Concentration-dependent rheology of chia seed mucilage across shear and extensional deformation

Abstract

Chia seed mucilage (CSM) is a plant-derived heteropolysaccharide with potential as a natural thickener, yet its structure–function relationships under combined shear and extensional flow remain unclear. In this study, CSM dispersions (0.01–2.0 g dL 1) were characterized under pH 3 and pH 7 buffered conditions using Fourier transform infrared (FTIR) spectroscopy, steady shear, small-amplitude oscillatory shear, and dripping-onto- substrate (DoS) extensional rheometry. FTIR spectra confirmed the polysaccharide-rich character of the extracted material and revealed differences in the carboxyl/carboxylate region between the two buffered conditions. Specific viscosity measured at 0.1 s 1 exhibited an apparent concentration crossover capp ) at approximately 0.14 g dL 1 at pH 3 and 0.30 g dL 1 at pH 7. Above these concentrations, the dispersions showed pronounced shear-thinning. At 0.3 g dL 1, G″ exceeded G′ throughout the investigated frequency range, whereas at 1.0–1.5 g dL 1 G′ exceeded G″ at low-to-intermediate frequencies, indicating weak and transient elastic structures rather than a permanent gel. Deviations from Cox–Merz rule (η* >η) suggest the presence of transient, shear-sensitive networks with η* exceeding η. Extensional relaxation time increased with concentration and reached approximately 4.4 ms at pH 3 and 5.4 ms at pH 7 at 2.0 g dL 1. Overall, CSM forms weak, dynamically reorganizable structures primarily governed by transient interactions, while differences between the two buffered conditions were most evident near the onset of intermolecular connectivity. Because buffer identity and ionic strength were not independently controlled, these differences cannot be attributed exclusively to pH.