Underpinning hierarchical anisotropic structure formation in soy protein high-moisture extrudates by Magnetic...

Plant-based meat alternatives are fibrous anisotropic food protein products that mimic the texture and palatability of animal-based foods. The main industrially scalable production technique for manufacturing such structured protein materials is high-moisture extrusion, which involves the thermo-mechanical processing, and subsequent cooling, of a hot protein melt in the presence of shear and temperature gradients. Yet, the complex mechanisms governing fibrous structure formation in plant-protein extrudates are poorly characterised, in turn limiting the rational design and optimisation of the extrusion process. In the present work, we aimed at characterising the hierarchical nature of anisotropic structures in pH-shifted high-moisture extrudates of soy protein concentrate (SPC) by Magnetic Resonance Imaging (MRI). $T_2$ -weighted MRI data, and corresponding Rotated Fourier Transform (RFT) analysis results, showed phase-separated protein-rich (PR) and water-rich (WR) lamellae at the 10² $\mu m$ length scale. Chemical Exchange Saturation Transfer (CEST) MRI measurements showed the presence of dissolved proteins in the WR regions. Diffusion Tensor (DT)-MRI, and resulting Principal Diffusion Direction (PDD) maps, probe the hindrance of water self-diffusion down to the 10^-1 $\mu m$ length scale and, here, revealed the presence of an underlying anisotropic structure in the PR lamellae. The latter finding was confirmed by Confocal Laser Scanning Microscopy (CLSM) measurements. The detailed features of RFT and PDD(DT) maps of these extrudates could be reproduced by simulating the effect of the cross-sectional cooling die geometry. Additionally, we found that the presence of structural order at the smallest length scale is a prerequisite for observing water diffusional anisotropy. We foresee that, in the future, the developed measurement methods will be deployed in the design and characterisation of protein-based products with a fibrous texture.