheoplast (R), allowing for simulation of an extrusion process, was used to determine the viscosity of 60/40 pea protein isolate/gluten blends previously extruded at different temperatures (130 degrees C, 140 degrees C, 150 degrees C) and moisture contents (50%, 55%, 60% w/w). Electronic microscopy and mechanical testing showed that the materials displayed distinct anisotropic and fibrous structures. Pressure profiles were determined over an apparent shear rate range of 10-10(4) s(-1), using capillary dies with length/diameter (L/D) ratios 8, 16, and 32 (D = 1 mm). For all materials, the pressure profiles are regular, suggesting the absence of wall slip. All flow curves could be fitted with the Ostwald-de Waele model. The values of consistency index (K), ranging from 480 to 5000 Pa.s(n), and those of the flow index (n), from 0.25 to 0.7, were inversely correlated, reflecting the effect of material structuring. Furthermore, the time-temperature superposition principle was extended to account for the influence of water content by introducing the glass transition temperature (Tg) of the protein mixture with curve shift factors depending linearly on Tg/T. All results can then be fitted using a Carreau model, describing the viscous behavior within the range [1, 10(5) s(-1)] with plateau viscosity (eta(0)) values varying between 240 and 1050 Pa.s. Finally, by extrapolating Bagley plots to L/D = 0, entry pressure was derived, and, consequently, apparent elongational viscosity was determined, leading to Trouton number values around 100. Results are interpreted by structural differences and the viscosity model can be used for computer simulation of flow in the die.