Magnetic resonance methods for quantifying multiscale anisotropic structure in soy protein extrudates

High-moisture extrusion is a proven industrial technique used to create anisotropic structured plant protein materials, involving the mixing and hydration of protein, followed by thermomechanical treatment and subsequent cooling under shear in a cooling die. However, the complex structure formation mechanisms influencing the final fibrous anisotropic texture of the extrudate are not yet fully understood [1]. Measurement techniques able to characterise phase-separated anisotropic protein extrudates at a multi-scale level, and thus verify structure formation hypotheses, are lacking, or have yet to be validated. Low-Field Time Domain Nuclear Magnetic Resonance (LF TD-NMR) methods are widely used to non-invasively characterize the water molecular mobility within optically-opaque food matrices [2]. The interpretation of such spatially-unresolved data, however, can be challenging without the required spatial information about the  respective microstructure. In this work, ¹H TD-NMR and high-field Magnetic Resonance Imaging (HF MRI) are used to investigate, from inter-molecular up to sub-mm level, the multiscale structure of phase-separated domains in soy protein extrudates. Swelling sample in water increases signal-to-noise ratio and emphasizes lamellar regions, whereas freeze-thawing enhances phase separation due to freeze concentration. HF TD-MRI measurements reveal two distinct populations of mobile water associated with protein-rich and water-rich phases. Diffusion Tensor Imaging (DTI) measurements unveil water diffusional anisotropy within these phase-separated regions. Additionally, LF TD-NMR unravels two signal components unaffected by water-swelling or freeze-thawing, respectively  assigned to the immobile semi-crystalline cellulose or to gelled protein phases. HF-MRI of SPC deadstop ribbon shows how structural anisotropic structure increases along the length of the cooling die. We conclude that the combination of our selected TD-NMR and MRI approaches provides a non-invasive and multi-scale ex situ characterization of phase separation in soy protein extrudates as a function of the adopted processing conditions. This will serve as a necessary springboard for translating these, and other, methods to in situ multi-scale characterization, as well as for generating robust experimental evidences for the underlying structure formation mechanisms thus far hypothesized [1,3].

References

[1] van der Sman, et al. Curr. Res. Nutr. Food Sci. 2023, 6, 100510.

[2] van Duynhoven et al. Annu. Rep. NMR Spectrosc. 2010, 69:3, 145.

[3] Kaunisto, et al. J. Food Eng. 2024, 362, 111760.