Particulate rheology and acid-induced gelation of oxidised cellulose

The rheological properties of a commercial preparation of microdispersed oxidised cellulose (MDOC) from Alltracel Pharmaceuticals have been explored as background to potential applications in functional foods. In the sample studied ∼85% of the particles had diameter <30 μm, with ∼40% below 5 μm, ∼75% of the glucose residues were converted to glucuronate, and solubility was restricted by Ca2+ cations present in the particles at 2:1 equivalent ratio with Na+. Aqueous dispersions of MDOC were pourable, but gave gel-like mechanical spectra at concentrations far below those required for close-packing, indicating formation of a “weak gel” structure by association (adhesion) of the particles. The critical concentration (co) for network formation (G′ > G″) by freshly prepared dispersions was ∼4.0 wt%; at values of concentration (c) well above co, G′ showed the c2-dependence commonly seen for gelling biopolymers. Appreciable increases in moduli were observed on holding (15 h at 20 °C), particularly at low concentrations, consistent with progressive association. The “weak gel” networks remained intact when subjected to small stresses (<1.1 Pa at c = 5 wt % and <13 Pa at c = 10 wt%), but ruptured and flowed in response to higher stress. Breakdown of network structure occurred as a progressive process, beginning when the strain generated by applied stress exceeded a threshold value of ∼50%. Dispersions prepared at concentrations below co had very low viscosities (e.g. ∼0.01 Pa s at c = 3 wt%), but the effective viscosities at c > co (attributed to re-arrangement of network structure) were massive (e.g. 104-105 Pa s at c = 10 wt%). A strong “true” gel structure was obtained by converting MDOC to the water-soluble sodium salt form and reducing pH with d-glucono-δ-lactone (GDL). Gel formation is attributed to suppression of electrostatic repulsion between the polymer chains by conversion of glucuronate residues to the uncharged acid form, and began as the pH dropped below ∼3.3.