Unexpected Rheological Behavior of a Hydrophobic Associative Shellac-Based Oligomeric Food Thickener

Sodium shellac constituted of a "surfactant" monomer, which is sensitive to shear stress, exhibits shear-thickening behavior at a low concentration (5 wt %), and reacts with H⁺ to retain transient high viscosity under shear, is introduced in this study. The steady shear flow test proved that, under a high shear rate, sodium shellac suspension could change from Newtonian fluid to continuous shear-thickening non-Newtonian fluid. The dynamic oscillation test suggested that the sodium shellac solution at low concentrations (0.1 and 1 wt %) under a low shear rate behaved as the viscous fluid ( G″ > G'), and the solution at high concentrations (5, 10, and 15 wt %) behaved as the elastic fluid ( G″ < G'). Moreover, a high shear rate caused a cross-linking point between the G″ and G' curve. At a low concentration, it could be the sol-gel point. At a high concentration, it could be the gel-sol point. All of these transforming points were related to the interaction between the sodium clusters. This interaction should be the hydrophobic association between the particles. To prove the assumption, hydrophilic polymer poly(ethylene oxide) (PEO) was employed as the disrupting factor to the hydrophobic association. As expected, the shear-thickening behavior vanished after mixing with PEO, which verified our assumption. On the other hand, the high viscosity of the suspension under shear could be retained by reaction with H⁺ to solidify the transient hydroclusters under shear. Meanwhile, sodium shellac had great potential as the functional shear thickener, which could modify the rheological property of the polymer with carboxyl groups, e.g., pectin, alginate, or poly(acrylic acid). Thus, this natural and green thicker has great potential in food, medical gel, green adhesive, or cosmetic products.