Observations from capillary and closed cavity rheometry on the apparent flow behavior of a soy protein isolate dough used in meat analogues

Abstract Twin-screw extrusion of plant proteins for meat analogue production has gained significant attention. However, understanding the underlying mechanisms remains challenging due to the complex flow behavior of protein doughs and limited measurement techniques. This study investigates the temperature-dependent flow behavior of a soy protein isolate dough (55 wt% water) under extrusion-relevant conditions. Apparent shear viscosity was measured between 95 and 140°C using a capillary rheometer and, for the first time in protein doughs, by performing a ramp test in a closed cavity rheometer (CCR). The Bagley correction was applied, and the resulting correction factors were analyzed and discussed. Flow curves from both methods were fitted with a power-law model. The ramp test resulted in higher consistency indices ( K = 3,412–32,237 Pa s n ) and lower flow behavior indices ( n = 0–0.43) at all temperatures than the uncorrected data from the capillary rheometer ( K = 708–13,013 Pa s n , n = 0.1–0.48). Visual analysis of extruded strands and CCR samples revealed ongoing reactions, stick-slip-like surface patterns, and melt-fracture-like appearances in the extruded strands. In the CCR samples, bulk fracture was evident at 95 and 110°C. At 125°C, deformations of the sample were visible, which could be attributed to partial flow, limited relaxation, or rearrangements within the protein network. Both methods were affected by protein reactions and water evaporation at 140°C, which partially falsified the measurements. Thus, this publication presents two methods for examining viscosity under extrusion-relevant conditions, demonstrating their potential and challenges, and discusses how they contribute to understanding high moisture extrusion for meat analogue production.