Quantification of phase separation in high moisture soy protein extrudates by NMR and MRI
Sam A. Kuijpers, Gert-Jan Goudappel, Thom Huppertz, John van Duynhoven, Camilla Terenzi
Abstract
• MRI visualises phase-separation in high moisture extrudates of soy proteins. • Water swelling enhances 1 H NMR signal and contrast between phase-separated domains. • Freeze-thawing enhances delamination of phase-separated domains. • Bulk low field time domain NMR can quantify phase-separated structures seen by MRI. • MRI quantifies development of anisotropic structure within cooling die. High-moisture (HM) extrusion is the dominant industrial process to create structured plant-based protein products that can be used for animal-free meat alternatives. Yet, the underlying mechanisms, such as phase separation, that govern structure formation in plant-protein extrudates, are still poorly understood. Current hypotheses require experimental data in order to be verified, but measurement techniques able to quantify phase-separated anisotropic protein extrudates are lacking, or have yet to be validated. In this study, Low-Field Time Domain (LF TD)-NMR and High-Field (HF) MRI techniques have been employed to unravel phase separation in HM extrudates of soy proteins. Results show that swelling with water enhances the 1 H NMR/MRI signal-to-noise ratio in the measurements and unveils the presence of lamellar regions, while freeze-thawing enhances phase separation due to freeze concentration. Phase separation could be quantified by the observation of two distinct populations by LF TD-NMR T 2 measurements. MRI images of dead-stop ribbon samples from interrupted HM extrusion revealed how the thickness of the aligned lamellar regions increases during passage of the protein melt through the cooling die. We conclude that TD-NMR can quantify phase separation, while spin-echo MRI can spatially resolve the lamellar structure conformation of HM extrudates. Thus, NMR and MRI are powerful techniques for non-invasively characterizing ex situ structure formation during HM extrusion, and for validating hypotheses on shear- and temperature-induced phase separation.