Plant-based meat analogues that mimic the characteristic structure and texture of meat are becoming increasingly popular. They can be produced by means of high moisture extrusion (HME), in which protein-rich raw materials are subjected to thermomechanical stresses in the extruder at high water content (>40%) and then forced through a cooling die. The cooling die, or generally the die section, is known to have a large influence on the products’ anisotropic structures, which are determined by the morphology of the underlying multi-phase system. However, the morphology development in the process and its relationship with the flow characteristics are not yet well understood and, therefore, investigated in this work. The results show that the underlying multi-phase system is already present in the screw section of the extruder. The morphology development mainly takes place in the tapered transition zone and the non-cooled zone, while the cooled zone only has a minor influence. The cross-sectional contraction and the cooling generate elongational flows and tensile stresses in the die section, whereas the highest tensile stresses are generated in the transition zone and are assumed to be the main factor for structure formation. Cooling also has an influence on the velocity gradients and, therefore, the shear stresses; the highest shear stresses are generated towards the die exit. The results further show that morphology development in the die section is mainly governed by deformation and orientation, while the breakup of phases appears to play a minor role. The size of the dispersed phase, i.e., size of individual particles, is presumably determined in the screw section and then stays the same over the die length. Overall, this study reveals that morphology development and flow characteristics need to be understood and controlled for a successful product design in HME, which, in turn, could be achieved by a targeted design of the extruders die section.
The steady-state shear viscosity η ( γ ̇ ) \eta (\dot{\gamma }) is required in controlling processing parameters for the extrusion processing of polymer melts. A new method, the so-called ramp test, is investigated in this study to obtain the steady-state shear viscosity with a closed cavity rheometer (CCR). To verify the method and the accuracy of the CCR data, three commercial polyolefin polymers, a low-density polyethylene (LDPE), a linear low-density polyethylene (LLDPE), and a polybutadiene (PBD), were used as model systems. Measurements of the magnitude of the complex viscosity ∣ η ⁎ ( ω ) ∣ | {\eta }^{\ast }(\omega )| were compared with the steady-state shear viscosity data obtained by capillary rheometer and CCR. Further, time–temperature superposition master curves of the magnitude of the complex viscosity and steady-state shear viscosity obtained by CCR were developed for LLDPE and PBD. The influence of the cavity sealing on the instrument’s accuracy to obtain the steady-state shear viscosity was investigated using the finite element method simulations. Thus, it was shown that the ramp test performed by CCR is a practical method to determine reliable and reproducible data of the steady-state shear viscosity within a wide range of temperatures (T = 50–180°C) for low and high viscous materials ( ∣ η ⁎ ( ω ) ∣ | {\eta }^{\ast }(\omega )| = 1.6–480 kPa s, M w = 144–375 kg mol−1).
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