Extrapolation of classical rheometry of plant protein pastes to extrusion conditions

Abstract

This study explores alternative methods to measure the rheology of pastes based on soy and pea proteins, which are commonly used as starting material in extrusion for the preparation of meat analogs. Currently, it is still a challenge to obtain consistent rheological data on such materials for steady shear at conditions that are relevant for the extrusion of such systems. Hence, possibilities are explored for the extrapolation of results obtained with classical rheometers operating at temperatures below boiling point, to conditions that are relevant for the extrusion of such systems. The above was combined, with findings from off-line and in-line capillary rheometers, which can provide steady shear measurements above boiling point and at hydration levels as used in extrusion.We have analysed the consistency of our new results with previous oscillatory shear measurements, performed at conditions similar to those of meat analog extrusion. For validation of the extrapolation we have analysed whether the new rheology data follow similar scaling rules. As classical rheometers can handle only moderate torques, the measurements are limited to hydration levels higher than those used in extrusion (75–85%). Despite this limitation, strain sweeps with classical rheometers show that in this regime the elastic modulus also scales with Tg/T, the ratio of glass transition, and actual temperature. Yet, its dependency on Tg/T appears differently from measurements performed at extrusion-like conditions. Moreover, our current results indicate a strong thixotropic nature of plant protein pastes at temperatures below boiling point. Consequently, flow curves are only stable if performed from high to low shear rates. Nevertheless, our data can be mapped to a single master curve following the Herschel-Bulkley model, with parameters also following a scaling rule dependent on Tg/T.This Tg/T scaling is used to construct master curves with data from the off-line and in-line capillary rheometers. The obtained shift factors for the flow curves are in agreement with old literature data obtained for SPI, but they deviate significantly from our steady shear measurements with classical rheometers. Based on the results of our current and previous study we have a critical, general discussion on the possibility of extrapolation of classical rheometry of plant protein pastes to process conditions used for production of meat analogs.