Structuration of plant proteins: a soft matter approach for the design of sustainable food products

Jean-Gabriel PICHON

In this thesis, we investigate the gelation of dispersions of two types of plant-based food proteins: wheat gluten proteins and potato proteins. Gluten protein dispersions undergo spontaneous gelation over time while remaining close to the critical state that characterizes the transition between the liquid and solid states. The mechanical response of these samples to a series of large deformations interspersed with relaxation phases was studied throughout the gelation process. As the gel point is approached, a singular response was identified during the first deformation of the samples. During a start-up shear, gluten starts to flow following a hardening induced by very large deformations. This strain hardening behavior disappears during subsequent deformations but reappears after long rest periods, revealing the self-healing ability of gluten. A densification of the network induced by successive large-amplitude shear cycles was also observed, highlighting the dynamic nature of the gel. Potato proteins, on the other hand, can gel through acidification or thermal treatment of colloidal solutions. Lowering the pH reduces electrostatic repulsions, leading to aggregation or even gelation, sometimes accompanied by syneresis at sufficiently high concentrations. Various kinetics are observed depending on the pH, but no steady state is reached under these conditions. In contrast, different stable gel states can be obtained in a controlled manner through thermo-induced gelation. We were thus able to study the impact of the proximity to the gel point on the mechanical properties of the gels in both linear and nonlinear regimes. Increased extensibility, accompanied by strain hardening, was identified near the gel point, regardless of the gelation pathway. Despite remarkable structural differences between the two types of proteins — intrinsically disordered proteins for gluten and globular proteins for potato — it appears that proximity to the gel point is an important criterion to consider for controlling the mechanical response of gels under small or large deformations. This is relevant for applications such as kneading, stabilizing gas bubbles in dough, or chewing a food gel.