Controlling Slip-Flow Onset of Hydrated Soy Protein Melts as a Basis for Selecting Cooling Die Temperature Profiles in High-Moisture Extrusion.

High-moisture meat analogs (HMMAs) acquire their fibrous texture primarily during cooling in an extrusion die, yet practical guidance for selecting die cooling media temperatures is scarce. Understanding melt fracture behavior from nonfood polymer processing can be applied to a viscous food protein suspension to identify physical material transitions that recent work has shown govern HMMA structure formation and heat transfer. A pressurized (6 bar) parallel-plate rheometer operated at constant shear rate (2.5 Hz) and ramped cooling (-2.5°C min-1) was used to measure the torque response of soy protein hydrated at two levels (50% and 60% w.b.). The shear stress response of the material decreased upon cooling and exhibited plateaus interpreted as slip regimes. We define Tcritical (slip onset, upper bound) and Tfreeze (maximum slip, lower bound) to demarcate a "texturization zone" where partial slip allows sustained deformation and fibrous alignment of the soy protein. Guided by these thresholds, we performed HMMA extrusion trials with a segmented cooling die, with the first section (of three) carrying heat transfer media at discrete levels between 100°C and 145°C, whereas the remaining sections were held at 36°C. Products extruded at 115-130°C (first-section media) showed visibly larger, more continuous fiber bundles and significantly higher anisotropy indices at higher moisture. In contrast, hardness was unaffected by the first-section temperature (p > 0.05) and was primarily influenced by moisture content. These findings demonstrate a rheology-to-process linkage, where maintaining product residence within the slip-bounded texturization zone by elevating the initial die-section temperature improves HMMA structural anisotropy.