Challenges in obtaining the steady-state shear viscosity of protein-water mixtures under extrusion-like conditions

Meat substitutes made from plant proteins have recently established themselves on the refrigerated shelves of supermarkets and mimic classic meat products in terms of taste and texture. The meat substitutes are mainly produced using the twin-screw extruder, with functionalization of the proteins taking place in the screw section and structuring taking place in the die section. While the die section of the extruder can be characterized and represented well, the screw section is challenging, which is why it is often regarded as a black box.
Numerical flow simulations offer the opportunity to investigate the screw section of the extruder. In addition to the numerical model, the success of the simulation is largely dependent on the material properties, in particular the steady-state shear viscosity of the material used. Determining the steady-state shear viscosity under extrusion-like conditions is a challenge for protein-water mixtures, as water loss and reactions taking place can influence the measurement. Closed-cavity rheometers (CCR) offer a possible solution, but so far they have hardly been used to investigate the steady-state shear viscosity.
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In this conference contribution, we shed light on the challenge of determining the steady-state shear viscosity of highly concentrated protein-water mixtures. The ramp test, a method for obtaining the steady-state shear viscosity of a CCR, will be presented. First, to verify the method and the accuracy of the CCR data, three polyolefin polymers, a low density and a linear low density polyethylene and a polybutadiene, were used as model systems. Measurements of the magnitude of the complex viscosity were compared with the steady-state shear viscosity data obtained by capillary rheometer and CCR.
Finally, the established method is used to analyze the protein-water mixtures. It is presented under which conditions the measurements are successful and what information can be obtained.